U.S. patent application number 14/497162 was filed with the patent office on 2016-03-31 for time transformation of local activation times.
This patent application is currently assigned to APN HEALTH, LLC. The applicant listed for this patent is APN HEALTH, LLC. Invention is credited to Donald Brodnick, Jasbir Sra.
Application Number | 20160089048 14/497162 |
Document ID | / |
Family ID | 55581914 |
Filed Date | 2016-03-31 |
United States Patent
Application |
20160089048 |
Kind Code |
A1 |
Brodnick; Donald ; et
al. |
March 31, 2016 |
TIME TRANSFORMATION OF LOCAL ACTIVATION TIMES
Abstract
An automatic method of determining local activation time (LAT)
from at least four multi-channel cardiac electrogram signals
including a ventricular channel, a mapping channel and a plurality
of reference channels. The method comprises (a) storing the cardiac
channel signals, (b) using the ventricular and mapping channel
signals and a first reference channel signal to compute LAT values
at a plurality of mapping-channel locations, (c) monitoring the
timing stability of the first reference channel signal, and (d) if
the timing stability of the monitored signal falls below a
stability standard, using the signal of a second reference channel
to determine LAT values. Substantial loss of LAT values is avoided
in spite of loss of timing stability.
Inventors: |
Brodnick; Donald;
(Cedarburg, WI) ; Sra; Jasbir; (Pewaukee,
WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
APN HEALTH, LLC |
Pewaukee |
WI |
US |
|
|
Assignee: |
APN HEALTH, LLC
Pewaukee
WI
|
Family ID: |
55581914 |
Appl. No.: |
14/497162 |
Filed: |
September 25, 2014 |
Current U.S.
Class: |
600/512 |
Current CPC
Class: |
A61B 5/04015 20130101;
A61B 5/0464 20130101; A61B 5/725 20130101; A61B 5/04525 20130101;
A61B 5/7257 20130101; A61B 5/7246 20130101; A61B 5/04011 20130101;
A61B 5/04018 20130101; A61B 5/0408 20130101; A61B 5/0432 20130101;
A61B 5/0452 20130101; A61B 5/7221 20130101 |
International
Class: |
A61B 5/04 20060101
A61B005/04; A61B 5/00 20060101 A61B005/00; A61B 5/0408 20060101
A61B005/0408; A61B 5/0452 20060101 A61B005/0452; A61B 5/0432
20060101 A61B005/0432; A61B 5/0464 20060101 A61B005/0464 |
Claims
1. An automatic method of determining local activation time (LAT)
from at least four multi-channel cardiac electrogram signals
including a ventricular channel, a mapping channel and a plurality
of reference channels, the method comprising: storing the cardiac
channel signals; using the ventricular and mapping channel signals
and a first reference channel signal to compute LAT values at a
plurality of mapping-channel locations; monitoring the timing
stability of the first reference channel signal; and if the timing
stability of the monitored signal falls below a timing-stability
standard, using a second reference channel signal to determine LAT
values and avoid substantial loss of LAT values in spite of loss of
timing stability.
2. The automatic LAT-determining method of claim 1 further
including computing one or more timing offsets using pairs of the
plurality of reference channel signals, a timing offset being
LAT.sub.K(J), the local activation time of a reference channel J
based on a reference channel K and used to transform an LAT value
based on reference channel J to an LAT value based on reference
channel K.
3. The automatic LAT-determining method of claim 2 wherein using
the second reference channel signal to determine LAT values
includes transforming future LAT values such that they are based on
the first reference channel.
4. The automatic LAT-determining method of claim 3 wherein:
LAT.sub.2(M) is a future LAT value of mapping channel M based on
the second reference channel; and future transformed values
LAT.sub.1(M) of mapping channel M based on the first reference
channel are equal to a timing offset LAT.sub.1(2) plus
LAT.sub.2(M).
5. The automatic LAT-determining method of claim 2 wherein using
the second reference channel signal to determine LAT values
includes transforming past LAT values such that they are based on
the second reference channel.
6. The automatic LAT-determining method of claim 5 wherein:
LAT.sub.1(M) is a past LAT value of mapping channel M based on the
first reference channel; and past transformed values LAT.sub.2(M)
of mapping channel M based on the second reference channel are
equal to a timing offset LAT.sub.2(1) plus LAT.sub.1(M).
7. The automatic LAT-determining method of claim 2 wherein the one
or more timing offsets are computed at a plurality of times, and
the value of each timing offset is replaced with its average over
the plurality of times.
8. The automatic LAT-determining method of claim 7 wherein the
average is computed over a predetermined number of times.
9. The automatic LAT-determining method of claim 2 wherein
monitoring the timing stability of the first reference channel
signal includes monitoring multiple timing offsets LAT.sub.1(X)
where X represents the channels with which timing offsets with the
first reference channel are computed.
10. The automatic LAT-determining method of claim 9 further
including computing a signal characteristic for the plurality of
reference channels and determining therefrom which one or more
channels among these reference channels has/have not lost timing
stability.
11. The automatic LAT-determining method of claim 10 further
including selecting the second reference channel signal from
channels which have not lost timing stability.
12. The automatic LAT-determining method of claim 11 wherein
selecting the second reference channel signal from channels which
have not lost timing stability includes computing signal
quality.
13. The automatic LAT-determining method of claim 10 wherein
computing a signal characteristic of a signal includes computing
the frequency content of the signal.
14. The automatic LAT-determining method of claim 13 wherein
computing the frequency content of the signal includes computing a
Fourier transform for a predetermined time period of the
signal.
15. The automatic LAT-determining method of claim 13 wherein
computing the frequency content of the signal includes computing a
fast Fourier transform for a predetermined time period of the
signal.
16. The automatic LAT-determining method of claim 15 wherein the
computed signal characteristic is the first moment of the signal
power determined from the fast Fourier transform.
17. The automatic LAT-determining method of claim 15 wherein the
signal is segmented into a plurality of time-overlapping segment
signals.
18. The automatic LAT-determining method of claim 17 wherein
weightings are applied to each of the segment signals.
19. The automatic LAT-determining method of claim 18 wherein
computing the fast Fourier transform of the signal includes (a)
computing a signal-segment fast Fourier transform for each segment
signal and (b) averaging each such signal-segment fast Fourier
transform to form the fast Fourier transform of the signal.
20. The automatic LAT-determining method of claim 19 wherein the
computed signal characteristic is the first moment of the signal
power determined from the fast Fourier transform of the signal.
21. The automatic LAT-determining method of claim 13 wherein
computing the frequency content of the signal includes computing a
Haar transform for a predetermined time period of the signal.
22. The automatic LAT-determining method of claim 21 wherein the
computed signal characteristic is the first moment of the signal
power determined from the computed Haar transform.
23. The automatic LAT-determining method of claim 21 wherein the
signal is segmented into a plurality of substantially-sequential
segment signals.
24. The automatic LAT-determining method of claim 23 wherein
computing the Haar transform of the signal includes (a) computing
Haar transform coefficients for each segment signal, (b) computing
absolute values of the coefficients, (c) computing a set of
frequency-selective aggregate magnitudes for each segment signal by
summing signal-segment Haar transform coefficients having like time
scales, and (d) averaging the sets of frequency-selective aggregate
magnitudes to form a single set of frequency-selective aggregate
magnitudes for the signal.
25. The automatic LAT-determining method of claim 24 wherein the
computed signal characteristic is the first moment of the signal
power determined from the frequency-selective aggregate
magnitudes.
26. The automatic LAT-determining method of claim 10 wherein the
computed signal characteristic is the fraction of time within a
predetermined time period of the signal at which the absolute value
of signal velocity is above a predetermined threshold.
27. The automatic LAT-determining method of claim 10 wherein the
computed signal characteristic is the maximum signal amplitude
minus the minimum signal amplitude within a predetermined time
period of the signal.
Description
FIELD OF THE INVENTION
[0001] This invention is related generally to the field of
electrophysiology, and more particularly to technology for accurate
measurement of parameters within body-surface ECG, intracardiac,
and epicardial electrical signals such as heart rates and local
activation times and the assessment of the quality of such
measurements.
BACKGROUND OF THE INVENTION
[0002] The invention disclosed herein involves the processing of
multiple channels of electrical signals which are produced by the
heart. These channel signals include the ECG signals from
body-surface electrodes and signals from electrodes within the
body, i.e., intracardiac signals from within vessels and chambers
of the heart and epicardial signals from the outer surface of the
heart. Throughout this document, the term "multi-channel cardiac
electrogram" (or "MCCE") is used to refer to all of these types of
channels, and when specific types are appropriate, specific
nomenclature is used. This new terminology (MCCE) is used herein
since the term "ECG" sometimes only refers to body-surface
measurements of cardiac performance.
[0003] A major component in cardiac interventional procedures such
as cardiac ablation is the display of cardiac data which is
extracted from the MCCE signals captured by an array of electrodes
placed on the body surface and within and on the structures of the
heart itself. Among the important data which are displayed are
ventricular pulse interval (time between heart beats), intracardiac
cycle length (time between the activations in arrhythmias (such as
atrial fibrillation), relative time differences between related
activations in two intracardiac channels to generate activation
maps, and assessments of signal strength, variability, and other
measures of signal quality within MCCE signals.
[0004] Cardiac interventional electrophysiology procedures (e.g.,
ablation) can be extremely time-consuming, and the reliable
determination and presentation of such cardiac parameters is an
important element in both the quality of the procedures and the
speed with which they can be carried out. Often the data which are
presented to the electrophysiology doctor during such procedures
exhibit high variability contributed not only by the performance of
the heart itself but by unreliable detection of certain features of
the MCCE signals. Therefore there is a need for more reliable and
more rapid algorithms to process body surface and intracardiac
signals obtained during an electrophysiology (EP) procedure.
[0005] MCCE electrodes capture the electrical signals in the
cardiac muscle cells. As mentioned above, some MCCE electrodes may
be attached to the body surface (ECG) and some may be positioned
inside cardiac veins, arteries and chambers (intracardiac) and on
the outer surface of the heart (epicardial) as conductive elements
at the tips or along the lengths of catheters introduced into the
body and maneuvered into position by the EP doctor. The electrical
signals within the heart muscles and which flow therefrom to other
regions of the body have very low voltage amplitudes and therefore
are susceptible to both external signal noise and
internally-generated electrical variations (non-cardiac activity).
In addition, cardiac arrhythmias themselves may be highly variable,
which can make reliable extraction of cardiac parameters from MCCE
signals difficult.
[0006] One important cardiac parameter used during such procedures
is the time difference between the activations occurring within two
channels, both of which contain the electrical signals of an
arrhythmia. This measurement is called local activation time (LAT),
and measurement of a plurality of values of LAT is the basis for
the generation of an activation map. The map displays information
about the sequence of activations of cardiac muscle cells relative
to each other, and this sequence of information is combined with
physical anatomical position information to form the map. An
activation map then provides guidance to the EP doctor for the
process of applying therapies to heart muscle cells which can
terminate cardiac arrhythmias and permanently affect the heart to
prevent recurrence of such arrhythmias.
[0007] The entire process of determining LAT is referred to as
mapping because all of the information generated by analysis of the
MCCE signals is combined in a single computer display of a
three-dimensional figure that has the shape of the heart chamber of
interest and display of additional image qualities such as color
that convey the sequence of electrical activity (activation map) or
possibly other qualities of the electrical activity (e.g., voltage
map). These images are similar in style to weather maps common
today in weather-forecasting. Such a cardiac map becomes a focus of
attention for the EP doctor as he directs the motion of catheters
in the heart to new positions, and an algorithm which processes the
MCCE signals produces measurements from the electrodes in new
positions. As this process continues, the map is updated with new
colored points to represent additional information about the
electrical activity of the heart.
[0008] During a mapping procedure, the timing relationships of
muscle depolarizations typically must be determined for hundreds of
locations around a heart chamber which may be experiencing an
abnormal rhythm. The locations are often examined, one at a time,
by moving an exploring cardiac-catheter electrode (mapping-channel
electrode) from location to location, acquiring perhaps only a few
seconds of signal data at each location. To compare timing
relationships, a different electrode (reference-channel electrode)
remains stationary at a single location and continuously acquires a
reference signal of the rhythm.
[0009] The collection of timing relationships and anatomical
locations constitutes an activation map (LAT map). As described
above, a relatively large number of individual LAT values are used
to generate a useful LAT map. Many different locations may serve
adequately as alternative reference locations, but it has been
critical in the present state-of-the-art that whatever location is
used as the reference, one activation map is committed for the
entire duration to that reference location only.
[0010] U.S. patent application Ser. No. 13/922,953 (Brodnick) filed
on Jun. 20, 2013 discloses several aspects of improved methods for
determining LAT. (Such application and the invention of the present
application are commonly-owned, and Donald Brodnick is also an
inventor of the present invention.) The Brodnick application
discloses LAT-determination methods which include replacement of
cardiac channels when the quality of such channel signals falls
below a standard measure of channel-signal quality. Much of the
disclosure of the Brodnick application is included herein since it
provides excellent background information for the improved
LAT-determination methods disclosed herein.
[0011] Occasionally a reference electrode is bumped or becomes
disconnected. In these cases, additional data cannot be collected
to extend the map (add more LAT values to the map) because the
timing relationships are no longer comparable (based on the same
reference channel signal). The doctor either makes his
interpretation of the map based on an incomplete map or establishes
a new reference and begins to create a new map, having lost the
time and effort which to this point in the procedure had been
expended. At a few seconds of signal acquisition per location, a
few seconds of catheter motion between locations, and hundreds of
locations, the amount of time and effort wasted if a map must be
restarted can be very significant. Furthermore, extending the total
procedure time adds more risk of complications for the patient.
[0012] Because the heart is constantly contracting and other
catheters are continually being repositioned, a procedure may last
for several hours, during which time the patient even may need to
be moved. Occasionally a reference electrode either makes poor
contact or may shift position, in which case the constant timing
relationship is disrupted (timing stability is lost) and additional
locations cannot be studied in relationship to accumulated data. As
described above, the resulting incomplete activation map may be
worthless, requiring a new map, extending the procedure and adding
cost and risk to the patient.
[0013] Thus there is a need for an automatic method of determining
local activation time (LAT) from multi-channel cardiac electrogram
signals which avoids substantial loss of LAT values in spite of
losses of timing stability in reference channels during a local
activation time mapping procedure.
[0014] The generation of position information and its combination
with cardiac timing information is outside the scope of the present
invention. The focus of the present invention is the processing of
MCCE signals to measure time relationships within the signals, the
two most important of which are cycle length (CL) and local
activation time (LAT).
[0015] Currently-available MCCE-processing algorithms are
simplistic and often provide inaccurate measurements which cause
the activation map and many other cardiac parameter values to be
misleading. A misleading map may either (1) compel the doctor to
continue mapping new points until apparent inconsistencies of the
map are corrected by a preponderance of new, more-accurately
measured map points or (2) convince the doctor to apply a therapy
to a muscle region which actually makes little or no progress in
the termination of an arrhythmia, again prolonging the procedure
while the EP doctor maps more points in an attempt to locate new
regions where therapy may be effective.
[0016] Currently, computer systems which assist doctors in the
mapping process have manual overrides to allow a technician, or
sometimes the EP doctor himself, to correct the measurements made
by the system automatically. This requires a person to observe a
computer display called the "Annotation Window" which shows a short
length of the patient's heart rhythm, perhaps 3-5 heart beats as
recorded in 3-8 channels (signals from MCCE electrodes).
[0017] The channels of the annotation window are of several types.
Usually there will be a body surface ECG lead such as lead II that
identifies when ventricular activity is occurring. (It is also
possible that the ventricular activity may be sensed by an
intracardiac channel electrode.) There is one channel, identified
as a reference channel, the electrode of which ideally remains in a
fixed position during the entire map-generating procedure. There is
at least one other intracardiac channel (the mapping channel) which
senses the electrical signal at a catheter tip, the precise
three-dimensional position of which is determined by other means.
The electrical activity in the mapping channel is compared to the
activity in the reference channel to determine the local activation
time (LAT) which is used to color the map at that precise
three-dimensional position.
[0018] Intracardiac channels may be of either the bipolar or
unipolar recording types, and the inventive measurement method
disclosed herein can be applied to both types of signals. Also,
since it is possible during arrhythmias for some chambers of the
heart to be beating in a rhythm different from other chambers of
the heart, the annotation window often contains additional channels
to aid the doctor's interpretation of the data presented.
OBJECTS OF THE INVENTION
[0019] It is an object of this invention, in the field of
electrophysiology, to provide an automatic method for accurate
measurement of several parameters which characterize MCCE
signals.
[0020] Another object of this invention is to provide an automatic
method for such measurements which operates rapidly enough to not
hinder an electrophysiologist performing procedures which utilize
such a method.
[0021] Another object of this invention is to provide an automatic
method for rapid and reliable measurement of cardiac parameters to
reduce the length of time certain cardiac procedures require and
also reduce the X-ray exposure times for the patients.
[0022] Another object of this invention is to provide an automatic
method for rapid and reliable measurement of local activation times
which are provided for the rapid generation of local activation
time maps, determining the precise phase relationship between a
reference channel and a mapping channel.
[0023] Still another object of this invention is to provide an
automatic method for cardiac parameter measurement which can be
used in real time during certain interventional cardiac
procedures.
[0024] Another object of this invention is to provide an automatic
method for rapid and reliable activation mapping which can continue
providing LAT measurement when a reference signal degrades in
timing stability such that it is no longer useable as a reference
signal.
[0025] Another object of the invention is to provide an automatic
method for measuring cardiac parameters which is largely
insensitive to the amplitude of the MCCE signals and almost
entirely dependent on the timing information contained in such
signals.
[0026] Another object of this invention is to provide an automatic
method for measurement of local activation times which avoids the
loss of LAT values which have been determined prior to a loss of
timing stability of the reference channel signal used to determine
such LAT values.
[0027] Yet another object of the present invention is to provide an
automatic method for generating a single map throughout an LAT
mapping procedure even when all of a plurality of reference channel
signals fail intermittently at different times as long as at least
one reference channel is functioning properly at any time during
the mapping procedure.
[0028] And yet another object of the inventive method is to provide
reliable and accurate automatic determination of cardiac-channel
timing stability and signal quality.
[0029] These and other objects of the invention will be apparent
from the following descriptions and from the drawings.
SUMMARY OF THE INVENTION
[0030] The term "digitized signal" as used herein refers to a
stream of digital numeric values at discrete points in time. For
example, an analog voltage signal of an MCCE channel is digitized
every millisecond (msec) using an analog-to-digital (A/D) converter
to generate a series of sequential digital numeric values one
millisecond apart. The examples presented herein use this sampling
rate of 1 kHz, producing streams of digital values one millisecond
apart. This sampling rate is not intended to be limiting; other
sampling rates may be used.
[0031] The term "velocity" as used herein refers to a signal the
values of which are generally proportional to the
time-rate-of-change of another signal.
