U.S. patent application number 12/637489 was filed with the patent office on 2011-01-13 for arterial blood pressure monitoring devices, systems and methods using cardiogenic impedance signal.
Invention is credited to Gene A. Bornzin, Taraneh Ghaffari Farazi, Wenbo Hou, Edward Karst, Allen J. Keel, Brian Jeffrey Wenzel.
Application Number | 20110009754 12/637489 |
Document ID | / |
Family ID | 43428007 |
Filed Date | 2011-01-13 |
United States Patent
Application |
20110009754 |
Kind Code |
A1 |
Wenzel; Brian Jeffrey ; et
al. |
January 13, 2011 |
ARTERIAL BLOOD PRESSURE MONITORING DEVICES, SYSTEMS AND METHODS
USING CARDIOGENIC IMPEDANCE SIGNAL
Abstract
Provided herein are implantable systems, and methods for use
therewith, for monitoring a patient's arterial blood pressure.
Electrode(s) implanting within and/or on the patient's heart are
used to obtain a cardiogenic impedance (CI) signal indicative of
cardiac contractile activity. Additionally, a signal (e.g., PPG or
IPG signal) indicative of changes in arterial blood volume remote
from the patient's heart is obtained using a sensor or electrodes
that are implanted remote from the patient's heart. One or more
metrics indicative of pulse arrival time (PAT) are determined,
where each metric can be determined by determining a time from one
of the detected features of the CI signal to one of the detected
features of the signal indicative of changes in arterial blood
volume. Based on at least one of the metric(s) indicative of PAT,
arterial blood pressure is estimated, which can include determining
values indicative of systolic blood pressure, diastolic blood
pressure, pulse pressure and/or mean arterial blood pressure,
and/or changes in such values.
Inventors: |
Wenzel; Brian Jeffrey; (San
Jose, CA) ; Keel; Allen J.; (San Jose, CA) ;
Karst; Edward; (S. Pasadena, CA) ; Hou; Wenbo;
(Lancester, CA) ; Farazi; Taraneh Ghaffari; (San
Jose, CA) ; Bornzin; Gene A.; (Simi Valley,
CA) |
Correspondence
Address: |
STEVEN M MITCHELL;PACESETTER INC
701 EAST EVELYN AVENUE
SUNNYVALE
CA
94086
US
|
Family ID: |
43428007 |
Appl. No.: |
12/637489 |
Filed: |
December 14, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61223992 |
Jul 8, 2009 |
|
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|
Current U.S.
Class: |
600/485 |
Current CPC
Class: |
G03F 7/0002 20130101;
A61B 5/1459 20130101; A61B 5/7239 20130101; A61B 5/0215 20130101;
A61B 5/02125 20130101; A61N 1/36585 20130101; A61B 5/0535 20130101;
A61B 5/0295 20130101; B82Y 10/00 20130101; B82Y 40/00 20130101 |
Class at
Publication: |
600/485 |
International
Class: |
A61B 5/0215 20060101
A61B005/0215 |
Claims
1. For use with an implantable system, a method for monitoring a
patient's arterial blood pressure, the method comprising: (a) using
one or more electrodes implanting within and/or on the patient's
heart to obtain a cardiogenic impedance (CI) signal indicative of
cardiac contractile activity; (b) using an implanted sensor or
implanted electrodes, remote from the patient's heart, to obtain a
signal indicative of changes in arterial blood volume remote from
the patient's heart; (c) detecting one or more predetermined
features of the CI signal; (d) detecting one or more predetermined
features of the signal indicative of changes in arterial blood
volume remote from the patient's heart; (e) determining one or more
metrics indicative of pulse arrival time (PAT), each metric
indicative of PAT determined by determining a time from one of the
detected features of the CI signal to one of the detected features
of the signal indicative of changes in arterial blood volume remote
from the patient's heart; and (f) estimating the patient's arterial
blood pressure based on at least one of the one or more metrics
indicative of PAT.
2. The method of claim 1, wherein step (b) comprises one of the
following: using an implanted photoplethysmography sensor, remote
from the patient's heart, to obtain a photoplethysmography (PPG)
signal indicative of changes in arterial blood volume remote from
the patient's heart; and using implanted electrodes, remote from
the patient's heart, to obtain an impedance plethysmography signal
(IPG) indicative of changes in arterial blood volume remote from
the patient's heart.
3. The method of claim 1, wherein step (f) comprises determining a
value indicative of systolic blood pressure (SBP) based on at least
one of the one or more metrics indicative of PAT.