[0032] The term "velocity-dependent signal" as used herein refers
to a set of possible signals which relate to the velocity of a
channel signal, and in particular, retain certain properties of
channel velocity. Channel signals are filtered to generate
velocity-dependent signals which contain signal information which
does not lose either the positive or negative activity in a channel
signal. One such velocity-dependent signal is the absolute value of
channel velocity; such a velocity-dependent signal is used in some
embodiments of the inventive method to preserve the magnitude of
the activity in a signal. Other possible velocity-dependent signals
are even powers of velocity (squared, 4.sup.th power, etc.) which
retain both the positive and negative signal activity in a velocity
signal--the relative magnitudes are not critical in the present
invention as long as both positive and negative activity within the
signals are not masked by the filtering. Numerous other possible
filtering strategies may be used to generate velocity-dependent
signals, such as comparison of positive portions of the velocity
with respect to a positive threshold and similarly, comparison of
negative portions of the velocity with respect to a negative
threshold. With respect to their use in the present invention, all
velocity-dependent signals as defined herein are fully equivalent
to absolute-value velocity filtering in every relevant respect.
[0033] The term "two differenced sequential boxcar filters" as used
herein refers to two boxcar filters which operate in tandem and
then the difference between the two boxcar filter values is
computed. Such a filtering operation is one embodiment by which a
low-pass filter followed by a first-difference filter is applied.
Two differenced sequential boxcar filters are illustrated in FIG.
3A and described in detail later in this document.
[0034] The term "dot-product autocorrelation" as used herein refers
to a mathematical operation applied to a time series of digital
values, and this operation is generally a signal-processing
application of conventional autocorrelation. Applying conventional
autocorrelation to a fixed-length time series of numeric values
x.sub.i generates another series of numeric values a.sub.j which
represents how well the signal x.sub.i correlates with itself as a
function of the time difference between the signal and the same
signal displaced in time by a period of time called lag. In
conventional autocorrelation of a fixed-length signal x.sub.i
having n values in a time interval,
a.sub.j=.SIGMA.(x.sub.ix.sub.i-j)
where the symbol .SIGMA. indicates the sum over all n-j values of
x.sub.i, represents values of time, and j represents values of lag.
As used herein, the dot-product autocorrelation may be adjusted by
a scale factor K as a computational convenience, in which case,
a.sub.j=K.SIGMA.(x.sub.ix.sub.i-j)
again where the symbol .SIGMA. indicates the sum over all n-j
values of x.sub.i, represents values of time, and j represents
values of lag. Such an adjustment is not intended to be limiting to
the meaning of the term. The maximum value of a.sub.j is, of
course, a.sub.0 since at lag=0, the signal perfectly correlates
with itself. One such form of scale factor may include a.sub.0 such
that K=k/a.sub.0 where k is a constant and its value is set for
computational convenience.
[0035] The term "magnitude-coincidence autocorrelation" as used
herein refers to a modification of dot-product autocorrelation. As
used herein, magnitude-coincidence autocorrelation operates on
signals which first have been rectified (an absolute-value filter
has been applied). Each numeric value of a fixed-length time series
x.sub.i (all values of x.sub.i.gtoreq.0) is replaced by a 1 if the
value x.sub.i is equal to or greater than a threshold value
T.sub.AC and by a 0 if x.sub.i is less than T.sub.AC. Further,
threshold T.sub.AC is set at some multiple of the median of all n
values of x.sub.i in the fixed-length time series. Rectified
cardiac signals such as those processed by the present invention
contain noise which is typically substantially smaller than the
peaks within such signals. Furthermore, over all n values of such a
signal, a large number of values will be close to the noise level
since there are substantial periods of time between signal
(electrical events) representing a heart beat. Therefore, if
threshold T.sub.AC=pmedian(x.sub.i) and p is, e.g., 4, threshold
T.sub.AC will be just above the baseline noise in the signal
x.sub.i, and the thresholded signal X.sub.i will be equal to 1 only
if a signal value is present which is generally not noise. Then,
the magnitude-coincidence autocorrelation will have peaks for
values of lag at which the time-distribution of the noise-free
signal aligns (correlates well) with itself. Magnitude-coincidence
autocorrelation is particularly useful when the "time" information
in a signal is of more interest than the "shape" or amplitude
information in a signal.
[0036] The term "normal median" as used herein refers to the
numeric value determined from a set of numeric values, such numeric
value (median) being computed according to the commonly-understood
mathematical meaning of the term median. The normal median of a
finite set of numeric values can be determined by arranging all the
numeric values from lowest value to highest value and picking the
middle value from the ordered set. If there is an even number of
numeric values in the set, the normal median is defined to be the
mean of the two middle values of the ordered set.
[0037] The term "set-member median" as used herein refers to the
numeric value determined from a set of numeric values in a manner
modified from the above-described method of median determination.
In this modified determination, if there is an even number of
numeric values in the set, the set-member median is either one of
the two middle values in the ordered set such that the set-member
median is always a member of the set of numeric values. As a
practical matter, in almost all sets of real data, there are a very
large numbers of data values near the median, and there is little
if any difference between the two middle values.
[0038] The term "intracardiac channel" as used herein refers to a
channel of a set of MCCE signals which is connected to an internal
lead, i.e., connected to a internal-surface electrode such as is at
the end or along the tip of a cardiac catheter. For example, such
an electrode may be in a blood vessel or in a chamber of a
heart.
[0039] The term "ventricular channel" as used herein refers to a
channel of a set of MCCE signals which exhibits the dominant
response of the ventricles. This may most often be a channel which
is connected to an external lead, i.e., connected to a body-surface
electrode. An epicardial or intracardiac channel may also sometimes
be a ventricular channel.
[0040] The term "activation" as used herein refers to a time
segment within an MCCE signal which represents the passage of a
depolarization wavefront within muscle cells adjacent to an MCCE
electrode. An activation may sometimes be referred to as an
activity trigger. Note that the terms "activations" and "activation
times" may herein be used interchangeably since each activation has
an activation time associated with it.
[0041] The term "cycle length" as used herein refers to the time
between neighboring activations in an MCCE signal, particularly in
a reference-channel or mapping-channel signal. As used herein, the
term "pulse interval" is used to connote the cycle length for a
ventricular channel. The terms "ventricular pulse interval" and
"intracardiac cycle length" are used to distinguish between these
two measures of repetitive signals. For example, if a cardiac
patient is in a period of atrial fibrillation or flutter, there may
be a significant difference between the rate of occurrence of
electrical events in a ventricular channel and in some intracardiac
channels. The ventricular cycle length, herein called ventricular
pulse interval to further distinguish it from intracardiac cycle
length, may be two or three times as long as the intracardiac cycle
length.
[0042] As used herein, the terms "method" and "process" are
sometimes used interchangeably, particularly in the description of
the preferred embodiment as illustrated in the figures. The
algorithms described as embodiments of the inventive automatic
method of measuring parameters of multi-channel cardiac electrogram
signals are presented as a series of method steps which together
comprise processes.
[0043] As used herein, the terms "signal" and "channel" may be used
interchangeably since the inventive automatic method described
herein uses signal values in the channels of MCCE signals. For
example, often as used herein, the term "channel" implies the
addition of the word "signal" (to produce "channel signal") but for
simplicity and textual flow, the word "channel" is used alone.
[0044] The term "timing stability" as used herein refers to the
degree to which a timing parameter, such as LAT, changes from one
value to the next value during a cardiac procedure, based on a
standard for timing stability. For example, an LAT may be said to
be stable if has not changed from its past value (or a composite of
past values) by more than a predetermined percentage or by more
than a multiple of its standard deviation. Measurement of a timing
parameter may of course also be affected by noise in one or more of
the MCCE signals such that a determination of such parameter is
degraded beyond usefulness. Such an occurrence will also be seen as
a loss of timing stability.
[0045] The term "substantial loss of LAT values is avoided" as used
herein refers to largely preventing the loss of the time and effort
invested by the EP doctor in capturing LAT values and not narrowly
to whether or not specific numerical values for LAT are retained.
Avoiding substantial loss of LAT values may mean (a) that specific
LAT values are used in an unchanged form, (b) that specific LAT
values are corrected in order to be useful, and/or (c) that
specific LAT values are replaced by other LAT values determined
from already-existing cardiac electrogram signals. In all of these
situations, the LAT values, whether in changed or unchanged form,
are still available to be used by the EP. Changed LAT values are
herein referred to as having been transformed.
[0046] The term "base reference channel" as used herein refers to
the reference channel used in an LAT computation. LAT is computed
using a mapping channel, a ventricular channel and a reference
channel, and the reference channel in this group of three channels
is sometimes referred to herein as the base reference channel.
[0047] The term "signal characteristic" as used herein refers to a
metric of a signal by which differences between signals may be
distinguished.
[0048] The term "center-of-power frequency" as used herein refers
to the first moment of power computed from a signal frequency
spectrum.
[0049] The term "frequency-selective aggregate magnitude" as used
herein refers to a value formed by combining multiple Haar
transformation coefficients having differences based on the same
time scale into a single value.
[0050] The present invention is an automatic method of determining
local activation time (LAT) from at least four multi-channel
cardiac electrogram signals which include a ventricular channel, a
mapping channel and a plurality of reference channels. The method
comprises: (a) storing the cardiac channel signals; (b) using the
ventricular and mapping channel signals and a first reference
channel signal to compute LAT values at a plurality of
mapping-channel locations; (c) monitoring the timing stability of
the first reference channel signal; and if the timing stability of
the monitored signal falls below a stability standard, using a
second reference channel signal to determine LAT values and avoid
substantial loss of LAT values in spite of loss of timing
stability.
[0051] Some preferred embodiments of the inventive automatic
LAT-determining method include computing one or more timing offsets
using pairs of the plurality of reference channel signals, a timing
offset being LAT.sub.K(J), the local activation time of a reference
channel J based on a reference channel K and used to transform an
LAT value based on reference channel J to an LAT value based on
reference channel K.
[0052] In certain preferred embodiments, using the second reference
channel signal to determine LAT values includes transforming future
LAT values such that they are based on the first reference channel.
In some of these embodiments, LAT.sub.2(M) is a future LAT value of
mapping channel M based on the second reference channel, and future
transformed values LAT.sub.1(M) of mapping channel M based on the
first reference channel are equal to a timing offset LAT.sub.1(2)
plus LAT.sub.2(M).
[0053] Some other preferred embodiments using the signal of a
second reference channel to determine LAT values by transforming
LAT values include transforming past LAT values such that they are
based on the second reference channel. In some of these
embodiments, LAT.sub.1(M) is a past LAT value of mapping channel M
based on the first reference channel, and past transformed values
LAT.sub.2(M) of mapping channel M based on the second reference
channel are equal to a timing offset LAT.sub.2(1) plus
LAT.sub.1(M).
[0054] In some highly-preferred embodiments, the one or more timing
offsets are computed at a plurality of times, and the value of each
timing offset is replaced with its average over the plurality of
times. In some of these embodiments, the average is computed over a
predetermined number of times.
[0055] In highly-preferred embodiments of the inventive automatic
LAT-determining method, monitoring the timing stability of the
first reference channel signal includes monitoring multiple timing
offsets LAT.sub.1(X) where X represents the channels with which
timing offsets with the first reference channel are computed. Some
of these embodiments further include computing a signal
characteristic for the plurality of reference channels and
determining therefrom which one or more channels among these
reference channels has/have not lost timing stability. Some
embodiments also include selecting the second reference channel
signal from channels which have not lost timing stability, and in
some of these embodiments, selecting the second reference channel
signal from channels which have not lost timing stability includes
computing signal quality.
[0056] In some preferred embodiments, computing a signal
characteristic includes computing the frequency content of the
signal. In some of these embodiments, computing the frequency
content of the signal includes computing a fast Fourier transform
(FFT) for a predetermined time period of the signal. In some such
embodiments, the computed signal characteristic is the first moment
of the signal power determined from the computed fast Fourier
transform.
[0057] In some highly-preferred embodiments, computing frequency
content of a signal includes segmenting the signal into a plurality
of time-overlapping segment signals. In some of these embodiments,
weightings are applied to each of the segment signals. In some such
embodiments, computing the fast Fourier transform of the signal
includes (a) computing a signal-segment fast Fourier transform for
each segment signal and (b) averaging each such signal-segment fast
Fourier transform to form the fast Fourier transform of the signal.
In some of these embodiments, the computed signal characteristic is
the first moment of the signal power determined from the fast
Fourier transform of the signal.
[0058] In other embodiments, computing the frequency content of the
signal includes computing a Haar transform for a predetermined time
period of the signal, and in some such embodiments, the computed
signal characteristic is the first moment of the signal power
determined from the computed Haar transform. In some of these
embodiments, the signal is segmented into a plurality of
substantially-sequential segment signals. In some embodiments,
computing the Haar transform of the signal includes (a) computing
Haar transform coefficients for each segment signal, (b) computing
absolute values of the coefficients, (c) computing a set of
frequency-selective aggregate magnitudes for each segment signal by
summing signal-segment Haar transform coefficients having like time
scales, and (d) averaging the sets of frequency-selective aggregate
magnitudes to form a single set of frequency-selective aggregate
magnitudes for the signal. In some of these embodiments, the
computed signal characteristic is the first moment of the signal
power determined from the frequency-selective aggregate
magnitudes.
[0059] In certain other embodiments of the inventive automatic
LAT-determining method, the computed signal characteristic is
fraction of time within a predetermined time period of the signal
at which the absolute value of the signal is above a predetermined
threshold.
[0060] In certain other embodiments, the computed signal
characteristic is the maximum signal amplitude minus the minimum
signal amplitude within a predetermined time period of the
signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] FIG. 1 is a schematic block diagram of a method for
measuring parameters of MCCE signals, including intracardiac cycle
lengths and local activation times and estimates of signal and
measurement quality. The steps of the method as illustrated in the
block diagram of FIG. 1 are further detailed in several other
schematic block diagrams.
[0062] FIG. 2 is a schematic block diagram illustrating steps of a
method to generate absolute-value velocity data of a selected
digitized MCCE signal.
[0063] FIG. 3A is an illustration of the operation of the filtering
which occurs by applying two differenced sequential boxcar filters
to a digitized signal.
[0064] FIG. 3B depicts the absolute value of the output signal of
the filtering operation illustrated in FIG. 3A.
[0065] FIG. 4A is a schematic block diagram of the process of
determining activations (activity triggers) in the absolute-value
velocity signal from an MCCE channel. The steps of this process are
applied to more than one channel signal in the method.
[0066] FIG. 4B illustrates the process of identifying activations
in an example absolute-value velocity channel signal as processed
by the process of FIG. 4A.
[0067] FIG. 5 is a schematic block diagram of the process of
determining the ventricular-channel cycle length, herein called
"pulse interval" when associated with a ventricular channel.
[0068] FIGS. 6A and 6B together are a schematic block diagram of
the process of determining the reference-channel cycle length in
the embodiment of FIG. 1.
[0069] FIGS. 7A and 7B together are a schematic block diagram of
the process of determining local activation time (LAT) for a single
mapping point in the embodiment of FIG. 1.
[0070] FIG. 7C is schematic block diagram of an alternative
embodiment to determine LAT for a single mapping point, using
additional fiducial times within a reference-channel signal.
[0071] FIG. 8A is a set of MCCE signal plots illustrating an
example of the process of determining LAT for a single mapping
point as shown in FIGS. 7A and 7B.
[0072] FIG. 8B is a table which illustrates the process by which a
specific mapping-channel activation is selected for the
determination of LAT for the example of FIG. 8A.
[0073] FIG. 8C-1 through FIG. 8C-4 is a set of plots illustrating
in detail a selected mapping-channel activation and its
corresponding portion of the reference-channel signal which, as
illustrated in FIGS. 8A and 8B, are used to determine LAT for a
single mapping point.
[0074] FIG. 8D is a table illustrating an embodiment of a method to
assess measurement confidence in the method of FIG. 1, using the
examples of FIG. 8A through FIG. 8C-4. FIG. 8D also illustrates a
second alternative method embodiment to determine an LAT value for
a single mapping point.
[0075] FIG. 9 is a schematic diagram illustrating the inclusion of
automatic selection of the ventricular and reference channels in
the automatic method of measuring parameters of MCCE signals.
[0076] FIG. 10A is a schematic block diagram of the process of
automatically selecting a ventricular channel from a set of
candidate MCCE channels, specifically illustrating the
determination of parameters for a single candidate ventricular
channel.
[0077] FIG. 10B is a schematic block diagram of the process of
automatically selecting a ventricular channel from a set of
candidate MCCE channels, specifically illustrating the automatic
selection from among candidate ventricular channels which have had
parameters determined in the automatic process of FIG. 10A.
[0078] FIG. 11A is a schematic block diagram of the process of
automatically selecting a reference channel from a set of candidate
MCCE channels, specifically illustrating the determination of
parameters for a single candidate reference channel.
[0079] FIG. 11B is a schematic block diagram of the process of
automatically determining the variability parameter for a single
candidate reference channel as used within the process illustrated
in FIG. 11A.
[0080] FIG. 11C is a schematic block diagram of the process of
automatically selecting a reference channel from a set of candidate
MCCE channels, specifically illustrating the automatic selection
from among candidate reference channels which have had parameters
determined in the automatic process of FIGS. 11A and 11B.
[0081] FIG. 12 is a matrix which schematically illustrates a series
of reference channels and mapping channels among a set of MCCE
signals which in an aspect of the inventive method may be processed
in parallel to generate multiple LAT maps by various combinations
of reference and mapping channels.
[0082] FIG. 13 is a schematic block diagram illustrating an
alternative embodiment of the monitoring of cardiac channel
quality. The alternative embodiment replaces a portion of the
schematic block diagram of FIG. 1.
[0083] FIG. 14 is a schematic block diagram illustrating an
alternative embodiment of the channel selection method illustrated
in FIG. 9, adding elements of the automatic channel selection steps
within initialization to the real-time operation of the inventive
method for measuring parameters of MCCE signals such that cardiac
channels may be replaced when the quality of a channel signal
degrades during operation of the inventive method.
[0084] FIG. 15 is a high-level schematic block diagram illustrating
the steps of an embodiment of the inventive method for transforming
LAT values using a second reference channel signal when the timing
stability of a first reference channel signal degrades below a
stability standard, in order to avoid substantial loss of LAT
values in spite of the loss of timing stability.
[0085] FIG. 16 is a table showing exemplary values for timing
offsets computed in one step within the embodiment of the inventive
method for determining local activation time shown in FIG. 15. The
table of FIG. 16 also presents an example determination of whether
or not a loss of timing stability has occurred in the example of
the method embodiment of FIG. 15.
[0086] FIG. 17 is schematic block diagram illustrating the steps of
a method for computation of a signal characteristic for use within
the method embodiment of FIG. 15. This signal characteristic
computation generates an FFT-based (fast Fourier transform-based)
parameter called herein the epoch center-of-power frequency.
[0087] FIG. 17A is an exemplary six-second epoch of a
representative cardiac channel electrogram signal.
[0088] FIG. 17B is a plot illustrating one embodiment of weightings
used to divide signal epoch data samples into overlapping segments
of data. The weightings are applied to the signal data of FIG. 17A
for use within the FFT-based signal characteristic computation of
FIG. 17.
[0089] FIGS. 17C-17G are five plots illustrating the resulting
segments having weightings applied to the cardiac channel signal of
FIG. 17A, to be used in the computation of an epoch center-of-power
frequency as illustrated in FIG. 17.
[0090] FIGS. 17H-17L are five plots illustrating the segment
spectra computed with a fast Fourier transform of the five weighted
segment signals of FIGS. 17C-17G.
[0091] FIG. 17M is a plot of the average signal spectrum of the
five segment spectra of FIGS. 17H-17L and from which an epoch
center-of-power frequency is computed.
[0092] FIG. 17N is a table illustrating a method embodiment of the
determination of which particular channel(s) have caused the loss
of timing stability which was determined to have occurred in the
example of FIG. 16. The signal characteristic used in this example
embodiment is the FFT-based signal characteristic computation
alternative of FIG. 17.
[0093] FIG. 18 is schematic block diagram illustrating a first
alternative method for computation of a signal characteristic for
use within the method embodiment of FIG. 15. This alternative
signal characteristic computation generates a signal characteristic
called epoch activity duration.
[0094] FIG. 18A is a plot illustrating the application of an
absolute-value velocity filter to the exemplary six-second epoch
cardiac channel electrogram signal of FIG. 17A.
[0095] FIG. 18B is a plot illustrating the computation of an
activity duration signal characteristic for the absolute-value
velocity epoch signal of FIG. 18A.
[0096] FIG. 19 is schematic block diagram illustrating a second
alternative method for computation of a signal characteristic for
use within the method embodiment of FIG. 15. This alternative
signal characteristic computation generates a signal characteristic
epoch peak-to-peak amplitude.
[0097] FIG. 19A is a table illustrating the second alternative
embodiment of FIG. 19, showing the peak-to-peak determination of
the exemplary six-second epoch of a representative cardiac channel
electrogram signal of FIG. 17A.
[0098] FIG. 20 is schematic block diagram illustrating a third
alternative method for computation of a signal characteristic for
use within the method embodiment of FIG. 15. This alternative
signal characteristic computation generates a Haar-transform-based
parameter called the epoch center-of-power frequency.
[0099] FIGS. 20A-20C are three plots of segments of the exemplary
six-second epoch of a representative cardiac channel electrogram
signal of FIG. 17A.
[0100] FIGS. 20D-20F are three plots of the Haar transformation
coefficients for the three data segments of FIGS. 20A-20C,
respectively, resulting from the Haar transformation of such three
segments of data.
[0101] FIG. 20G is a table detailing the computation of a Haar
transformation of a cardiac electrogram signal consisting of 2,048
signal values and resulting in 2,048 Haar transformation
coefficients H.sub.i.
[0102] FIG. 20H is a table detailing the computation of a set of
eleven frequency-selective aggregate magnitudes A.sub.i from the
2,048 Haar transformation coefficients H.sub.i of FIG. 20G.
[0103] FIGS. 20I-20K are three plots of the absolute values of the
Haar transformation coefficients shown in FIGS. 20D-20F for the
three data segments of FIGS. 20A-20C, respectively.
[0104] FIGS. 20L-20N are three bar charts of the eleven
frequency-selective aggregate magnitudes A.sub.i for each of the
three segments of signal data of FIGS. 20A-20C.
[0105] FIG. 20P is a bar chart presenting the eleven
frequency-selective aggregate magnitudes used in the determination
of a COP-frequency signal characteristic using the alternative
method of FIG. 20.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0106] FIG. 1 illustrates one embodiment of a method for measuring
parameters of multi-channel ECG signals. FIG. 1 is a high-level
schematic block diagram of a method which measures intracardiac
cycle lengths and local activation times on a near-real-time basis
as part of a system to generate maps (e.g., computer displayed 3D
presentations of the distribution of voltages and activation times
across cardiac structures) and provides feedback regarding signal
quality and measurement confidence.
[0107] Several other figures in this document relate to the method
of FIG. 1, and the steps presented in the schematic block diagrams
of these other figures are nested within the high-level schematic
block diagram of FIG. 1, as will be described below. In addition,
the method also includes initial channel selection steps which
occur prior to the steps of FIG. 1. These are illustrated and
described later in this document, in FIGS. 9 through 11C.
[0108] Referring to FIG. 1, an embodiment 10 of the method includes
a flow loop of method steps which is initiated by a request 12 to
map a point, and each time a mapping-point request 12 is generated,
the method proceeds through the steps shown in FIG. 1. The flow
chart element labeled with reference number 14 indicates that the
flow loop waits to receive request 12. During a procedure in which
the method is used, an electrophysiologist (EP doctor) is
maneuvering an electrode-tipped catheter (mapping catheter) through
and around the chambers, arteries and veins of a patient's heart.
The electrode on this maneuvered catheter provides the
mapping-channel signal. When the EP doctor determines that the
maneuvered catheter electrode is in a desired position, the EP
doctor activates a signal as request 12 to map a point. A plurality
of map points constitute the map.
[0109] Generating the map during this procedure involves time
measurements made between the MCCE signals of the mapping electrode
and a reference electrode. (As used herein, electrodes are
positioned to provide signals to channels. Thus, for example, the
mapping electrode provides the signal for the mapping channel.) The
reference electrode is positioned before mapping begins in a
location that is expected to remain constant during the mapping
process and that will generate stable and repetitive electrical
signals.
[0110] Each electrode develops an electrical signal when muscle
cells in contact with the electrode change their cell membrane
potentials. These electric potentials change as the cells
mechanically contract. Nerve cells, which do not contract, also can
be in contact with electrodes and produce electrical signals.
[0111] The map being generated represents a particular heart rhythm
being studied, such as tachycardia. The reference-channel and
mapping-channel signals are both cyclical and have substantially
the same cycle length (CL). The reference-channel signal represents
a zero-phase or index moment of the particular cardiac cycle, and
the local activation time (LAT) measurements (time difference
between mapping and reference-channel signals) indicate the
sequence of muscle and nerve cell activation of various points (map
points) in the cardiac structure. This time sequence and its
physical course around the anatomy of the heart are the information
the EP doctor needs to determine how to apply therapy. The term
"local" refers to the fact that the measurement applies to the
heart cells in contact with the electrode and to signals with
respect to a reference-channel signal, and this information is
translated to a position on a three-dimensional (3D) image of the
heart chamber.
[0112] Activation time is measured relative to one or more
activations at the reference electrode and may be positive or
negative. A local activation time which is negative by more than a
half of one cycle length may also be recognized as being positive
at a corresponding time less than a half of one cycle length. Local
activation times may be defined as being relative to the nearest
activation in the reference channel.
[0113] Positioning of the mapping catheter is guided at times by
fluoroscopic imaging. At a position of interest, the EP doctor
generates request 12 to trigger the system to make measurements
from the MCCE signals available from the maneuvered catheter and
other more stationary catheters and body surface electrodes. These
measurements at mapping points are represented graphically, usually
by color, on a 3D image of the heart chamber of interest. These
points may be requested at irregular intervals of several seconds
to perhaps minutes, depending on when the EP doctor maneuvers the
mapping catheter to a point at which measurements should be
taken.
[0114] When request 12 is received, measurements are made using an
"epoch" of the most recent 6 seconds of MCCE signals. In embodiment
10, the 6-second length of this epoch should not be taken as
limiting. The epoch is a preset time window of MCCE signals, and
its 6-second length is chosen here in embodiment 10 such that
selected signals during the preset time window contain a suitable
number of electrical events to permit the analysis to be performed.
During such mapping procedure, at least one mapping channel and at
least one reference channel are used. At some points within
embodiment 10, as will be described later in this document, the
epoch is divided into three equal periods of time, and six seconds
is chosen here since a 2-second period will almost always contain
at least one heart beat (or cell activation) for all heart rates
above 30 beats per minute.
[0115] As the mapping catheter is moved, it is important that its
electrode be in place at the selected location for a period of time
(dwell time) long enough to obtain a suitable signal. In embodiment
10, such dwell time is about 2 seconds. Thus, when request 12 is
received, the epoch consists of 6 seconds of data on other channels
being used and 2 seconds of data on the mapping channel. (The 6
seconds of data may consist of the immediate past 4 seconds of the
data plus 2 seconds of data generated after request 12 occurs. The
6 seconds of data in an epoch may also be the 6 seconds of data
immediately preceding the request 12, since it may be that the
mapping catheter has already been in a stable position for the 2
seconds prior to the triggering of request 12. Other possible
strategies for acquiring the epochs of data are also possible.)
[0116] In the high-level schematic block diagram of FIG. 1, after
request 12 is received, ending the wait in method step 14, a
determination of the pulse interval in the ventricular channel is
performed in method step 15. Details of ventricular pulse-interval
determination 15 are detailed in the schematic block diagrams and
example signals of FIG. 2 through FIG. 5, all of which will be
described later in this document.
[0117] Following ventricular pulse-interval determination 15, a
determination 16 of the intracardiac cycle length in the reference
channel is performed. (Method step 16 is shown in FIG. 1 as
determining tachycardia cycle length since the method is intended
primarily for monitoring cardiac parameters in the treatment of
patients in tachycardia. The use of the term "tachycardia" is not
intended to be limiting. The method is applicable to measurement of
all types of cardiac arrhythmias as well as normal heart rhythms.)
Details of intracardiac cycle length determination 16 are detailed
in the schematic block diagrams and example signals of FIGS. 2
through 4B and FIG. 6, all of which will be described later in this
document. Intracardiac cycle length determination 16 depends on the
value of ventricular pulse interval established in determination
15.
[0118] Decision step 18 follows determination 16 such that the
cycle length determined in step 16 is compared to a
cycle-length-change criterion in decision step 18, and if the cycle
length has not exceeded the cycle-length-change criterion, the
method proceeds. If, however, the cycle-length-change criterion is
exceeded, the EP doctor is alerted in method step 20 in order that
steps may be taken by the EP doctor during the mapping procedure to
evaluate the impact of such a change.
[0119] A cycle-length-change criterion applied in method step 18
may be based on an absolute time difference in cycle length from a
previous cycle length or on the average of a plurality of previous
cycle lengths. Or it may be based on a percentage change from such
quantities. One useful previous cycle length is the initial or
starting cycle length of the reference channel, established at the
beginning of the mapping procedure. A local activation time map is
related to a particular rhythm so that if there is too great a
change in cycle length, the EP doctor may choose to start a new
map, or in fact may determine that mapping is no longer appropriate
at such time. A value for the percentage change which triggers an
alert in method step 20 may be that the current reference-channel
cycle length (determined in method step 16) is found to differ from
the starting cycle length by more than 10%. Such value is not
intended to be limiting; other values may be found to provide
adequate warning to the EP doctor.
[0120] Embodiment 10 of the method then proceeds to a computation
22 of the local activation time (LAT) associated with the map point
being analyzed. Details of local activation time computation 22 are
detailed in the schematic block diagram of FIGS. 7A-7C which will
be described later in this document, and examples of such
determination are illustrated in FIGS. 8A through 8D.
[0121] Embodiment 10 of the method for measuring parameters of MCCE
signals includes steps for evaluation 24 of signal quality and
evaluation 26 of measurement confidence, both of which are applied
within embodiment 10 to monitor the measurement process. In each
case, that is, reduced signal quality as determined in step 24 and
reduced measurement confidence in step 26, the EP doctor is alerted
(user alerts 28 and 30, respectively) that such conditions have
been detected. One embodiment of a method to measure signal quality
in method step 24 is included in the steps illustrated in FIG. 4A
and will be discussed later in this document. One embodiment of a
method to assess measurement confidence in method step 26 is
illustrated in the example of FIG. 8D described later in this
document.
[0122] As shown in FIG. 1, the method of embodiment 10 provides (in
step 32) the map point and its related measurement data to a
computer at least for display to the EP doctor during the procedure
and for storage in memory for later analysis. The system then
returns via loop path 33 to wait for the next mapping point request
12 at step 14.
[0123] FIGS. 2, 3A and 3B illustrate an embodiment of a portion of
the steps of the method further detailed in FIGS. 4A-11C. FIG. 2 is
a schematic block diagram illustrating detailed steps within
embodiment 10 by which a selected digitized MCCE channel signal 36i
is filtered to generate a corresponding absolute-value velocity
signal 36o. The steps of FIG. 2 are applied to various signals
within method embodiment 10, as indicated later in the description
below.
[0124] In FIG. 2, the combined steps, low-pass filter 38,
first-difference filter 40, and absolute-value filter 42, are
together shown as an absolute-value velocity filter 34. The first
two steps of absolute-value velocity filter 34, low-pass filter 38
and first-difference filter 40, are together shown as a bandpass
filter 44 which generates a filtered velocity signal 41 of an input
signal.
[0125] As shown in FIG. 2, an input signal 36i is a 6-second preset
time window (epoch) of digitized data from a selected MCCE signal.
Low-pass filter 38 operates on input signal 36i followed by
first-difference filter 40, and together these two filters generate
a digital stream of data 41 which corresponds to the filtered
velocity (first derivative) of input signal 36i with certain low
and high frequencies filtered out. That is, filter 44 is a bandpass
filter. Absolute-value filter 42 simply applies an absolute-value
operation (rectification) to filtered velocity signal 41 from
first-difference filter 40 to generate output signal 36o which is
an absolute-value velocity signal of input signal 36i.
[0126] One embodiment of applying a combination 44 of low-pass
filter 38 and first-difference filter 40 to a digitized signal is
what is called herein "two differenced sequential boxcar filters,"
and such filtering embodiment is illustrated in FIG. 3A, in which
an example digitized signal 46 is shown both graphically (46g) and
numerically (46n). Seven pairs of "boxcars" 48b illustrate the
sequential operation of boxcar filter 48.
[0127] Referring to FIG. 3A, each pair of boxcars 48b in boxcar
filter 48 is four time samples in length, and two boxcars 48b are
such that one follows the other immediately in time. (Only two of
the 14 boxcars 48b are labeled.) The sum of the four time samples
of digitized signal 46 in each boxcar 48b is calculated. Thus, for
example, the left boxcar 48b of the uppermost (first in time) pair
as shown holds the sum of the four time samples it subtends, and
the right boxcar 48b of this pair holds the sum of the four time
samples it subtends. These two sums are 7 and 12, respectively, and
the difference between the right boxcar value and the left boxcar
value is 12-7=5. This differenced value 5 is shown to the right of
the uppermost boxcar 48b pair, and seven such values, indicated by
reference number 50, are shown to the right of the seven example
sequential boxcar 48b pairs. This output signal 50 is shown both
numerically as 50n and graphically as 50g. Filter output signal 50
is shown for the seven time samples between the dotted lines
labeled 52a and 52b.
[0128] In the example of FIG. 3A, each boxcar 48b has a
boxcar-width w.sub.B of four samples. The value of w.sub.B
determines the frequency response of boxcar filter 48, or the
amount of smoothing provided by boxcar filter 48. Larger values of
w.sub.B produce a lower central frequency of boxcar filter 48 and
therefore more smoothing of the signal on which it operates. Such
relationships are well-known to people skilled in the art of
digital filtering. Any specific value for w.sub.B used herein is
not intended to be limiting. However, for the embodiments
exemplified herein, it has been found that values of w.sub.B of
around 4 are appropriate for use on intracardiac signals, and
values of w.sub.B around 20 are appropriate for ventricular
channels. For MCCE signals digitized every 1 millisecond and for
sequential four-sample-long boxcar filters 48 (w.sub.B=4)
illustrated in FIGS. 3A, 3B and 5, the resulting bandpass filter
has a center frequency of 125 Hz.
[0129] The operation of the two differenced sequential boxcar
filters 48 performs low-pass filtering and differentiation to input
signal 46 such that filter output 50 is proportional to the
velocity of bandpass-filtered digitized signal 46. No scaling has
been applied in this example, but such lack of scaling is not
intended to limit the meaning of the term two differenced
sequential boxcar filters.
[0130] FIG. 3B, shown below and to the left of FIG. 3A, simply
graphically illustrates the absolute value of output signal 50 as
exampled in FIG. 3A. The absolute value of output signal 50 is
processed by absolute-value filter 42 as shown in FIGS. 3A and 3B.
This absolute-value velocity signal is output signal 36o of FIG.
2.
[0131] Some steps of the method as illustrated in embodiment 10
include the identification of activations or activity triggers
within one or more channel signals of MCCE signals. Activations
(activity triggers) are the electrical activity associated with the
initiation of the depolarization of the heart muscle cells which
occurs during a heart beat, progressing like a wave through the
various portions of the cardiac structure and causing the heart to
pump.
[0132] FIG. 4A is a schematic block diagram of a process 58 of
determining activations (activity triggers) in an absolute-value
velocity signal. The steps of process 58 may be applied to more
than one channel signal.
[0133] In the embodiment of FIG. 4A, a signal 60 which is 6 seconds
in duration (6-sec epoch) and is the absolute-value velocity of an
MCCE channel signal, is divided into three 2-second "chunks" in
method step 62. In method steps 64-1, 64-2 and 64-3, these three
chunks are processed to find three signal maxima (max1, max2, and
max3), one for each of the three signal chunks. These three values
(max1, max2, and max3) are inputs to method step 66 which selects
the maximum MAX among the three inputs and method step 68 which
selects the minimum MIN among the three inputs. The values MAX and
MIN are in turn inputs to method step 70 which determines an
estimate SI for signal irregularity. In method step 70, signal
irregularity SI is estimated as SI=MAX-MIN. A larger difference
between the maximum (MAX) and minimum (MIN) values of the chunk
maxima (max1, max2, and max3) indicates that there is more
irregularity among the heart beats within epoch 60 being processed.
Signal irregularity SI is related to the variations in the "shape"
of the activations in MCCE signals while other measurements
described later in this document relate to variations in the time
of activations.
[0134] The value MIN represents an estimate SS of signal strength.
SS is multiplied by 0.5 (threshold factor) in method step 72 to
determine a value for an activation threshold AT to be used in step
74 to determine the occurrence of activations within the MCCE
signal being processed. The value (0.5) of the threshold factor
applied in method step 72 of this embodiment is not intended to be
limiting. Other values for the threshold factor maybe be applied in
embodiments of the method.
[0135] Signal irregularity SI and signal strength SS are used in
conjunction with an estimate of signal noise N.sub.S to provide an
estimate of signal quality SQ in method step 79. In method step 78,
signal 60 (provided by flow path 60a) is processed to compute its
median over the entire six-second epoch, and such median is
multiplied by 2 to produce estimate N.sub.S of signal noise. In
method step 78, the calculation of the median of signal 60 may be
done using a normal median or a set-member median. For such large
data sets (e.g.; 6 seconds at 1,000 samples per second), it has
been found that using the set-member median is computationally
convenient and highly suitable. In step 79, signal quality SQ is
computed as SQ=SS-SI-2N.sub.S.
[0136] The factor of 2 applied in method step 78 and the factor of
2 applied in method step 79 are both not intended to be limiting.
Other values for such factors may be used. The size of the factor
in step 78 is related to ensuring that the estimate of noise
N.sub.S in signal 60 is a good representation of the noise level in
signal 60. The size of the factor in step 79 is related to the
relative weight given to noise estimate N.sub.S compared to those
given to signal strength SS and signal irregularity SI in
generating the estimate for signal quality SQ. The values of 2 for
both of these factors have been found to provide good performance
for estimating noise N.sub.S and signal quality SQ.