4. The method of claim 1, wherein step (f) comprises determining a
value indicative of diastolic blood pressure (DBP) based on at
least one of the one or more metrics indicative of PAT.
5. The method of claim 1, wherein step (f) comprises determining a
value indicative of mean arterial blood pressure (MAP) based on at
least one of the one or more metrics indicative of PAT.
6. The method of claim 1, wherein step (f) comprises: determining a
value indicative of systolic blood pressure (SBP) based on at least
one of the one or more metrics indicative of PAT; and determining a
value indicative of diastolic blood pressure (DBP) based on at
least one of the one or more metrics indicative of PAT.
7. The method of claim 6, wherein step (f) further comprises
determining a value indicative of mean arterial blood pressure
(MAP) based on the value indicative of SBP and the value indicative
of DBP.
8. The method of claim 1, wherein the one or more predetermined
features of the CI signal detected at step (c) is/are selected from
the group consisting of: minimum amplitude of the CI signal;
maximum upward slope of the CI signal; maximum amplitude of the CI
signal; and maximum downward slope of the CI signal.
9. The method of claim 1, wherein the signal indicative of changes
in arterial blood volume remote from the patient's heart obtained
at step (b) comprises a photoplethysmography (PPG) signal or an
impedance plethysmography (IPG) signal, and the one or more
predetermined features of the PPG or IPG signal detected at step
(d) is/are selected from the group consisting of: minimum amplitude
of the PPG or IPG signal; maximum upward slope of the PPG or IPG
signal; maximum amplitude of the PPG or IPG signal; dicrotic notch
of the PPG signal or IPG; maximum downward slope of the PPG or IPG
signal prior to the dicrotic notch; and maximum downward slope of
the PPG or IPG signal following the dicrotic notch.
10. The method of claim 9, wherein step (d) includes: filtering the
PPG or IPG signal to reduce effects of respiratory noise, motion
artifacts and baseline drift; grouping a plurality of cycles of the
PPG or IPG signal together and performing an outlier removal
process to remove cycles of the PPG or IPG signal that are more
than a specified threshold away from a mean of the plurality of
cycles of the PPG or IPG signal; averaging the cycles of PPG or IPG
signal remaining after the performance of the outlier removal
process to thereby determine an averaged PPG or IPG signal;
determining a first derivative and a second derivative of the
averaged PPG or IPG signal; and detecting, based on the first and
second derivatives of the averaged PPG or IPG signal, the one or
more predetermined features of the PPG or IPG signal.
11. The method of claim 1, further comprising: causing paced
cardiac events by pacing the patient's heart, using implanted
electrodes, with a voltage sufficient to cause capture; and wherein
steps (a), (b), (c) and (d) are performed while the patient's heart
is being paced to cause capture.
12. An implantable system, comprising: a cardiogenic impedance
measurement circuit configured to obtain a cardiogenic impedance
(CI) signal indicative of cardiac contractile activity; a
plethysmography sensor configured to obtain a signal indicative of
changes in arterial blood volume remote from the patient's heart;
an arterial blood pressure monitor configured to detect one or more
predetermined features of the CI signal; detect one or more
predetermined features of the signal indicative of changes in
arterial blood volume remote from the patient's heart; determine
one or more metrics indicative of pulse arrival time (PAT) based on
the detected features of the CI signal and the signal indicative of
changes in arterial blood volume remote from the patient's heart;
and estimate the patient's arterial blood pressure based on at
least one of the one or more metrics indicative of PAT.
13. The implantable system of claim 12, wherein the plethysmography
sensor is selected from the group consisting of: a
photoplethysmography sensor configured to obtain a
photoplethysmography (PPG) signal indicative of changes in arterial
blood volume remote from the patient's heart; and circuitry
configured to obtain, using electrodes implanted remote from the
patient's heart, an impedance plethysmography signal (IPG)
indicative of changes in arterial blood volume remote from the
patient's heart.
14. The implantable system of claim 12, wherein the arterial blood
pressure monitor is configured to estimate the patient's arterial
blood pressure by determining one or more values selected from the
group consisting of: a value indicative of systolic blood pressure
(SBP) based on at least one of the one or more metrics indicative
of PAT; a value indicative of diastolic blood pressure (DBP) based
on at least one of the one or more metrics indicative of PAT; a
value indicative of pulse pressure (PP) based on at least one of
the one or more metrics indicative of PAT; and a value indicative
of mean arterial blood pressure (MAP) based on at least one of the
one or more metrics indicative of PAT.