[0137] FIG. 4B illustrates method step 74 of FIG. 4A, the process
of identifying activations in an example absolute-value velocity
channel signal 60 as processed by the method steps of FIG. 4A. The
signal of epoch 60 being processed is an input to method step 74 as
indicated by the signal flow path 60a. A portion of example epoch
60 is illustrated in FIG. 4B. Activation threshold AT is shown as a
dotted line AT parallel to the time axis and intersecting signal 60
at points 76. (Eleven signal crossings are shown; one such point is
labeled 76a, one is labeled 76b, and one is labeled 76c).
[0138] As indicated in method step 74 of FIG. 4A, activations in
epoch 60 being processed are indicated by identifying threshold
crossings 76 before which signal 60 does not cross activation
threshold AT for at least T.sub.BT milliseconds. The value of
before-threshold time T.sub.BT chosen may vary according to the
type of MCCE signal 60 being processed. For example, it has been
found that T.sub.BT=120 msec is an appropriate value when a
ventricular channel is being analyzed, and that T.sub.BT=90 msec is
an appropriate value when an intracardiac channel is being
analyzed. These values for T.sub.BT are not intended to be
limiting; the selection of a value for T.sub.BT is based on
choosing a value by which a reliable differentiation between
subsequent activations and among threshold crossings 76 within an
individual activation can be achieved.
[0139] In the example of FIG. 4B, the activation labeled 75 shown
includes six threshold crossings 76 as indicated by dotted circles
occurring in rapid succession, the first being threshold crossing
76b and the last being threshold crossing 76c. A portion of a
previous activation 77 within signal 60 is also shown in FIG. 4B.
In activation 77, five threshold crossings 76 occur in rapid
succession, the last of which is labeled 76a.
[0140] The time difference between threshold crossing 76a
associated with activation 77 and threshold crossing 76b associated
with activation 75 is about 185 msec as shown in FIG. 4B. In this
example, 185 msec is longer than either of the example values for
T.sub.BT; thus threshold crossing 76b is determined to be the
leading edge of activation 75, and the time at which threshold
crossing 76b occurs is determined to be activation time t.sub.ACT.
In this example, threshold 76b is the only such threshold crossing
illustrated in FIG. 4B.
[0141] FIG. 5 is a schematic block diagram of an embodiment 80 of a
process of determining the ventricular-channel pulse interval
(heart rate). In the automatic method of measuring parameters of
MCCE signals, ventricular-channel pulse-interval information is
used in the calculations to determine intracardiac-channel
parameter calculations. The steps of embodiment 80 of FIG. 5
analyze an absolute-value velocity ventricular-channel signal epoch
82, again 6 seconds in duration. In method step 84, activations
within epoch 82 are identified by applying steps 58 as illustrated
in FIG. 4A. As indicated in method steps 82 and 84, when processing
a ventricular channel, values for boxcar-width w.sub.B and
before-threshold time T.sub.BT may differ from those used for
intracardiac channels. In the embodiment of FIG. 5, these values
are w.sub.B=20 and T.sub.BT=120 msec.
[0142] Activations identified in method step 84 each have an
activation time and for purposes of description, there are n such
activation times. In method step 86, all activation intervals
I.sub.i are computed. There are n-1 activation intervals I.sub.i
computed as follows:
I 1 = t 2 - t 1 .cndot. I i = t i + 1 - t 2 .cndot. I n - 1 = t n -
t n - 1 ##EQU00001##
[0143] In method step 88, a maximum interval MAX.sub.PI of the n-1
activation intervals I.sub.i is computed, and in step 90, the
minimum interval MIN.sub.PI of the n-1 activation intervals I.sub.i
is computed. In method step 92, a range R.sub.PI for activation
intervals I.sub.i is computed as the difference between MAX.sub.PI
and MIN.sub.PI.
[0144] The n activation times t.sub.i are also used in method step
94 to compute all double-intervals D.sub.i of ventricular-channel
signal epoch 82. There are n-2 double-intervals D.sub.i, and such
double-intervals D.sub.i are computed as follows:
D 1 = t 3 - t 1 .cndot. D i = t i + 2 - t 2 .cndot. D n - 2 = t n -
t n - 2 ##EQU00002##
In method step 96, the normal median M.sub.DI of all
double-intervals D.sub.i is computed, and in step 98, the estimate
PI of ventricular-channel pulse interval is computed as
PI=M.sub.DI/2
Thus, method steps of process 80 generate an estimate of
ventricular pulse interval PI and provide an estimate of the range
R.sub.PI over which ventricular pulse interval PI varies. The value
of pulse interval PI is used in the determination of
reference-channel and mapping-channel cycle lengths and is reported
as a heart rate HR for the patient being monitored. Heart rate HR
in beats per minute (bpm) is determined in method step 99 from
pulse interval PI (in msec). (For computational convenience in step
96, a set-member median calculation may be used in place of the
normal median calculation.)
[0145] FIGS. 6A and 6B together are a schematic block diagram of a
process 220 of determining the reference-channel cycle length CL.
(Method step 16 of FIG. 1 includes process 220 along with other
elements of the reference-channel cycle-length determination as
illustrated in the embodiments of FIGS. 2 through 4B.) The steps of
FIGS. 6A and 6B process an absolute-value velocity
reference-channel signal epoch 222, again 6 seconds in duration. In
step 224, reference-channel signal strength SS, signal irregularity
SI, and noise N.sub.S are computed using the steps of FIG. 4A.
Method steps 72 ands 74 of FIG. 4A are not used in the processing
of reference-channel absolute-value velocity signal 222; individual
activations within signal 222 are not detected. Signal strength SS,
signal irregularity SI, and noise N.sub.S are used to determine
signal quality SQ, and noise N.sub.S is used in the computation of
the magnitude-coincidence autocorrelation function ACF in method
step 226.
[0146] In method step 226, a magnitude-coincidence autocorrelation
is performed on the data in absolute-value velocity
reference-channel signal epoch 222. (The computed autocorrelation
function is indicated by the term ACF.) The threshold value for the
magnitude-coincidence autocorrelation is dependent on noise N.sub.S
in signal 222 as described in the summary section above which
defines magnitude-coincidence autocorrelation. As applied in method
step 226, the value of the threshold T.sub.AC is set to ensure that
the thresholding process selects events which are significant
events within input signal 222. In one embodiment,
T.sub.AC=2N.sub.S where noise N.sub.S=2(median(input)+1).
The "1" is added to the median for computational convenience and to
avoid singular conditions within the system. Values other than 1
may be used and other ways to set threshold T.sub.AC may be used;
this specific expression for T.sub.AC is not intended to limit the
scope of this method.
[0147] The remaining method steps of process embodiment 220 in
FIGS. 6A and 6B, steps 228-252, encompass analysis of ACF to
determine reference-channel cycle length CL. As described above,
ACF is a function of lag, and in this analysis, values of lag in
ACF are analyzed with respect to ventricular-channel pulse interval
PI such that reference-channel cycle-length CL is determined in
process 220 based on estimates of pulse interval PI.
[0148] In method step 228, a minimum of ACF at values of lag less
than about 200 msec is identified. (200 msec is a preset lag
threshold.) The lag at this minimum in ACF is labeled W and is an
estimate of activity width. The lag threshold value of 200 msec for
searching for activity width is chosen such that the width of
activations expected for most intracardiac-channel signals will be
found at lag values less than 200 msec. The search window (preset
lag threshold) should be shorter than the shortest expected value
of reference-channel cycle length and longer than the width of
activations in the reference-channel signal. Since activations
typically are significantly shorter than CL, it is straightforward
to set the range to an appropriate value. 200 msec has been found
to be a useful value. However, the specific value of 200 msec for
the preset lag threshold is not intended to be limiting.
[0149] In method step 230, the maximum peak P.sub.1 is found in ACF
for values of lag greater than W; CL is set at the value of lag
CL.sub.1 where ACF has its maximum peak P.sub.1 for lag greater
than W; and an interim peak amplitude P.sub.CL is set to P1.
(P.sub.CL, P.sub.1, P.sub.2, P.sub.3, CL.sub.1, CL.sub.2 and
CL.sub.3 are interim values in the steps of process 220.) Then in
method step 232, if CL.sub.1 is very near (within .+-.20 msec)
double the ventricular pulse interval PI, then process 220 proceeds
to method step 234. If CL.sub.1 is not very near 2PI, then process
220 proceeds to method step 242 in FIG. 6B. (Points A and B in
process 220 represent common points joining FIGS. 6A and 6B.) The
situation of CL.sub.1 being very near double the ventricular pulse
interval PI may occur in a condition in which a slight alternation
of ventricular pulse interval PI in a pattern of bigeminy (long,
short, long, short, etc.) causes the autocorrelation to be slightly
stronger at the repeating double interval. Consecutive single
intervals are only slightly different, but magnitude-coincidence
autocorrelation is very sensitive to the slight timing
differences.
[0150] Throughout process embodiment 220 of determining
reference-channel cycle length CL, there are several time intervals
which are used to identify certain values in ACF such as the .+-.20
msec "nearness" criterion in method step 232. These occur in method
steps 232, 234, 238, 242, 248, and 250. In each such occurrence,
these specific values have been found to perform well in the
embodiment of process 220. (The "nearness" criteria are also
referred to as lag intervals. The lag intervals in the method steps
of process 220 are: step 232, a first lag interval; step 234, a
second lag interval; step 238, a third lag interval; step 242, a
fourth lag interval; and steps 248 and 250, a fifth lag
interval.)
[0151] In method step 234, the maximum amplitude P.sub.2 of ACF is
identified within a lag interval of .+-.40 msec of ventricular
pulse interval PI, CL.sub.2 is set to the value of lag at maximum
P.sub.2, and process 220 proceeds to method step 236. In method
step 236, if the amplitude P.sub.2 is greater than half of peak
amplitude P.sub.CL and if, in method step 238, CL.sub.2 is within
20 msec of CL.sub.1/2, then in method step 240, CL is set to
CL.sub.2, P.sub.CL is set to P.sub.2, and process 220 proceeds to
step 242. If both of these two conditions (in steps 236 and 238)
are not true, process 220 proceeds to step 242 without setting CL
to CL.sub.2 and P.sub.CL to P.sub.2. Method step 238 distinguishes
peak P.sub.2 from a maximum on one of the boundaries of the .+-.40
msec lag interval in method step 234. If the P.sub.2 is not greater
than half of P.sub.1, then the process proceeds to method step
242.
[0152] In method step 238, if CL.sub.1/2 is within 20 msec of
CL.sub.2, then CL is set to CL.sub.2 in method step 240 and the
process proceeds to method step 242. If CL.sub.1/2 is not within 20
msec of CL.sub.2, then the process proceeds to method step 242
without setting CL to CL.sub.2.
[0153] In method step 242, if CL (set in method step 230 or method
step 240) is within 60 msec of ventricular pulse interval PI, then
process 220 proceeds to method step 244. If CL is not within 60
msec of PI, then the process ends and the reference-channel cycle
length is either CL=CL.sub.1 as set in method step 230 or CL.sub.2
as set in method step 240.
[0154] In method step 244, the maximum amplitude P.sub.3 of ACF is
identified within the lag interval between lag=CL/6 and lag=2CL/3,
interim value CL.sub.3 is set to the lag at maximum P.sub.3, and
process 220 proceeds to method step 246. In method step 246, if
amplitude P.sub.3 is greater than half of the amplitude P.sub.CL at
CL (CL is either the lag CL.sub.1 at peak P.sub.1 or the lag
CL.sub.2 at peak P.sub.2), then process 220 proceeds to method
steps 248 and 250. If the amplitude P.sub.3 does not satisfy the
criterion in method step 246, then process 220 ends and the value
of reference-channel cycle length CL is either CL=CL.sub.1 as set
in method step 230 or CL.sub.2 as set in method step 240.
[0155] It is possible that there may be a significant peak in ACF
between lag=CL/6 and lag=2CL/3. Method steps 248 and 250 are
parallel steps which, if either of the criteria in these steps is
satisfied, process 220 proceeds to method step 252 in which the
reference-channel cycle length CL is set to CL=CL.sub.3 and process
220 ends. If neither of these two criteria is satisfied, process
220 ends and reference-channel cycle length CL is either
CL=CL.sub.1 as set in method step 230 or CL.sub.2 as set in method
step 240. The criteria in method steps 248 and 250 check whether
peak P.sub.3 has a value of lag wherein CL is within 20 msec of
either 2CL.sub.3 or 3CL.sub.3. If either condition is true, then,
as stated above, reference-channel cycle length CL is set to
CL.sub.3 and process 220 ends. The situation of a proper
reference-channel cycle length CL being at 1/3 or 1/2 of
ventricular pulse interval PI is related to 3:1 or 2:1
atrio-ventricular conduction with the artificial enhancement of the
ACF peak at pulse interval PI because of the ventricular artifact
that occurs for some of the atrial activations.
[0156] The methods just described can be summarized as three
distinct and separable strategies. First is the use of the
autocorrelation function to identify repeating cycles in the
cardiac rhythm with maximum use of all the data available and
little dependance on shape, no dependance on threshold-crossing
jitter, and robust-to-occasional noise glitches. The second
important strategy is avoiding the choice of a false cycle length
at twice the ventricular pulse interval because the ventricular
response slightly alternates in a pattern of bigeminal timing. The
third important strategy is avoiding the choice of a cycle length
equal to the ventricular pulse interval because ventricular
far-field distortions may occur in atrial signals during 2:1 or 3:1
atrio-ventricular conduction. These three strategies are useful
separately but more so in combination.
[0157] FIGS. 7A and 7B together are a schematic block diagram of an
embodiment 100 of the process of determining local activation time
(LAT) for a single mapping point in the method of measuring
parameters of MCCE signals. FIG. 7A illustrates three MCCE signals
(6-sec epochs) on which computations are performed, as has been
described above, in order to provide results which are used in the
determination of LAT for a single mapping point. A
ventricular-channel epoch 102 and a reference-channel epoch 108 are
coincident in time, and a mapping-channel 2-sec epoch 114 is
coincident with the last 2 seconds of epochs 102 and 108. FIG. 7A
includes a legend which defines the terminology used in FIGS. 7A
and 7B.
[0158] In method step 104, ventricular-channel epoch 102 is
processed with the steps of FIG. 4A and produces a set of
ventricular-channel activation times t.sub.V-ACT and estimates of
signal quality SQ and signal irregularity SI for epoch 102. The
ventricular-channel activation times t.sub.V-ACT are used in the
determination of LAT as indicated by the circle labeled V which is
common with the same such circle in FIG. 7B. In method step 106,
ventricular-channel pulse interval PI and range of pulse intervals
R.sub.PI are computed for presentation to the EP doctor using the
steps shown in FIG. 5.
[0159] In a similar fashion, in method step 110, reference-channel
epoch 108 is processed with the steps of FIG. 4A and produces
estimates of signal quality SQ and signal irregularity SI for epoch
108. Within the method steps of FIG. 4A, a velocity signal for the
reference channel is computed, and it is used in the determination
of LAT as indicated by the circle labeled R.sub.vel which is common
with the same such circle in FIG. 7B. In method step 112,
reference-channel cycle length CL is determined using the steps
shown in FIGS. 6A and 6B, and reference-channel cycle length CL is
used in the determination of LAT as indicated by the circle labeled
CL which is common with the same such circle in FIG. 7B.
[0160] In method step 114, mapping-channel epoch 114 is processed
with the steps of FIG. 4A and produces a set of mapping-channel
activation times t.sub.M-ACT and estimates of signal quality SQ for
epoch 114. (Epoch 114 is not sufficiently long to determine a
useful estimate of signal irregularity SI. However, if a longer
epoch length is used, SI may be estimated in method step 116.)
Mapping-channel activation times t.sub.M-ACT are used in the
determination of LAT as indicated by the circle labeled M which is
common with the same such circle in FIG. 7B. Within the steps of
FIG. 4A, a velocity signal for the mapping channel is computed, and
it is used in the determination of LAT as indicated by the circle
labeled M.sub.vel which is common with the same such circle in FIG.
7B.
[0161] FIG. 7B shows a continuation of the flow chart of embodiment
100. Inputs to the method steps of FIG. 7B have been computed in
the method steps of FIG. 7A, and these inputs are illustrated by
the circles labeled as described above. In method step 118, a
mapping-channel activation for LAT determination is selected from
among the activations and corresponding mapping-channel activation
times t.sub.M-ACT determined in method step 116. The selection of
such activation in step 118 includes the maximization of an
activation selection score A.sub.SC, a value for which is computed
for each candidate activation among the set of mapping-channel
activations. Details of method step 118 are described later in this
document in the discussion of the example of FIGS. 8A and 8B.
[0162] After selecting the specific mapping-channel activation to
be used to determine LAT in method step 118, a mapping-channel
fiducial time t.sub.M is found in method step 120. In determining
LAT, a more precise representation of event times is required than
the threshold-crossing determination of activation detection in
method step 74. In this document, "fiducial time" is the term used
to indicate such a more precise determination of an event
(activation) time. "Fiducial time" as used herein represents the
instant within an MCCE signal at which a depolarization wavefront
passes below the positive recording electrode in either a bipolar
or unipolar MCCE signal.
[0163] As is well-known to those skilled in the field of
electrophysiology, one good representation of fiducial time is the
instant at which a signal exhibits its maximum negative velocity.
Thus, one embodiment of method step 120 includes determining
mapping-channel fiducial time t.sub.M as the time at which the
maximum negative velocity occurs within the selected activation of
the mapping channel. In a similar fashion, a reference-channel
fiducial time t.sub.R is found in method step 122.
Reference-channel fiducial time t.sub.R is the time at which the
maximum negative velocity occurs within .+-.CL/2 of mapping-channel
fiducial time t.sub.M.
[0164] The use of the time of maximum negative velocity as the
fiducial time is not intended to be limiting. Other indications of
precise depolarization event times may be used in determining the
fiducial times.
[0165] In method step 124, the local activation time LAT for a
position at which the mapping-channel electrode is located within
the heart is computed as LAT=t.sub.M-t.sub.R. Local activation time
LAT is determined relative to the selected reference channel, and
values of LAT at a plurality of locations within the region of the
heart being mapped are determined during the process of building an
LAT map. If the quality of the channel signals being processed
degrades before mapping is complete such that mapping cannot be
continued, a new map must be generated. Local activation times may
be positive or negative times (occurring after or before the
corresponding activation event in the reference channel).
[0166] FIG. 7C is a schematic block diagram of an alternative
embodiment 122' of the process by which an LAT value is determined
for a single mapping point. (Embodiment 122' of FIG. 7C is an
alternative embodiment to method steps 122 and 124 of FIG. 7B.)
FIG. 7C will be described later in this document, after the example
of FIGS. 8A-8D is described.
[0167] FIG. 8A through FIG. 8D together illustrate in more detail
the process of determining LAT for a single mapping-channel
electrode location. FIG. 8A is a set of exemplary MCCE signal
plots. At the top of FIG. 8A is a six-second epoch of an ECG
reference-channel signal 108. At the bottom of FIG. 8A is a
six-second epoch of an ECG ventricular-channel signal 102
time-coincident with reference-channel signal 108. In the middle
and to the right of FIG. 8A is a 2-second epoch of an MCCE
mapping-channel signal 114 time-coincident with the final 2 seconds
of reference-channel signal 108 and ventricular-channel signal 102.