15. The implantable system of claim 12, wherein the arterial blood
pressure monitor is configured to detect one or more predetermined
features of the CI signal selected from the group consisting of:
minimum amplitude of the CI signal; maximum upward slope of the CI
signal; maximum amplitude of the CI signal; and maximum downward
slope of the CI signal.
16. The implantable system of claim 12, wherein the signal
indicative of changes in arterial blood volume remote from the
patient's heart comprises a photoplethysmography (PPG) signal or an
impedance plethysmography (IPG) signal, and wherein the arterial
blood pressure monitor is configured to detect one or more
predetermined features of the PPG or IPG signal selected from the
group consisting of: minimum amplitude of the PPG or IPG signal;
maximum upward slope of the PPG or IPG signal; maximum amplitude of
the PPG or IPG signal; dicrotic notch of the PPG signal or IPG;
maximum downward slope of the PPG or IPG signal prior to the
dicrotic notch; and maximum downward slope of the PPG or IPG signal
following the dicrotic notch.
Description
PRIORITY CLAIM
[0001] This application claims priority under 35 U.S.C. 119(e) to
U.S. Provisional Patent Application No. 61/223,992, filed Jul. 8,
2009, entitled "Arterial Blood Pressure and Electromechanical Delay
(EMD) Monitoring Devices, Systems and Methods" (Attorney Docket No.
A09P3005), which is incorporated herein by reference.
RELATED APPLICATIONS
[0002] This application is related to the following commonly
assigned applications, each of which is incorporated herein by
reference: U.S. patent application Ser. No. 12/474,276, entitled
"Standalone Systemic Arterial Blood Pressure Monitoring Device,"
filed May 28, 2009 (Attorney Docket No. A09P3002); U.S. patent
application Ser. No. 11/848,586, entitled "Implantable Systemic
Blood Pressure Measurement Systems and Methods," filed Aug. 31,
2007 (Attorney Docket No. A07P3032); U.S. patent application Ser.
No. 12/______, entitled "Arterial Blood Pressure Monitoring
Devices, Systems and Methods for use while Pacing," filed the same
day as the present application (Attorney Docket No. A09P3006); and
U.S. patent application Ser. No. 12/______, entitled "Cardiac
Electromechanical Delay (EMD) Monitoring Devices, Systems and
Methods," filed the same day as the present application (Attorney
Docket No. A09P3007).
FIELD OF THE INVENTION
[0003] Embodiments of the present invention relate to devices,
systems and methods for monitoring arterial blood pressure.
BACKGROUND OF THE INVENTION
[0004] A person's circulatory system includes both systemic and
pulmonary circulation. Pulmonary circulation supplies the lungs
with blood flow, while the systemic circulation takes care of all
the other parts of the body. The heart serves as a pump that
circulates the blood, while blood vessels act as the conduits that
deliver blood to tissue. Both the pulmonary and systemic
circulatory systems are made up of arteries, arterioles,
capillaries, venules and veins. The arteries take the blood from
the heart, while the veins return the blood to the heart
[0005] Blood pressure is defined as the force exerted by the blood
against any unit area of the vessel wall. The measurement unit of
blood pressure is millimeters of mercury (mmHg). Pulmonary and
systemic arterial blood pressures are pulsatile, having systolic
and diastolic blood pressure values. The highest recorded pressure
reading in a cardiac cycle is called systolic blood pressure, which
results from the active contraction of the ventricle. Although the
arterial blood pressure and indeed flow in the arteries is
pulsatile, the total volume of blood in the circulation changes
little over a cardiac cycle. The lowest arterial pressure reading
in a cardiac cycle is called diastolic blood pressure which is
maintained by the resistance created by the smaller blood vessels
still on the arterial side of the circulatory system (arterioles).
Stated another way, the systolic blood pressure is defined as the
peak pressure in the arteries, which occurs near the beginning of a
cardiac cycle, where a cardiac cycle can be said to begin when
blood is ejected from the ventricles. In contrast, the diastolic
blood pressure is the lowest pressure, which occurs at the resting
phase of the cardiac cycle. The pulse pressure reflects the
difference between the maximum and minimum pressures measured
(i.e., the difference between the systolic blood pressure and
diastolic blood pressure). The mean arterial blood pressure is the
average pressure throughout the cardiac cycle.