(Note that in FIG. 8A, the signal traces illustrated are the MCCE
voltage signals, not absolute-value velocity signals which are
created during signal processing represented in the steps 100 of
FIG. 7A.)
[0168] Also illustrated in FIG. 8A is magnitude-coincidence
autocorrelation ACF 126 of the absolute-value velocity of
reference-channel signal 108. ACF 126 is used to determine
reference-channel cycle length CL in a process such as embodiment
220 in FIGS. 6A and 6B. ACF 126 in FIG. 8A is annotated to show
activity width W. Peak P.sub.1 occurs at a lag value of 342 msec,
and the lag at peak P.sub.1 is the reference-channel cycle length
CL in this example.
[0169] Ventricular-channel activations identified in method step
104 are shown in FIG. 8A. Nine such activations, represented by
ventricular-channel activation times 128, were identified in
ventricular-channel signal 102. In a similar fashion,
mapping-channel activation times 130 are shown in FIG. 8A; four
mapping-channel activations were identified in method step 116.
Note that in this example, the first and last activations in the
2-second epoch of mapping-channel signal 114 were not identified by
the threshold-crossing activation detection process of method step
74 of FIG. 4A. (For description purposes, all six activations in
mapping-channel signal 114 are labeled 132a through 132f in FIG. 8A
even though only four such activation were detected. The number 132
is not repeated with the letters a-f for simplicity in the figure.
Activations 132a and 132f were not detected.) FIG. 8B presents a
table detailing method steps 118 by which a specific
mapping-channel activation is selected for the determination of LAT
for the example of FIG. 8A. FIG. 8B includes a legend further
defining the terms utilized in construction of and computations
within the table for this example. In method step 112 of FIG. 7A,
reference-channel cycle length CL was determined to be 342
milliseconds, shown in FIG. 8B at the top of the table.
[0170] As mentioned above, local activation time (LAT) is measured
by the time difference between a fiducial time t.sub.M in an
activation in the mapping channel and its corresponding fiducial
time t.sub.R in the reference channel. As part of this
determination, an activation within the mapping-channel signal 114
must be selected for such computation, in method step 118. This
selection process includes: (a) for each mapping-channel activation
i, determining the time t.sub.NV(i) to the nearest
ventricular-channel activation for each mapping-channel activation;
(b) for each mapping-channel activation i, determining the
deviation D.sub.P(i) from CL of the time to the previous
mapping-channel activation i-1; and (c) for each mapping-channel
activation i, determining the deviation D.sub.F(i) from CL of the
time to the next (future) mapping-channel activation i+1. The
mathematical representations of these determinations are shown in
the legend of FIG. 8B.
[0171] To generate a full map of local activation times, often a
large number of individual points must be determined. This can be a
time-consuming process. It is therefore desirable to determine each
individual value of LAT as quickly as possible once a new position
of the mapping-channel electrode being manipulated by the EP doctor
is established. It has been found that about 2 seconds is often
required to make a good determination. At typical intracardiac
heart rates being measured, only a few activations occur in the
mapping channel during a 2-second epoch period, so it is helpful to
increase the number of candidate activations by adapting to
situations where an activation is "missing" due to a failed
activation detection or to a simple epoch-end timing situation. The
method includes a beginning-of-data rule and an end-of-data rule to
increase the number of candidate mapping-channel activations. These
special rules are as follows:
[0172] Beginning-of-data rule: In some cases, the first detected
activity may be very near the beginning of available data. If the
expected previous activity to a detected activity would be located
before the beginning of the mapping-channel epoch, then there is no
evidence that detections failed and the value for D.sub.P(i) for
such a candidate activation is presumed to be 0. However, if the
amount of time in the available data in the mapping-channel epoch
is longer than the expected cycle length CL, then it is likely that
an activation failed to be detected due to some kind of noise in
the mapping-channel signal, an irregular signal, or an
insufficiency in the detection algorithm. In this case, D.sub.P(i)
is set to t.sub.M-ACT(i)-CL, but not less than 0, where CL is the
reference-channel cycle length.
[0173] End-of-data rule: This rule is symmetrical to the
beginning-of-data rule and is created to handle the same available
data constraint at the end of the data. WO for only the last
candidate mapping-channel activation is set to 0 if the last
detected activity is within one reference-channel cycle length CL
of the end of data. However, there may be more time in the
available mapping-channel epoch data than one CL after the last
detected activation. In this case, it is very likely that some kind
of noise in the mapping-channel signal, an irregular signal, or an
insufficiency in the detection algorithm caused a failed activation
detection. In this case, the value of D.sub.F(i) is set to the
length of available following data minus CL or
D.sub.F(i)=t.sub.ME-t.sub.M-ACT(i)-CL, but not less than 0, where
t.sub.ME is the mapping-channel epoch length, in this example, 2000
msec, and CL is the reference-channel cycle length. Two such
situations are illustrated in the example of FIGS. 8A and 8B.
[0174] The mapping-channel activation which is selected is the
activation for which activation selection score A.sub.SC(i) is a
maximum. As shown in FIG. 8B,
A.sub.SC(i)=t.sub.NV(i)-D.sub.P(i)-D.sub.F(i).
It is desirable that the selected mapping-channel activation be far
in time from a ventricular-channel activation and that the
neighboring cycle lengths in the mapping channel be close to
reference-channel cycle length CL. This mathematical construction
of the activation selection score A.sub.SC(i) accomplishes this
desired relationship.
[0175] The computations outlined above and represented in FIG. 8B
were carried out for the four candidate mapping-channel activations
132b through 132e (having activation times labeled 130). The
activations labeled 132a and 132f were not detected; activation
132a occurred too close to the beginning of epoch 114, and
activation 132f was not large enough to trigger the threshold in
activation detection step 74. Thus, as shown FIG. 8B, both the
beginning-of-data rule and the end-of-data rule were applied in
this example to determine values for activations 132b and 132e. In
the row of data in the table for mapping-channel activation 132b,
the value of D.sub.P=40 was determined by application of the
beginning-of-data rule, and in the row of data in the table for
mapping-channel activation 132e, the value of D.sub.F=247 was
determined by application of the end-of-data rule.
[0176] Mapping-channel activation 132c is selected based on its
maximum activation selection score A.sub.SC=290 among the candidate
mapping-channel activations.
[0177] FIGS. 8C-1 through 8C-4 are a set of plots illustrating in
detail method steps 120, 122, and 124 in which LAT is computed
based on selected mapping-channel activation 132c and a
reference-channel activation 134. (Note that reference-channel
activation 134 is not detected in the method, but it is clear in
FIG. 8A that activation 134 is an activation near in time to
t.sub.M.) As indicated above, in this example, fiducial times
t.sub.M and t.sub.R are the instants in the mapping-channel and
reference-channel activations at which the maximum negative
velocity occurs. FIG. 8C-1 illustrates an expanded signal of
mapping-channel activation 132c, and FIG. 8C-3 illustrates an
expanded signal of mapping-channel activation velocity 132c:v.
Mapping-channel fiducial time t.sub.M is indicated by the dotted
line labeled 136. The value of t.sub.M in this example is 4716 msec
as indicated to the left of mapping-channel activation 132c in FIG.
8A.
[0178] Reference-channel activation 134 is the activation in
reference-channel signal 108 which is located within .+-.CL/2 along
the time axis of reference-channel signal 108. FIG. 8C-2
illustrates an expanded signal of reference-channel activation 134,
and FIG. 8C-4 illustrates an expanded signal of reference-channel
activation velocity 134:v. Reference-channel fiducial time t.sub.R,
indicated by the dotted line labeled 138, is clearly located within
.+-.171 msec (.+-.CL/2) of t.sub.M.
[0179] In this example, reference-channel activation 134 occurs
after mapping-channel activation 132c, and the local activation
time LAT=t.sub.M-t.sub.R=-15 msec. This value of LAT provides a
single point in the generation of an LAT map. As mentioned above,
an LAT map is based a single reference channel with its electrode
placed at the same point in the cardiac structure throughout the
entire generation of the map. A plurality of LAT measurements is
used to generate an LAT map, each such point being made available
for display by the system.
[0180] In FIG. 7C, an alternative embodiment 122' for LAT
determination is illustrated. Alternative embodiment 122' takes
advantage of the fact that multiple fiducial times t.sub.R are
available in reference-channel signal 108 to which mapping-channel
fiducial time t.sub.M may be compared. In method step 200, the
times t.sub.R of maximum negative velocity generated in method step
110 of FIG. 7A are identified in reference-channel signal 108. (As
described above, activations in the reference channel are not
detected using a threshold-crossing technique; rather a simple
numerical search method may be used to find the times t.sub.R of
local relative negative-velocity maxima in signal 108.) In method
step 202, the four nearest values t.sub.R to mapping-channel
fiducial time t.sub.M are selected from among the values of
t.sub.R, and in method step 204, each of these four times t.sub.R
are adjusted by adding or subtracting multiples of
reference-channel cycle length CL until each adjusted value
t.sub.RA is within .+-.CL/2 of t.sub.M so that t.sub.RA now
represents its relative time position within one cycle length CL.
In method step 206, these four values are averaged and this average
av.sub.RA is included with the set of four values of t.sub.RA in
method step 208, creating a set of five time values. In method step
210, the median med.sub.RA of this set is found and in method step
212, LAT is determined as LAT=t.sub.M-med.sub.RA.
[0181] Referring again to FIG. 8A, reference-channel signal 108 is
shown with a set of numerical values 135 next to each activation in
signal 108. (Only one such triplet 135 of values is labeled with
reference number 135 to avoid clutter within FIG. 8A.) There are 14
such triplets 135 of values, and each triplet consists of (1) a
fiducial time t.sub.R of the maximum negative velocity in signal
108 (the number in parentheses), (2) the adjusted time t.sub.RA by
adding or subtracting multiples of cycle length CL to place
t.sub.RA within .+-.CL/2 of t.sub.M, and (3) the time difference
between t.sub.M and t.sub.RA, also referred to as intermediate LAT
values. This set of time values is also shown in the table of FIG.
8D.
[0182] Referring to FIG. 8D, rows A, B, and C correspond to the
time value triplets described in the above paragraph. Row D is an
ordered list of the intermediate LAT values in row C. Due to
variations in measurement conditions or sources of variability,
outlier values may be present at either end of this ordered list.
To avoid such outliers (such as the two highest values in row D, 44
and 161), an interquartile set of values is selected in row E,
dropping the lowest and highest 25% of values from the ordered
list. This interquartile list may then be used to provide an
estimate of a measurement confidence interval for method step 26 in
FIG. 1. The range of this interval is indicated by the end values
of row E which range from -15 to -11, or a .+-.2 msec LAT
measurement confidence interval. Further, a measurement-confidence
criterion for method step 26 in FIG. 1 may be as follows: if the
width of the measurement confidence interval (in this example, 4)
is greater than some percentage of cycle length CL, then alert the
EP doctor in method step 30. This width percentage criterion may be
about 5% but this value is not intended to be limiting. An absolute
width, say 15 msec, may also be used as the criteria; again such
absolute width criterion value is not intended to be limiting.
[0183] Referring to FIG. 8D to illustrate the alternative method of
LAT determination described in FIG. 7C, the four times t.sub.R
indicated by reference number 214 are the four nearest times
t.sub.R which encompass mapping-channel fiducial time t.sub.M,
forming a set 214 of four values t.sub.R (in row A) as described in
method step 202. The four values are adjusted as described above
and form the set of four values (in row B) described in method step
204. The average of these four values t is computed in method step
206 (av.sub.RA=4730.25 msec), This value av.sub.RA is added to set
214 to form a set of five values as indicated in method step 208.
The set of five values is now the set (4727, 4729, 4730.25, 4731,
4734). The median med.sub.RA of this set is 4730.25 as found in
method step 210, and LAT is determined by LAT=t.sub.M-med.sub.RA,
or LAT=4716-4730.25=-14.25 msec. One advantage of such an
embodiment is that some additional computational precision occurs
with the use of the averaging step.
[0184] The use of the four nearest times t.sub.R which encompass
t.sub.M is not intended to be limiting. Other choices for the
number of values t.sub.R used in the LAT determination may be
employed.
[0185] Additionally, the steps described with respect to FIG. 8D
also form a second alternative embodiment for LAT determination,
potentially taking into account even more reference-channel
fiducial times t.sub.R in the estimate of LAT. In this second
alternative embodiment, the median value of the interquartile set
of intermediate LAT values may be used as the LAT value for the
current mapping point. As shown in FIG. 8D, the median value in row
D is -13, the LAT value for this second alternative method of LAT
determination. Again as above, other choices may be made for the
number of values of t.sub.R used in the determination of LAT.
[0186] Signal quality SQ as determined in method step 79 of FIG. 4A
is also applicable for use within method step 24 of FIG. 1 in which
signal quality is monitored to provide alert 28 when signal quality
SQ degrades. One criterion by which to assess signal quality in
method step 79 of FIG. 1 is simply to determine if all three signal
quality values (ventricular channel, reference channel and mapping
channel) are positive. The signal quality SQ determination for the
mapping channel differs from that of the other two channels in that
it does not include a signal irregularity SI term (i.e.,
SQ=SS-2N.sub.S for the mapping channel) since the 2-second epoch of
the mapping channel is too short to generate a meaningful value for
signal irregularity SI. The decision of method step 79 is in the
affirmative if any of the three signal quality SQ values is
negative, at which time an alert is given to the EP doctor. Other
criteria may be used in method step 24 to trigger user alert
20.
[0187] As described above, activation maps are used during certain
cardiac procedures. But during such procedures, a variety of other
cardiac parameters may advantageously displayed. Among these may
be: (1) a value for starting reference-channel cycle length; (2) a
value for current reference-channel cycle length CL with a
confidence interval; (3) a value for LAT with a confidence
interval; and (4) a value for ventricular-channel pulse interval PI
with a confidence interval. All of these quantities are generated
by the method disclosed herein. For example, a confidence interval
for current reference-channel cycle length CL may be determined
from the lag L2 of a peak in ACF near twice the cycle length CL,
with the confidence interval being .+-.(L2-2CL) interval. A
confidence interval for the LAT measurement may be .+-.half the
interquartile range as described above. A confidence interval for
ventricular pulse interval PI may be represented by range R.sub.PI
(.+-.R.sub.PI/2) as computed in method step 92 of FIG. 5.
[0188] As described above, an activation map comprises a plurality
of LAT measurements all of which are made relative to a particular
reference-channel signal. One aspect of the inventive automatic
method of measuring parameters of multi-channel cardiac electrogram
signals includes the ability to compensate for signal degradation
in the reference channel during the creation of an activation map.
Since LAT maps are made relative to a specific reference channel,
if the reference-channel signal being used degrades during mapping
below a useful level of signal quality, the inventive method
enables another reference channel to be selected and recreates the
set of LAT measurements based on the new reference channel and
generates a new map. This is possible since the inventive method
computes reference-channel parameters as described above for
several reference channels in real-time and stores the necessary
parameters for use if needed. Very fast computation available with
present computing equipment enables these "extra" channels to be
recorded and analyzed in real-time without hindering the operation
of the "current" channels being used to create a map.
[0189] As seen above, a ventricular channel and a reference channel
from among the channels of the MCCE signals are used in the
automatic method of the present invention. The processes of
selecting these channels automatically are among the various
aspects of the inventive automatic method. FIG. 9 is a schematic
diagram illustrating the inclusion of automatic selection of the
ventricular and reference channels to the inventive automatic
method of measuring parameters of MCCE signals. The steps of this
overall combination are indicated by reference number 140.
[0190] Referring to FIG. 9, these automatic initialization steps
include automatic selection of a ventricular channel, indicated by
reference number 144. The method steps of an embodiment of
automatic process 144 are illustrated in FIGS. 10A and 10B and
described in detail below. Following the selection of a ventricular
channel, a reference channel is automatically selected as indicated
by reference number 164, and the method steps of an embodiment of
automatic process 164 are illustrated in FIGS. 11A-11C.
[0191] The entire automatic method of the invention disclosed
herein is under the control of the electrophysiologist (EP doctor)
as indicated above. At the time of a medical procedure, there may
be overriding medical or technical reasons for the EP doctor to
reject a channel or the channels which have been automatically
selected, so automatic method 140 includes a confirmation step 142
in which the EP doctor performing the procedure may accept or
reject the channels which have been automatically selected. If the
EP doctor rejects one or both of these selections, indicated by the
"N" option on confirmation step 142, channel selection may be done
manually or channels may be selected automatically as indicated by
pathway 142n.
[0192] Upon final selection of ventricular and reference channels,
automatic process 140 continues with the method steps of mapping as
indicated by reference number 10 and as described in detail
above.
[0193] FIGS. 10A and 10B are schematic diagrams of embodiment 144
of the process of automatically selecting a ventricular channel
from a set of candidate MCCE channels. The ventricular channel is
selected from a set of candidate channels for use within the
inventive automatic method of measuring parameters of MCCE signals.
It is desirable that the ventricular channel selected be a channel
which exhibits high signal quality and provides a stable
representation of the action of the ventricles of the heart. As
mentioned above, the ventricular channel of choice is most often
connected to a body surface electrode, but other channels such as
an epicardial electrode or an intracardiac electrode may provide
the most useful signal among the set of candidate ventricular
channels.
[0194] Referring to FIG. 10A, in one embodiment of automatic
process 144 for selection of the ventricular channel, an initial
time period for capturing and assessing signal characteristics from
all candidate channels is employed. Data is captured during the
time period represented by five epochs E1 through E5. (Epochs E1
through E5 are also called sub-signals E1 through E5.) In the
embodiment of FIG. 10A, this initial period is 30 seconds long. The
signal from each candidate channel is an absolute-value velocity
signal generated as described above. During the ventricular-channel
selection process, there may be numerous possible channels being
evaluated, the candidate set of channels including body-surface
channels as well as some epicardial and intracardiac channels,
depending on the strength of the ventricular signal in such
channels. The process illustrated in FIG. 10A is applied to the
waveform signals of each channel individually, generating for each
channel two measures by which the ventricular-channel selection is
made.
[0195] Referring again to FIG. 10A, a 30-second signal 148 from
each candidate channel is divided into five six-second epochs
(sub-signals) E1 through E5. The 30-second period, six-second epoch
length and number of epochs in the initial period are not intended
to be limiting. FIG. 10A illustrates the process for just one such
candidate channel; a plurality of candidate channels is processed
concurrently within ventricular-channel selection process 144.
[0196] As illustrated in FIG. 10A, the signals in each epoch E1-E5
are processed to determine the corresponding signal quality
SQ.sub.i in method step 150. (Five such separate signal quality
calculations are performed; only one is labeled with reference
number 150, but the marking SQ.sub.i indicates this calculation
being performed sequentially five times, each producing a value
SQ.sub.i for the signal quality of the corresponding epoch
E.sub.i.) Signal quality calculation 150 is carried out as
illustrated in FIG. 4A.
[0197] In method step 154, the five signal quality values SQ.sub.i
are summed to produce an overall signal quality value SQ.sub.VC for
each candidate ventricular channel.
[0198] Also illustrated in FIG. 10A, the signals in each epoch
E1-E5 are processed in method steps 152 to determine a value for
pulse-interval variability VAR.sub.i for each of the five epochs
(as found in the steps of FIG. 5). Five such variability
calculations are performed; one is labeled with reference number
152, but the markings indicate that this calculation is performed
sequentially five times, each producing a value VAR.sub.i for the
variability of the corresponding epoch. In this embodiment,
variability VAR.sub.i for each epoch is the difference between its
maximum MAX.sub.E1 and minimum MIN.sub.E1 pulse-interval values
during the six-second epoch. This is illustrated for epoch E1 in
the detailed method step 152a as follows:
VAR.sub.1=MAX.sub.E1-MIN.sub.E1
Similar relationships for each epoch are calculated to generate the
variability VAR.sub.i for each epoch E1 through E5.
[0199] In method step 156, the maximum value of variability among
the five values of variability is set as the variability VAR.sub.VC
of the candidate ventricular channel.
[0200] At this stage in the automatic ventricular-channel selection
process, each ventricular channel in the set of candidate
ventricular channels has a channel signal quality assessment value
SQ.sub.VC and a channel pulse-interval variability assessment value
VAR.sub.VC which will be used to complete the automatic
ventricular-channel selection process.
[0201] FIG. 10B illustrates the final portion of
ventricular-channel selection process 144 for this embodiment. In
general, it is desirable to select a ventricular channel with high
signal quality and acceptable variability. Thus, FIG. 10B
illustrates an embodiment which selects the channel with the
highest signal quality but excludes channels from this choice which
have especially high variability. Variability in such signals is
expected. However, the reason that some channels are excluded is
that these channels have variability values which are remarkably
higher than those of other channels, indicating that the algorithm
may be failing to detect activations properly in such channels.
[0202] In FIG. 10B, the wide arrows labeled 156a (two such arrows)
and 154e represent a plurality of values as indicated. Arrows 156a
therefore represent that the channel variability values for each of
the channels are all inputs to the method steps 158 and 160. In
method step 158, a ventricular-channel variability threshold
T.sub.VC is calculated as follows:
T.sub.VC=2[median(VAR.sub.VC)+.DELTA..sub.VC]
where .DELTA..sub.VC is a small increment of time which may be
added into this calculation simply as a computational convenience,
such as to avoid singular calculations or to avoid excluding too
many channels when the variability of some channels is extremely
small. The inclusion of increment .DELTA..sub.VC in the embodiment
of FIG. 10B is not intended be a limitation; .DELTA..sub.VC may
have a value of 0 or other computationally-convenient value. In
FIG. 10B, .DELTA..sub.VC has a value of 5 milliseconds.
[0203] Ventricular-channel variability threshold T.sub.VC is a
threshold value above which the variability of a channel is deemed
to be unacceptably high. In method step 160, the variability
VAR.sub.VC for each channel is compared with ventricular-channel
variability threshold T.sub.VC, and channels for which VAR.sub.VC
is equal to or exceeds threshold T.sub.VC are excluded from being
the selected ventricular-channel VC.sub.S.
[0204] Other computational assessments of signal quality and
variability for each channel and for the exclusion of channels on
the basis of high variability are of course possible. The specifics
of these assessment embodiments are not intended to be
limiting.
[0205] Wide arrow 154e represents one or more ventricular-channel
signal quality values SQ.sub.VC for channels which have not been
excluded in method step 160. Each channel represented in the set of
values 154e is a possible selected ventricular-channel VC.sub.S. In
method step 162, the channel with the highest value of channel
signal quality SQ.sub.VC is then selected as the
ventricular-channel VC.sub.S within the automatic method of
measuring parameters of MCCE signals. (The method therefore also
knows which channels are, for example, "second best" and "third
best" among the candidate channels.)
[0206] After ventricular-channel VC.sub.S has been selected using
ECG signals over an initial period of time (30 seconds in the
embodiment of FIGS. 10A and 10B), MCCE signals from the initial
period of time or from an additional period of time are used to
select a reference channel to be used within the automatic method
of measuring parameters of MCCE signals. FIGS. 11A through 11C are
schematic diagrams of embodiment 164, 166 of the process of
automatically selecting a reference channel from a set of candidate
channels.
[0207] Body surface electrode channels are generally known not to
be good choices for reference channels for many arrhythmias; thus,
the reference channel is typically selected from the remaining set
of MCCE channels for use within the automatic method of measuring
parameters of MCCE signals. It is desirable that the reference
channel selected be a channel which exhibits high signal quality
and low cycle-length variability and also which exhibits a fast
heart rate. For physiological reasons related to the cardiac
measurements for which the present invention is intended to be
used, it is also desirable that the selected reference channel
indicate the shortest cycle length CL. All of these criteria are
used to select a reference channel from among the set of candidate
reference channels.
[0208] Referring to FIG. 11A, in automatic process 164 for
selection of the reference channel, an initial time period for
capturing and assessing signal characteristics from all candidate
channels is employed. Data are captured during the time period
represented by five epochs, labeled E6 through E10. (Epochs E6
through E10 are also called sub-signals E6 through E10.) In the
embodiment of FIG. 11A, this additional initial period is 30
seconds long. The signal from each candidate reference channel is
an absolute-value velocity signal generated as described above.
During the reference-channel selection process, there may be
numerous possible channels being evaluated. What specific channels
are candidate reference channels for cardiac mapping are well-known
to those skilled in electrophysiology. The process illustrated in
FIG. 11A is applied to the waveform signals of each candidate
channel individually, generating for each channel two measures by
which the reference-channel selection is made.
[0209] Referring again to FIG. 11A, a 30-second signal 168 from
each candidate channel is divided into five six-second epochs
(sub-signals) E6 through E10. The 30-second period, six-second
epoch length and number of epochs in the initial period are not
intended to be limiting in any way. FIG. 11A illustrates the
process for just one such candidate channel; a plurality of
candidate channels is processed concurrently within
reference-channel selection process 164.
[0210] As illustrated in FIG. 11A, the signals in each epoch E6-E10
are processed to determine the corresponding signal quality
SQ.sub.i in method step 170. (Five such separate signal quality
calculations are performed; only one is labeled with reference
number 170, but the marking SQ.sub.i indicates this calculation
being performed sequentially five times, each producing a value
SQ.sub.i for the signal quality of the corresponding epoch
E.sub.i.) Signal quality calculation 170 is carried out as
illustrated in the steps of FIG. 4A.
[0211] In method step 174, the five signal quality values SQ.sub.i
are summed to produce an overall signal quality value SQ.sub.RC for
each candidate reference channel.
[0212] Also illustrated in FIG. 11A, the signals in each epoch
E6-E10 are processed in method steps 172 to determine a cycle
length CL.sub.i and a value VAR.sub.i for cycle-length variability
for each of the five epochs. The cycle length determination is
performed using the method steps described in detail above and
illustrated in FIGS. 6A and 6B (and as referred to in FIG. 11B).
The determination of cycle-length variability (and cycle length)
for each epoch is described using the schematic diagram of FIG.
11B, which illustrates this process 172a for epoch E6 as indicated
by the dotted-line ellipse in FIG. 11A. Five such cycle length and
cycle-length variability calculations as illustrated in FIG. 11B
are performed; one is labeled with reference number 172a, but the
markings of each such element in FIG. 11A indicate that this
calculation is performed sequentially five times, each producing a
value VAR.sub.i for the cycle-length variability of the
corresponding epoch.
[0213] In the method steps illustrated in FIG. 11B, F6 is the
magnitude-coincidence autocorrelation function of epoch signal E6
as calculated in method step 178. The threshold value for the
magnitude-coincidence autocorrelation is a function of the noise in
signal being processed as described in the summary section which
defines magnitude-coincidence autocorrelation and with respect to
method step 226 of FIG. 6A. Cycle length CL is computed in method
step 182 as indicated, using steps outlined in FIGS. 6A and 6B.
Then, in method step 186, a peak in F6 near a value of lag near 2
times CL is identified, and an interim variable DCL is set to the
lag at such peak. In method step 188, another interim variable
CL.sub.A is set to DCL-CL. The value for the variability VAR.sub.6
of epoch E6 is then calculated as follows in method step 190:
VAR.sub.6=|CL-CL.sub.A|
Similar relationships for each epoch are calculated to generate the
variability VAR.sub.i for each epoch E6 through E10.
[0214] Referring again to FIG. 11A, in method step 176, three
additional values (in addition to SQ.sub.RC) are determined for
each epoch E6 through E10 by which to select from among the
candidate reference channels: VAR.sub.RC; MAX.sub.CL; and
MIN.sub.CL. MAX.sub.CL is the maximum cycle length CL.sub.i among
the five cycle-length values. MIN.sub.CL is the minimum cycle
length CL.sub.i among the five cycle-length values. VAR.sub.RC is
the maximum variability value VAR.sub.i among the five variability
values.
[0215] At this stage in the automatic reference-channel selection
process, each reference channel in the set of candidate reference
channels has a channel signal quality assessment value SQ.sub.RC, a
channel variability assessment value VAR.sub.RC, and maximum and
minimum cycle length values MAX.sub.CL and MIN.sub.CL which will be
used to complete the automatic reference-channel selection
process.
[0216] FIG. 11C illustrates the final portion of reference-channel
selection process 164 for this embodiment. In general, it is
desirable to select a reference channel with high signal quality,
low variability, and short cycle length, as indicated above. FIG.
11C illustrates an embodiment which selects the channel with these
characteristics.
[0217] In FIG. 11C, the wide arrows labeled 174a, 176v, 176 x, and
176n each represent a plurality of values as indicated. Wide arrow
174a represents all values of SQ.sub.RC from the candidate
reference channels; wide arrow 176v represents all values of
VAR.sub.RC from the candidate reference channels; wide arrow 176x
represents all values of MAX.sub.CL from the candidate reference
channels; and wide arrow 176n represents all values of MIN.sub.CL
from the candidate reference channels.
[0218] In method step 192, a figure-of-merit FM.sub.RC is evaluated
for each candidate reference channel. FM.sub.RC for each candidate
reference channel is computed as follows:
FM.sub.RC=SQ.sub.RC/S.sub.RC-MAX.sub.RC-MIN.sub.RC-S.sub.VARVAR.sub.RC
where S.sub.RC is an arbitrary scale factor and S.sub.VAR is an
arbitrary scale factor. The two scale factors are chosen such that
a useful tradeoff within the figure-of-merit FM.sub.RC is created.
When signal quality values SQ.sub.RC are in microvolts and cycle
lengths are in milliseconds, a value of S.sub.RC of 32 and a value
of S.sub.VAR of 2 have been found to yield a useful tradeoff among
cycle lengths, variability, and signal quality and also to be
computationally convenient.
[0219] The FM.sub.RC values for each candidate reference channel
are output from method step 192 as indicated by wide arrow 194. In
method step 196, the channel with the highest value of FM.sub.RC is
the selected reference-channel RC.sub.S.
[0220] Other computational assessments of signal quality,
variability, and cycle length for each channel are of course
possible. The specifics of these assessment embodiments are not
intended to be limiting.
[0221] As described above, one aspect of the inventive automatic
method of measuring parameters of multi-channel cardiac electrogram
signals includes the ability to compensate for signal degradation
in the reference channel during the creation of an activation map
by selecting a new reference channel and recreating the set of LAT
measurements based on the new reference channel and generating a
new map. During the initial selection process for the reference
channel, the inventive method keeps track of the reference channels
which have values for FM.sub.RC just below the selected reference
channel RC.sub.S so that if necessary, these "second best"
reference channels can be substituted for the selected reference
channel and the mapping process can continue without losing the
valuable time and effort that has already been spent on the mapping
process.
[0222] In another aspect of the inventive method, multiple mapping
channels may also be employed, and the processing steps outlined
herein applied to multiple mapping channels as well as multiple
reference channels. Some catheters which are used in cardiac
procedures may include multiple electrodes in a variety of
configurations. In addition, multiple catheters may be employed.
The speed of computer processing available enables numerous
calculations to be done very rapidly such that multiple mapping
channels may be supported to generate a plurality of maps as the EP
doctor moves the mapping electrodes throughout chambers and vessels
of the heart.
[0223] FIG. 12 is a matrix which schematically illustrates a series
of reference channels and mapping channels from among a set of MCCE
signals which in an aspect of the inventive method may be processed
in parallel to generate multiple LAT maps as well as track the
other cardiac parameters measured with the inventive method in a
variety of combinations of reference and mapping channels. Some
channels may be displayed for the EP doctor in parallel while
others may serve as possible backups in case of signal degradation
as explained above.
[0224] Referring to FIG. 12, the table 260 shown is an 8.times.8
matrix (eight reference channels R.sub.1 through R.sub.8 and eight
mapping channels M.sub.1 through M.sub.8. The number of channels
illustrated is not intended to be limiting, but rather to
illustrate the concept of flexible configuration combinations of
reference and mapping channels. In fact, a set of MCCE signals is
typically much larger than the eight illustrated here. Eight
channels in the MCCE set of signals is only for purposes of
illustration.) The "" symbols indicate that the mapping channel and
reference channel at such an intersection together can generate a
map. In other words, as shown in table 260 of FIG. 12, each channel
M.sub.1 through M.sub.8 can be paired with any other channel
R.sub.1 through R.sub.8, and any combination of these pairings can
be created. The "-" simply indicate redundancy in table 260. The
"-" symbol is located along the diagonal of table 260 indicating
that any channel is not usefully paired with itself. Further, some
channels among the set of channels may in fact be ventricular
channels (primarily exhibiting ventricular signal characteristics)
and therefore also not good candidates for either a reference or
mapping channel.
[0225] The advantages of such multiple-channel processing
configurations are that procedure time may be shortened but also
that a much richer array of measurements may be obtained to provide
better information to the EP doctor to ameliorate the cardiac
deficiency being treated. Further, as described above, backup
channels can be available to deal with lost or degraded signals
during a procedure without the need to start the procedure over
again.
[0226] It is possible in some multi-channel configurations that
certain information may be shared among several parallel
computations. For example, it is possible that ventricular
pulse-interval values may be used for the determination of several
reference-channel cycle lengths, and ventricular-channel activation
times may be shared for use with more than one mapping channel. And
many other combinations other than those exampled here are possible
with the multi-channel processing of the inventive method described
herein.
[0227] In embodiment 10 of the inventive automatic method of
measuring parameters of multichannel cardiac signals described in
detail above, contiguous six-second epochs of MCCE signal data are
used. Alternatively, a moving-window format of selecting epoch
starting and end points may be used, such as the next epoch in a
series of epochs consisting of the last 5 seconds of the previous
epoch and a new sixth second. Other epoch-forming strategies may be
used, depending on the computational advantages which may be
possible and on the desired speed of analysis.
[0228] FIG. 13 is a schematic block diagram illustrating an
alternative embodiment of the monitoring of cardiac channel
quality. The alternative embodiment replaces a portion of the
schematic block diagram of FIG. 1. As previously described and as
illustrated in FIG. 1 and in FIG. 4A, signal quality is defined as
a quantity dependent on signal strength, signal irregularity and
noise. In addition to such a specific assessment of signal quality,
an overall channel-quality assessment may include information on
the variability of the cycle length (or pulse interval when
considering ventricular channels). Such an overall assessment for a
reference is represented by a figure-of-merit which reflects the
fact that higher signal quality values and lower cycle-length
variability are desirable.
[0229] In order to make an assessment of overall channel quality
during ongoing operation of the inventive method, one embodiment of
such a system includes applying the steps of automatic channel
selection for initialization in real-time to monitor channel
quality. These channel selection steps are fully described above
and illustrated in FIGS. 9-11C. As described in these embodiments,
assessment of overall channel quality includes calculation of
various signal quantities over a plurality of epochs. In these
embodiments, five contiguous six-second epochs are evaluated as
shown in FIGS. 10A-11C. The number of epochs may vary and the
length of the epochs may also differ from 6 seconds. Further,
signal parameters may be computed over moving time windows rather
than sequential (contiguous) epochs. The epochs may be of
fixed-length or of various lengths. Any number of variations of
time periods are possible in the computation of the elements of
channel figures-of-merit, signal quality and cycle-length or
pulse-interval variability.
[0230] The schematic of FIG. 13, labeled with reference number 24a,
replaces element 24 in FIG. 1 in the alternative embodiment of FIG.
13. The connections to the block diagram are shown as connections
with elements 22, 26 and 28 of FIG. 1. The functional elements 25
and 27 represent the alternative functions of calculating a
figure-of-merit FM (element 25) and using figure-of-merit FM in
comparison to a channel quality standard (element 27). Such a
standard may be a preset threshold value that figure-of-merit FM
must exceed or it may be a dynamically-determined standard such as
being the highest figure-of-merit FM among a set of candidate
channels. Other standards are also possible such as, for example, a
combination of a preset threshold value and the maximum among a set
of channels. In such a case, a channel has acceptable overall
quality when it has a figure-of-merit FM which is both the highest
in a set of channels and is above a preset threshold value.
[0231] The calculation of figure-of-merit FM as illustrated in FIG.
13 involves the application of the steps of FIGS. 10A and 10B for
the assessment of overall quality for candidate ventricular
channels and application of the steps of FIGS. 11A-11C for the
assessment of the overall quality for candidate reference channels.
As described above, various strategies for determining the timing
of signals used are possible.
[0232] In this inventive method, a plurality of channels are stored
and processed such that the monitoring of overall channel quality
is possible for cardiac channels as desired. The inventive method
includes a variety of strategies for monitoring overall cardiac
channel quality, including (a) performing the necessary
calculations in real-time for only one or more of the "active"
channels (the currently used mapping, ventricular, and reference
channels for LAT determination), (b) performing the necessary
calculations in real-time for the entire plurality of cardiac
channels, and (c) performing such calculations for a subset of
cardiac channels. Since channel signal data is stored, if a
strategy such as (a) is chosen, overall channel quality of other
("non-active") channels can be performed when necessary to
determine which channel will replace the current cardiac channel.
If a strategy such as (c) is employed, the inventive method
monitors every cardiac channel, making an up-to-date assessment of
the overall channel quality for every cardiac channel in the system
available at any time.
[0233] FIG. 14 is a schematic block diagram illustrating an
alternative embodiment of the channel selection method illustrated
in FIG. 9, adding to real-time operation the steps of automatic
channel selection which in the embodiment of FIG. 9 operate only
within the initialization process. The method steps of FIG. 14,
indicated together as 140a, include not only the initialization
steps from the embodiment of FIG. 9 but also elements 202 through
210. Element 10a, as shown, is a modification of element 10 in FIG.
9, modified to include the now real-time automatic steps embodied
in FIGS. 10A-11C and 13. Elements 202 through 210 describe an
embodiment which carries out cardiac channel replacement during
real-time operation of the inventive method, replacing a reference
or ventricular channel when the overall quality of such channel
degrades during operation.
[0234] Method element 10a encompasses the running (real-time)
operation of the inventive method as detailed in FIGS. 1-11C and
13. Connection 202 and method element 204 illustrate the step of
deciding if a cardiac channel needs to be replaced, the result of
calculating figure-of-merit FM in step 25 and making a comparison
with a channel-quality standard in step 27 (see FIG. 13). A user
alert is generated in step 28 (see FIG. 1), and decision step 204
may proceed in a completely automatic fashion if the user chooses
such automatic functioning. In such a case, an alert at step 28 is
still provided to the user. However, it is anticipated that the
user may intervene in the automatic steps of channel replacement to
exercise more control over the replacement process. An "N" decision
in decision step 204 returns the process to overall running-time
operation represented in element 10a.
[0235] The inventive method in embodiment 140a proceeds to method
element 206 in which a replacement channel is selected, either
automatically or manually by the user, based on assessments of
possible replacement cardiac channels. Embodiment 140a proceeds
then to method step 208 in which confirmation of the channel
replacement is carried out, either automatically or by user
intervention. With a "Y" decision at decision step 208, the process
continues channel replacement and updating in element 210 and then
the process continues with overall running-time operation
represented in element 10a. An "N" decision at decision step 208
returns the process to overall running-time operation represented
in element 10a without channel replacement and updating.
[0236] In the embodiments of FIGS. 10A through 11C, the assessments
of overall channel signal quality differ for candidate reference
channels and candidate ventricular channels. Skilled practitioners
in the use of cardiac channel signals and in electrophysiology are
able to determine which cardiac channels may serve as possible
reference channels as well as those cardiac channels which have the
properties of ventricular channels. Herein, the set of candidate
reference channels is also referred to as a first subset of cardiac
channels, and the set of candidate ventricular channels is also
referred to as a second subset of cardiac channels.
Transformation of Local Activation Times
[0237] FIG. 15 is a high-level schematic block diagram illustrating
the steps of an embodiment 300 of the inventive method for
transforming LAT values using a second reference channel signal
when the timing stability of a first reference channel signal
degrades below a timing-stability standard, in order to avoid
substantial loss of LAT values in spite of the loss of timing
stability. Method element 302 represents a plurality of reference
channel signals among the MCCE electrogram signals available for
processing. The ventricular channel and mapping channel signals 306
are also among this set of available MCCE signals.
[0238] Method elements 304 (three shown) represent the step of
storing these cardiac signals. Method embodiment 300 may be
realized within a computer programmed to carry out the steps as
described herein, and the divisions between the method elements may
vary based on the programming generated to carry out such steps.
Other quantities in addition to the cardiac electrogram signals may
be captured in computer memory, although not shown in FIG. 15
(other than timing offsets in method element 308), in order to
enable other computed values to be generated as desired, either
during a procedure or retrospectively.
[0239] Connection 302c is drawn as a wider arrow to represent that
the plurality of reference channels 302 are available within the
method structure along a connection 302c as shown. A connection
labeled 302c is shown in three other locations within the schematic
block diagram of FIG. 15 to indicate that reference channels 302
are also available at these points within the block diagram of
embodiment 300. In similar fashion, other connections are indicated
by reference numbers ending with the letter "c" to indicate that
multiple quantities are available thereon as indicated by the
reference number of the method element and/or the nature of the
multiple quantities available at such connections. For example,
method element 314, to be described later, is the source for the
quantities available on connection 314c.
[0240] Three method elements in FIGS. 15 (310, 314 and 318) involve
the application of a standard by which a determination is made
within embodiment 300 of the inventive automatic LAT-determining
method. These standards are indicated by reference numbers ending
with the letter "s" and which include the reference number of the
method element in which such a determination is made.
[0241] A first reference channel is established as the base
reference channel for the LAT computation carried out in method
element 320. Method element 320 is shown as having the three
requisite channel signals (ventricular, mapping and reference) as
indicated by connections from method elements 306 and 318. LAT
computations are carried out in method element 320 using a number
of possible ways of determining parameters within such three
signals. One such approach to LAT computation is described in this
document in FIGS. 7A and 7B, their related figures, and the
accompanying description. However, this example approach is not
intended to be limiting; numerous other approaches for determining
the necessary quantities for an LAT computation are within the
scope of the present invention.
[0242] In method element 308, timing offsets are computed to be
used in the determination of LAT values as required and also for a
determination in method decision element 310 of whether or not the
timing stability of the first reference channel signal (base
reference channel signal) has fallen below a timing-stability
standard 310s. (Timing-stability standard 310s will be described
later in this document.) In embodiment 300, the timing offsets used
to transform local activation times are themselves also local
activation times and are defined as LAT.sub.K(J) where LAT.sub.K(J)
is the local activation time of a reference channel J based on a
reference channel K. In method element 308, timing offsets are
computed for pairs of the plurality of reference channels 302. A
connection 306v is so labeled to indicate that only the ventricular
channel signal is needed for the computations of timing offsets in
method element 308.
[0243] For example, if there are five reference channels 302, then
there are ten pairs within reference channels 302 for which timing
offsets may be calculated. (In general, with n possible reference
channels available, there are (n.sup.2-n)/2 timing offsets which
may be computed.) Note that LAT.sub.J(K)=-LAT.sub.K(J) so that in
this case there are 20 timing offsets which may be available for
later use if each pair has had a period of time during which the
timing stability of both signals has been satisfactory. It is most
desirable that all possible pairs may be used for such timing
offsets computations, but alternatively only a subset of the
available pairs may be used.
[0244] In method decision element 310, if it is determined that
timing-stability standard 310s has been met, method embodiment 300
simply proceeds along the "Y" branch of method decision element 310
to process the next epoch of electrogram signal data in method
element 320 without replacing the first reference channel (the base
reference channel) with a second reference channel. However, if it
is determined that timing-stability standard 310s has not been met
(i.e., there likely has been a disruption caused, for example, by
electrode displacement, degraded electrode contact or noise), the
method of embodiment 300 proceeds along the "N" branch of method
decision element 310 to method element 312 in which certain
characteristics of the reference channel signals 302 are computed
to enable embodiment 300 to make a determination of which channels
among the reference channel signals 302 have been disrupted.
[0245] Method decision element 310 as described here, while
determining that timing stability has been degraded below
timing-stability standard 310s, does not determine which channel(s)
has/have caused this loss of timing stability. Such determination
is made in method elements 312 and 314. Note that four specific
embodiments of signal characteristic computations 312 are described
later in this document. These four embodiments are not intended to
be limiting; other computations of signal characteristics may be
used within the intent of method element 312. Connection 312c
indicates that the signal characteristic results of method element
312 are available to method element 314.
[0246] Method element 314 represent the method step of sorting
reference channels 302 according to whether or not specific
channels have been disrupted and are therefore not available to be
selected in method step 318 as the second reference channel. This
sorting is accomplished by comparison of computed signal
characteristics with a channel-sorting standard 314s. The output of
method element 314 is subset of reference channels available along
connection 314c which may be selected as the second reference
channel (new base reference channel for LAT determination).
[0247] In method step 316, the signal qualities of each available
reference channel are determined, and in method step 318, a
selection is made of the second reference channel based on a
signal-quality standard 318s. A number of signal quality
computations may be made in method element 316, examples of which
are high signal amplitude, low signal noise, low signal amplitude
variability, low cycle-length variability and short cycle length.
Some approaches to computing signal quality are described earlier
in this document, such as in sections related to automatic
selection of channel signals. None of these specific measures of
signal quality are intended to be limiting to the scope of this
invention; other computations of signal quality are possible within
the intent of method element 316. Connection 316c indicates that
the signal characteristic results of method element 312 are
available to method element 318.
[0248] During a cardiac mapping procedure, the ventricular channel
electrode and the electrodes of the reference channels 302 ideally
remain stationary. Since the reference-channel electrodes ideally
remain stationary, a set of timing offsets LAT.sub.J(K) are
computed in method element 308 using pairs from among the reference
channels 302, the values of the timing offsets being computed in
advance of their being needed.
[0249] MCCE signals are by their very nature noisy from a number of
sources, and thus the determination of LAT values, and of course,
timing offsets, is a statistical process. Transformation factors
may be established by averaging a set of LAT.sub.J(K) values for
the pair of reference channels (J,K) over a period of time during a
cardiac mapping procedure, and the statistical variations in
LAT.sub.J(K) may be used as part of a determination as to whether a
reference channel is suitable to remain as the first (base)
reference channel or to be available to be selected as the second
(new base) reference channel. Among the causes of stability
degradation is of course the physical movement of a reference
channel electrode such that its location has changed and therefore
cannot be relied upon as a suitable reference.
[0250] When a new base channel (second reference channel) has been
selected in method element 318, the computation of LAT values in
method element 320 changes from the straightforward use of the
ventricular channel, the mapping channel and the base reference
channel to include the use of one or more timing offsets. The
availability of the timing offsets in method element 320 is
indicated by the timing offsets being inputs along connection
308c.
[0251] The transformation process triggered by a change from the
first reference channel to the second reference channel is
represented by the equation LAT.sub.A(C)=LAT.sub.A(B)+LAT.sub.B(C)
where timing offset LAT.sub.B(C) is the local activation time of
reference channel C based on a reference channel B, and such timing
offset LAT.sub.B(C) transforms values LAT.sub.A(B) into LAT values
LAT.sub.A(C). As indicated, computation of LAT values in method
element 320 may include a straightforward calculation of an LAT
value without the application of a timing offset or it may include
transforming a past or current LAT value with the appropriate
timing offset. Method element 322 represents that LAT values may be
displayed in a map or other form as desired by the EP doctor.
[0252] One important aspect of the inventive automatic method is
that it is able to transform both past and future LAT values so
that an LAT map, once it has been started and considerable
investment of time (both patient and medical personnel) has been
undertaken along with possible exposure to X-ray radiation, the LAT
map is able to be completed without the loss of such investment
made in the capturing of LAT values.
[0253] As an example, an LAT map is partially generated based on
the first reference signal, such LAT values of mapping channel M
being represented by LAT.sub.1(M). Then, during this procedure the
quality of the first reference signal degrades, and the first
reference channel signal is replaced by a second reference channel
signal. Each of the past LAT values LAT.sub.1(M) may be transformed
into LAT values LAT.sub.2(M) by
LAT.sub.2(M)=LAT.sub.2(1)+LAT.sub.1(M). New LAT values LAT.sub.2(M)
may continue to be captured based on the second reference channel
signal, and the completed map in this case is now based on the
second reference channel. However, all of the points LAT.sub.1(M)
that were captured based on the first reference channel have been
used to build the new map without having to be recaptured. The
timing offset LAT.sub.2(1) has been established over time during
periods in which both the first and second reference channel
signals were signals having suitable timing stability and
quality.
[0254] In another example, an LAT map is partially generated based
on the first reference signal, such LAT values of mapping channel M
being represented by LAT.sub.1(M). Then, during this procedure the
quality of the first reference signal degrades, and the first
reference channel signal is replaced by a second reference channel
signal. New LAT values LAT.sub.1(M) may be determined by computing
LAT values LAT.sub.2(M) based on a second reference signal and then
transformed into LAT values LAT.sub.1(M) by
LAT.sub.1(M)=LAT.sub.1(2)+LAT.sub.2(M). The completed map in this
case is still based on the first reference channel, and all of the
points LAT.sub.1(M) that were captured based on the first reference
channel have been used in the map with new LAT values added to the
map having been transformed into LAT values based on the same
(first) reference signal. The timing offset LAT.sub.1(2) has been
established over time during periods in which both the first and
second reference channel signals were signals having suitable
timing stability and quality.
[0255] In the high-level schematic block diagram of FIG. 15, a
number of more detailed steps are implicitly embedded in the method
elements shown in embodiment 300. For example, among these
functions are: (1) the logic and control within method element 320
to determine whether a future-value transformation or a past-value
transformation is appropriate for the LAT computation; (2) the
sequential processing of the channel signals (e.g., from epoch to
epoch); and (3) the logic and control necessary to capture an LAT
value when the EP doctor decides that such a value should be
captured. The automatic nature of the inventive method of
determining LAT is not compromised by the manual intervention of
the EP doctor to "take a point" for mapping. The method proceeds
automatically, computing LAT values, monitoring timing stability,
and selecting replacement channels as described herein, with a
"take point" process operating over the top of such automatic
method.
[0256] Also implicitly included in the method elements of
embodiment 300 is logic control of the automatic method whereby
after a reference channel has been replaced due to a loss of timing
stability, a period of time is provided during which the statistics
of the timing offsets are reestablished with the new set of
electrode conditions such as might have been caused by a
reference-channel electrode having shifted its position. For
example, if six-second epochs are being used and twenty data points
is regarded as sufficient, a period of two minutes would be
required to reestablish such statistics.
[0257] FIG. 16 is a table 309 showing exemplary mean values for
timing offsets as computed in method element 308 of FIG. 15. This
example is of a reduced set of six cardiac channels. As described
above, the timing offsets are local activation times LAT.sub.J(K),
the local activation time of a reference channel K based on a
reference channel J where channels J and K are reference channels
within a set of possible or candidate reference channels in the
reduced set of MCCE channels. In this example, none of these
channels would intentionally be used as an exploring catheter
(mapping channel).
[0258] The first two columns from the left of table 309 indicate
the channel numbers for the timing offset calculations. Each entry
in the third column of table 309 contains an LAT value to be used
as a timing offset for the activation delay from one possible
reference electrode to another possible reference electrode. As
such, reference channel K takes the role of the mapping channel
within an LAT calculation in method element 308, and a negative LAT
value indicates that the activation arrives first at the channel K
electrode which is filling the role of mapping channel. As
mentioned above, LAT.sub.J(K)=-LAT.sub.K(J); thus, half of the
available timing offsets need not be shown in table 309. Since six
possible reference channels are available in this example, fifteen
timing offsets are computed, and thirty offsets are available.
[0259] The timing offset values shown in the third column in table
309 are the mean values of each computed timing offset, averaged
over a predetermined number of computed values. For this example,
the timing offsets are computed for six-second epochs of signal
data over a period of two minutes; therefore, 20 timing offset
values are averaged to produce the means values shown in table 309.
The predetermined number (20) of epochs and the 6-second length for
each epoch are not intended to be limiting; other values for these
two variables may be used.
[0260] It is assumed that repeated measurements of the timing
offset between two channels with stationary electrodes will
populate a Gaussian distribution with a mean and standard deviation
(stdev). The fourth column of table 309 shows the standard
deviation of each of the 15 timing offset values over the 20-sample
(two-minute) history of computed timing offset values. The
statistics of the third and fourth columns are determined on a
two-minute moving window to be used in the determination of method
element 310 as to whether or not timing stability has been lost.
Data from the epoch immediately following the two-minute window are
used in conjunction with the statistics of the two-minute moving
window to make determination 310. If it is determined in method
element 310 that timing stability has not been lost, the means and
standard deviations for the new two-minute moving window are
updated and are ready for the data from the next six-second epoch
to be processed.
[0261] The data in the fifth and sixth columns represent a next
epoch (epoch following the updating of the moving-window
statistics). These two columns represent data taken from an epoch
in which timing stability has not been lost, i.e., before loss of
timing stability. The fifth column of data, labeled X, contains 15
individual computed timing offset values, one for each pair of
channels in the set of timing offset pairs. For example, the timing
offset LAT.sub.1(2)=-24 msec and the timing offset LAT.sub.3(6)=-42
msec.
[0262] Method element 310s indicates the use of a timing-stability
standard (also indicated by reference number 310s) in the
determination of method element 310. Since it is assumed that the
timing offsets have Gaussian distributions, one example of a useful
timing-stability standard 310s is to compute Z-scores for the
timing offsets X for the epoch immediately after the current
two-minute moving window for which the statistics have been
calculated. This statistical test quantity is computed as
Z-score=(X-mean)/stdev
and values for the fifth column of timing offsets X are shown in
the sixth column. A Z-score exceeding .+-.2 indicates that a new
timing offset (in the set of values labeled X) has significantly
(with 95% confidence) deviated from the expected distribution,
thereby indicating that some change has occurred. As can be seen in
the sixth column, none of the Z-scores exceeds .+-.2, indicating
that it is unlikely that timing stability has been lost as defined
by this example timing-stability standard 310s.
[0263] The seventh and eighth columns of table 309 are outlined in
bold. The data in the seventh column of table 309 illustrates an
example of an epoch before which timing stability has likely been
lost. As method embodiment 300 proceeds and an epoch has been found
not to have indicated a loss of timing stability, the two-minute
moving-window statistics would be updated with the latest data
before proceeding to the next epoch data. In the simplified example
of table 309, the timing offsets values X in the seventh columns
are assessed against the statistics in columns three and four to
illustrate the detection of the loss of timing stability.
[0264] The eighth column shows the Z-scores for the timing offset
values in the seventh column, and it can be seen that two timing
offset values, LAT.sub.2(4) and LAT.sub.2(6), have Z-scores which
exceed -2, indicating that timing stability is very likely to have
been lost prior to this epoch. (These two Z-scores are highlighted
by gray shading.) Because timing offsets represent a relationship
between two channel signals, when a timing offset changes, it is
not known which channel (or both) have shifted or otherwise have
lost a satisfactory signal. In method element 312 of embodiment
300, signal characteristics of reference channel signals are
computed for the purpose of determining which channel(s) has/have
caused the loss of timing stability as detected in method element
310.
[0265] When timing stability has been found to have been lost, it
may become necessary to provide extra time during which the
statistics of some computed timing offsets are reestablished. This
could be achieved by enabling a timer to prevent such channels from
use during, for example, a two-minute period.
[0266] The timing stability standard 310s exampled in table 309 is
not meant to be limiting; other indications of loss of timing
stability may be used and are within the scope of the claimed
invention.
[0267] FIG. 17 is schematic block diagram illustrating the steps of
an FFT-based method embodiment 312(1) for computation of a signal
characteristic for use within method embodiment 300. This signal
characteristic computation 312(1) generates an FFT-based parameter
called the epoch center-of-power frequency freq.sub.COP and is used
within the example of FIGS. 16 through 17N illustrating embodiment
300. In the method steps of embodiment 312(1), a fast Fourier
transform (FFT) is computed for each relevant channel signal in an
epoch. Computation of an FFT is well-known to those skilled in the
art of signal processing and is not detailed herein.
[0268] FIG. 17A is an exemplary six-second epoch of a
representative cardiac channel electrogram signal which is used to
illustrate method embodiment 300, and in particular, to illustrate
four alternative methods 312(1)-312(4) for computation of a signal
characteristic for use within method embodiment 300. (The abscissa
for the exemplary epoch of FIG. 17A is in millisecond (msec) or
sample index for the 6,000 signal values in a six-second epoch.) In
method element 324 of FIG. 17, each such channel signal is divided
into overlapping segments of data for the purpose of a more
time-efficient computation of the FFT--by computing an FFT for each
of a plurality of shorter overlapping segments which span the time
range of the signal. (This method for estimating the power of a
signal as a function of frequency is similar to Welch's method or
Welch's periodogram.)
[0269] A six-second window of signal data with a sampling rate of
1,000 samples per second (sps) is divided into five overlapping
blocks of 2,048 values of the signal which span the entire 6,000
data values in the six-second epoch, as illustrated in FIG. 17B.
Each of the five curves 326(1)-326(5) is a raised and scaled
sinusoidal curve also illustrating the weightings applied in method
step 326. The weightings 326(1)-326(5) are applied individually to
segments of the signal data. Each curve has 2,048 non-zero values.
In FIG. 17B, curve 326(1) spans between data values 1 and 2,048;
curve 326(2) spans between data values 977 and 3,024; curve 326(3)
spans between data values 1,977 and 4,024; curve 326(4) spans
between data values 2,977 and 5,024; and curve 326(5) spans between
data values 3,953 and 6,000.
[0270] FIGS. 17C-17G are plots of the resulting weighted segments
of signal data. (Note that the abscissa of each of the five plots
in FIGS. 17C-17G span from 1 through 2,048 since the fast Fourier
transform to be performed is on each segment individually although
the weighted signal data is taken from different segments of the
epoch as indicated above.) The segmenting and the application of
weightings as illustrated in this example causes a net attenuation
of the total power in each segment since the maximum weighting in
each segment is 1.0 and the average across each segment is less
than 1.0. However, since the same computation is applied to each
epoch, the relative relationship of the resulting values is
preserved, and the computation is much more time efficient.
[0271] In method element 328, an FFT is computed for each of the
weighted signal segments. Each of the five segment FFT's computed
in method element 328 is a 1,024-point FFT, i.e., there are 1,024
frequency coefficients (bins) each being 0.488 Hz wide
(1,000/2,048). With a sampling rate of 1,000 sps, frequencies of up
to 500 Hz can be measured. The resulting FFT coefficients are
complex numbers, and in method element 330, these complex
coefficients are converted to magnitudes to form 1,024 real
coefficients.
[0272] FIGS. 17H-17L show the five resulting segment spectra for
the weighted segment signal data of FIGS. 17C-17G, respectively,
including the conversion of the FFT coefficients to magnitudes in
method element 330. The average of these individual spectra is
computed in method element 332 and results in the spectrum of FIG.
17M for the entire six-second epoch of signal data. (For
computational efficiency, a sum could also be computed in method
element 332 instead of an average, the only difference being the
scale factor; these approaches are equivalent.)
[0273] In method element 334, the first moment of power is computed
in order to determine the center-of-power frequency (freq.sub.COP),
as follows:
freq.sub.COP=0.488[.SIGMA.(c.sub.i.sup.2i]/.SIGMA.c.sub.i.sup.2
where the c.sub.i's equal the 1,025 spectral coefficients (the 0 Hz
and 0.488 Hz coefficients are set to 0 to remove these
frequencies), i equals the bin number of the frequency bin in the
spectrum, ranging from 0 to 1,024, and the sums are computed over
the range of 1,025 bins (the abscissa of each plot is bin number).
(i=1 is the 0.488 Hz bin number.) Conceptually, this
center-of-power frequency can be thought of as the average of the
bin distances (bin number) from 0 Hz with the bin distances being
weighted by the power at the corresponding frequency of the bin.
The corresponding frequency of each bin is 0.488 times the bin
number so the maximum frequency available in the 1,000 sps signal
is 500 Hz (the Nyquist frequency). Note that the 0 Hz and 0.488 Hz
(bin numbers 0 and 1) coefficients are set to zero since these
frequencies are substantially adulterated by the use of the
raised-cosine segmentation process and therefore are not useful for
determining the power spectrum.
[0274] The value of freq.sub.COP found for the spectrum of FIG. 17M
is 107 Hz. Note that the exemplary signal epoch data of FIG. 17A
has been captured through a 60 Hz notch filter as is often the case
for cardiac electrogram signals. This can be seen in FIGS. 17H-17M
as indicated by the deep V cut into the spectra around 60 Hz. As
long as the filter remains in place throughout a procedure, its use
is of little consequence to the ability of the inventive method to
discriminate between signals.
[0275] It has been determined that center-of-power-frequency
freq.sub.COP provides an excellent indication that a change has
occurred in a cardiac channel signal. FIG. 17N shows a table 311 of
the results of computations in method element 312 of the FFT-based
freq.sub.COP signal characteristic for the example reduced channel
set of FIG. 16. In the first column of table 311, further specifics
of the six channels are given. Cardiac electrogram channel 1 is
shown as HRA (high right atrium) and channel 2 as HIS (bundle of
His). These two channels are independent of all of the cardiac
channels. (The bundle of His is a slender bundle of modified
cardiac muscle that passes from the atrioventricular node in the
right atrium to the right and left ventricles by way of the septum
and that maintains the normal sequence of the heartbeat by
conducting the wave of excitation from the right atrium to the
ventricles. It is also called the atrioventricular bundle.)
[0276] The remaining four channels (A12, A34, A56, and A78) are all
bipolar signals between two adjacent electrodes of an octo-polar
catheter. (The signal epoch data of FIG. 17A is data from a
representative six-second epoch from channel 5-A56.) The electrodes
are at fixed spacings on the catheter and therefore are generally
constrained to move together. The second column of table 311,
labeled "Wire," shows an index of wires. Channels having the same
wire index are on the same wire or catheter, and all channels
having the same wire index are likely to be disrupted if the wire
or catheter is disrupted.
[0277] The third and fourth columns of table 311 present the
statistics (mean and standard deviation stdev) of each channel, the
mean and standard deviation of the signal characteristic
freq.sub.COP over a two-minute moving window (20 six-second
epochs). The fifth column of table 311 presents current values of
the freq.sub.COP (labeled X) for the epoch immediately after the
current two-minute moving window for which the statistics have been
calculated. This freq.sub.COP characteristic has units of Hertz
(Hz) and is the frequency at the center of the bulk of signal
power.
[0278] Table 311 of FIG. 17B also includes an embodiment of method
elements 314 and 314s. After computing signal characteristics for
the signals in method element 312, method element 314 sorts the
channels based on the results of method element 312 to provide an
automatic determination of which channel or channels caused the
loss of timing stability detected in method element 310. A
channel-sorting standard 314s is applied in the sorting of method
element 314. Since it is assumed that the FFT-based freq.sub.COP
values for each channel have Gaussian distributions, one example of
a useful channel-sorting standard 314s is to compute Z-scores for
the freq.sub.COP values X for the epoch immediately after the
current two-minute moving window for which the statistics have been
calculated. A Z-score exceeding 2 indicates that a new freq.sub.COP
(in the set of values labeled X) has significantly (with 95%
confidence) deviated from the expected distribution, thereby
indicating that some change has occurred.
[0279] The sixth, seventh and eighth columns of table 311 contain
absolute values of three different computed Z-scores. The isolated
Z-score is computed for each channel based only on the statistics
of that channel, as follows:
Isolated Z-score=abs[(X-mean)/stdev]
The Group Z-score is the average of the isolated Z-scores of all
the channels that share a common wire index, and the combined
Z-scores are the averages of the isolated and group Z-scores for
each channel.
[0280] In table 311, the channel 2-HIS and channel 6-A78 signals
(see grayed entries) both indicate a significant change in the
current measurement of the FFT-based freq.sub.COP; each has shifted
by more than two standard deviations. (The 2-HIS isolated Z-score
is 2.4, and the 6-A78 Z-score is 2.1.) However, the 6-A78 channel
is on a wire that constrains the 6-A78 electrode from moving much
differently than the electrodes capturing three other signals.
Because the three other signals have much lower Z-scores, there is
a strong indication that the channel 6-A78 isolated Z-score,
especially being only a little over threshold of 2, may simply be
due to natural variation and not of practical significance. (Even
with a criterion like abs(Z)>2, one in 20 measurements are
expected to be over threshold simply by random chance.) The
additional information about companion signals on the same wire
(the four "A" channels) therefore assists in the recognition of a
disrupted channel if the characteristic for that channel (6-A78) is
near the Z>2 criterion, if many of the other companion signals
less ambiguously satisfy the criterion, i.e., Z is much less than
2. Thus, in this particular example, the conclusion is that channel
2 (2-HIS) is the disrupted reference channel.
[0281] The specific computations illustrated in tables 309 and 311
are not intended to be limiting with respect to the ways in which
the loss of timing stability and the detection of which channel or
channels have caused such loss; other specific computational
approaches may be used within the scope of the inventive automatic
method of determining local activation time disclosed herein.
[0282] As indicated by the highlighted portion in gray shading in
table 311, cardiac electrogram channel 2-HIS is the channel which
embodiment 300 of the inventive automatic method applied to this
reduced channel-set example has identified as having caused the
loss of timing stability. In this case, if channel 2-HIS is the
current base reference channel, it will be replaced with a new
reference channel selected from the remaining set of candidate
reference channels, and such replacement channel is selected based
on signal quality computations in method element 316 which provide
method element 318 measures of signal qualities to be compared
based on a signal-quality standard 318s.
[0283] As indicated above, the qualities that may recommend a
signal for use as a reference channel are: 1) high signal
amplitude, 2) low signal noise, 3) low signal amplitude
variability, 4) low cycle-length variability, and 5) shorter cycle
length. Since several signal-quality measures determined in method
element 316 and assessed in method element 318 using corresponding
signal-quality standards 318s have been described above in this
document, no further examples are presented here. For the purpose
selecting a new best replacement reference channel, these five
channel qualities may be assessed over a longer time period since
in embodiment 300 of the inventive method, such assessments occur
in the background, not causing any delay in the LAT mapping
function. In this case, the assessment could be over a longer
period such as two minutes. The most recent epoch may be
incorporated into the assessment since only non-disrupted channels
will be considered.
[0284] FIG. 18 is schematic block diagram illustrating an
embodiment 312(2) of a first alternative method for computation of
a signal characteristic for use within the method embodiment of
FIG. 15. This alternative signal characteristic computation 312(2)
generates a signal characteristic called epoch activity duration.
In method element 340, values for the velocities of signal epoch
data samples are computed using a method such as described above
with respect to FIGS. 2-3B. The use of such method for computing
signal velocities is not intended to be limiting. A number of
approaches for signal velocity computation are well-known by those
skilled in the art of signal processing, and such approaches are
applicable within the inventive method claimed herein.
[0285] In method element 342, the absolute values of the velocities
are determined, and in method element 344, an activity threshold
act.sub.TH is computed. Since all MCCE signals contain some noise,
it is necessary to define signal activity as occurring with signal
levels above a threshold in order to avoid such noise corrupting
the determination of activity duration. One useful definition of
threshold act.sub.TH is four times the median of the data across
the entire six-second epoch. Such threshold definition is not
intended to be limiting to the present invention; other useful
definitions of act.sub.TH may be used.
[0286] A signal is said to be active when the absolute value of the
velocity is greater than act.sub.TH. This comparison with
act.sub.TH occurs within method element 346 which also counts the
number of signal data values which exceed act.sub.TH. Method
element 348 counts the total number of signal data values, and in
method element 350, the epoch activity duration is computed as the
fraction of the total signal data values exceeding act.sub.TH.
[0287] FIG. 18A is a plot illustrating the application of an
absolute-value velocity filter to the exemplary six-second epoch
cardiac channel electrogram signal of FIG. 17A. The median of the
epoch signal data of FIG. 17A is 15 .mu.volts resulting in a value
of act.sub.TH of 60 .mu.volts. The plot of FIG. 18A indicates the
value of act.sub.TH used, and FIG. 18B is a plot illustrating
computation of the activity duration signal characteristic for the
absolute-value velocity epoch signal of FIG. 18A. Of the 6,000
total signal data values, 1,103 values were found to exceed
act.sub.TH=60, resulting in a value of activity duration of 0.184
for the exemplary signal epoch data of FIG. 17A.
[0288] It has been found that such an activity duration measurement
is a useful signal characteristic by which to determine which
cardiac channel or channels has/have been disturbed. Another such
useful signal characteristic is simply a peak-to-peak measure
across an epoch of cardiac channel data, and such a determination
is described in FIG. 19 in method embodiment 312(3). Method
elements 352 and 354 find the minimum and maximum signal values,
respectively, across an epoch of signal data. Method element 356
computes the difference between the maximum and minimum values to
yield the peak-to-peak signal characteristic.
[0289] FIG. 19A is a table 358 illustrating second alternative
embodiment 312(3) of FIG. 19, showing the peak-to-peak
determination of the exemplary six-second epoch of the exemplary
cardiac channel electrogram signal of FIG. 17A. The peak-to-peak
value for the epoch of signal data values of FIG. 17A is found to
be 1,731 .mu.volts. The epoch of signal data in FIG. 17A provides a
good illustration of why an epoch duration of about six seconds is
desirable, and this can be seen from at least three perspectives.
Six seconds is long enough to span several complete cycles of
cardiac contraction; six seconds is long enough to span at least
one respiratory cycle; and six seconds is long enough to obviate
the need to carefully detect individual cardiac beats. By including
these sources of variability entirely within, and therefore common
to, an epoch, stability can be recognized from epoch to epoch. FIG.
17A clearly illustrates beat-to-beat variability within the span of
a six-second epoch.
[0290] FIG. 20 is schematic block diagram illustrating an
embodiment 313(4) of a third alternative method for computation of
a signal characteristic for use within the method embodiment of
FIG. 15. This alternative signal characteristic computation
generates a Haar-transform-based parameter called the epoch
center-of-power frequency freq.sub.COP. In FIG. 20, signal epoch
data is divided into sequential segments in method element 360. The
division of signal epoch data into segments is useful for
computational efficiency depending on the number of data values in
a epoch of data. A Haar transform requires that the number of
samples be a power of 2. Thus, for example, a six-second epoch of
data captured at 1,000 sps contains 6,000 values, and such an epoch
can be divided into three segments of 2,048 (2.sup.11) values with
only very modest overlap between the segments.
[0291] FIGS. 20A-20C illustrate the segmenting of the exemplary
data of FIG. 17A into three sequential segments, each consisting of
2,048 values. Note that each segment is shown having sample indices
from 1 to 2,048 although the actual index values range as follows:
the plot of FIG. 20A includes epoch samples 1 through 2,048; the
plot of FIG. 20B includes epoch samples 1,977 through 4,024; and
the plot of FIG. 20C includes epoch samples 3,953 through 6,000.
Again, as in FIGS. 17C-17G, the abscissa of each of the three plots
in FIGS. 20A-20C span from 1 through 2,048 since the Haar transform
to be performed is on each segment individually although the signal
data is taken from different segments of the epoch as indicated
above.
[0292] In method element 362, the Haar transformation coefficients
for each segment of data are computed. The details of a Haar
transformation are well known to those skilled in the art of signal
processing. However, some details of such a computation will be
described to illustrate certain aspects of method embodiment
312(4). FIGS. 20D-20F are three plots of the Haar transformation
coefficients for the three data segments of FIGS. 20A-20C,
respectively, resulting from the Haar transformation of such three
segments of data. These coefficients are computed as illustrated in
the table of FIG. 20G. A Haar transformation is based a series of
signal differences as shown in FIG. 20G, and a signal having 2,048
data values results in 2,048 Haar transformation difference-related
coefficients H.sub.i.
[0293] The abscissa of the plots of FIGS. 20D-20F represent the
2,048 Haar coefficients, and the plots are not frequency spectra.
Unlike an FFT, the Haar transformation coefficients do not
constitute a frequency spectrum but rather are a set of difference
terms as described further in FIG. 20G. H.sub.1 is the sum of all
2,048 values in the segment of epoch data, and as such it
represents the DC term in the signal (no differencing). H.sub.2 is
the difference between the sum of the second half of the signal
values minus the sum of the first half of the values, and as such,
represents information in the signal related to a cycle which is
the full length of the epoch segment (2.048 seconds--0.488 Hz--in
the exemplary data of FIG. 17A). The differencing continues by
powers of two as shown in FIG. 20G up to H.sub.2048 which is the
difference between the last two signal values. The last 1,024
coefficients (H.sub.1025 through H.sub.2048) are the differences
between neighboring pairs of signal values. These coefficients
relate to the highest frequency information available in the
transform which in the example being discussed is 500 Hz.
[0294] Note that in the description of FIG. 20G, the sign
convention may differ from some descriptions of the Haar transform,
but since the signal characteristic being computed is related to
power, the absolute value of the coefficients is used and the sign
convention is not relevant. Thus, in method element 364, an
absolute value filter is applied to the Haar coefficients.
[0295] FIG. 20H shows a table detailing the computation of a set of
eleven frequency-selective aggregate magnitudes A.sub.i from the
2,048 Haar transformation coefficients H.sub.i of FIG. 20G. The
Haar transformation coefficients are aggregated in method element
366 of method embodiment 312(4) of FIG. 20 by combining the
absolute values of the Haar transformation coefficients H.sub.i
having like time scales as shown in FIG. 20H. Thus, for example,
every difference of same-time-scale sets of signal values are
summed in method element 368 of method embodiment 312(4) of FIG. 20
to form the frequency-selective aggregate magnitude related to the
corresponding time scale. (The aggregate magnitudes are
frequency-selective because they are formed from differences having
the same time scales.)
[0296] Such aggregation results in a spectrum-like plot of
frequency-selective aggregate magnitudes A.sub.i which then relate
to certain frequencies which are assigned to the
frequency-selective aggregate magnitudes in method element 370 of
method embodiment 312(1). In the example being discussed, the
frequency F.sub.i related to A.sub.i is
F.sub.i=0.488.times.2.sup.(i-1), and these frequencies are shown in
the table of FIG. 20H.
[0297] The final step in method embodiment 312(4), in method
element 372, is a determination of a signal characteristic based on
the Haar transformation, a center-of-power frequency freq.sub.COP.
One way to determine freq.sub.COP is to compute a first moment
similar to that used for the FFT-based signal characteristic.
freq.sub.COP=[.SIGMA.(A.sub.i.sup.2F.sub.i)]/.SIGMA.A.sub.i.sup.2
where the sum is over the 11 frequency-selective aggregate
magnitudes A.sub.i and their corresponding frequencies F.sub.i.
[0298] FIGS. 20I-20K are three plots of the absolute values of the
Haar transformation coefficients shown in FIGS. 20D-20F for the
three data segments of FIGS. 20A-20C, respectively, and FIGS.
20L-20N are three bar charts showing the values frequency-selective
aggregate magnitudes A.sub.i for each of the three segments as
shown in FIGS. 20A-20C and determined by applying the computations
described in FIGS. 20G and 20H.
[0299] FIG. 20P is a bar chart presenting the sum of the three sets
of segments magnitudes A.sub.i from the Haar transformations of the
three data segments. The eleven frequency-selective aggregate
magnitudes of the abscissa are used in the determination of
frequency-related signal characteristic freq.sub.COP using the
Haar-transform-based method of FIG. 20. For the exemplary signal
data of FIG. 17A, freq.sub.COP is found to be 141 Hz, and this is
indicated in the chart of FIG. 20P.
[0300] Note that this frequency of 141 Hz differs from the 107 Hz
found for the FFT-base computation of freq.sub.COP. Such a
difference is expected due to the logarithmic nature of the
abscissa in the chart of FIG. 20P, but since comparisons are made
using the same signal characteristic computation, what is important
is the ability of the signal characteristic to be sensitive to
differences in the signals. All of the methods exampled above for
method element 312 of method embodiment 300 are able to detect such
differences in signals.
[0301] While the principles of this invention have been described
in connection with specific embodiments, it should be understood
clearly that these descriptions are made only by way of example and
are not intended to limit the scope of the invention.
* * * * *