[0006] Arterial pulse pressure, such as mean arterial blood
pressure (MAP), is a fundamental clinical parameter used in the
assessment of hemodynamic status of a patient. Mean arterial blood
pressure can be estimated from real pressure data in a variety of
ways. Among the techniques that have been proposed, one is
presented below. In this formula, SBP is the systolic blood
pressure, and DBP is diastolic blood pressure.
MAP = ( SBP + 2 DBP ) / 3 = 1 3 ( SBP ) + 2 3 ( DBP )
##EQU00001##
[0007] Systolic blood pressure and diastolic blood pressure can be
obtained in a number of ways. A common approach is to use a
stethoscope, an occlusive cuff, and a pressure manometer. However,
such an approach is slow, requires the intervention of a skilled
clinician and does not provide timely readings as it is a
measurement at only a single point in time. While systolic blood
pressure and diastolic blood pressure can also be obtained in more
automated fashions, it is not always practical to obtain measures
of pressure using a cuff and pressure transducer combination,
especially if the intention or desire is to monitor systemic
arterial blood pressure on a chronic basis.
[0008] Another approach for obtaining measures of arterial blood
pressure is to use an intravascular pressure transducer. However,
an intravascular device may cause problems, such as, embolization,
nerve damage, infection, bleeding and/or vessel wall damage.
Additionally, the implantation of an intravascular lead requires a
highly skilled physician such as a surgeon, electrophysiologist, or
interventional cardiologist.
[0009] Plethysmography, the measurement of volume of an organ or
body part, has a history that extends over 100 years.
Photoplethysmography (PPG) uses optical techniques to perform
volume measurements, and was first described in the 1930s. While
best known for their role in pulse oximetry, PPG sensors have also
been used to indirectly measure blood pressure. For example,
non-invasive PPG sensors have been used in combination with in an
inflatable cuff in a device known as Finapres. U.S. Pat. Nos.
4,406,289 (Wesseling et al.) and 4,475,940 (Hyndman) are exemplary
patents that relate to the Finapres technique. The cuff is applied
to a patient's finger, and the PPG sensor measures the absorption
at a wavelength specific for hemoglobin. After the cuff is used to
measure the individual's mean arterial blood pressure, the cuff
pressure around the finger is then varied to maintain the
transmural pressure at a constant predetermined pressure as
determined by the PPG sensor. The Finapres device tracks the
intra-arterial blood pressure wave by adjusting the cuff pressure
to maintain the optical absorption constant at all times.
[0010] There are a number of disadvantages to the Finapres
technique. For example, when there exists peripheral
vasoconstriction, poor vascular circulation, or other factors, the
blood pressure measured in a finger is not necessarily
representative of central blood pressure. Further, maintaining
continuous cuff pressure causes restriction of the circulation in
the finger being used, which is uncomfortable when maintained for
extended periods of time. Accordingly, the Finapres technique is
not practical for chronic use. Additionally, because of the need
for a pneumatic cuff, a Finapres device can not be used as an
implanted sensor.
[0011] Simple external blood pressure monitors also exist, but they
do not offer continuous measurement and data logging capability.
These devices can be purchased at a drug store, but patient
compliance is required to make regular measurements and accurately
record the data. Additionally, portable external miniature monitors
that automatically log blood pressure data exist, but these devices
can only store a day or so of data and require clinician
interaction to download and process the measured data.
[0012] As is evident from the above description, there is the need
for improved systems and methods for monitoring arterial blood
pressure, including systolic blood pressure, diastolic blood
pressure and mean arterial blood pressure.
[0013] Electromechanical delay (EMD) is the time delay between
onset of ventricular electrical activation and mechanical ejection
of blood from the heart. This delay is partly due to the time
required for the contractile elements of muscles to stretch the
series elastic components. EMD is believed to be affected by
conduction abnormalities, myocardial contractility and cardiac
diseases, including but not limited to heart failure (HF), mitral
stenosis, and hypertension. Accordingly, monitoring EMD can be
useful for monitoring conduction abnormalities, myocardial
contractility and cardiac diseases.
SUMMARY
[0014] Certain embodiments of the present invention related
implantable systems, and methods for use therewith, for monitoring
a patient's systemic arterial blood pressure. One or more
electrodes implanting within and/or on the patient's heart are used
to obtain a cardiogenic impedance (CI) signal indicative of cardiac
contractile activity. Additionally, an implanted sensor or
implanted electrodes, remote from the patient's heart, are used to
obtain a signal indicative of changes in arterial blood volume
remote from the patient's heart. In an embodiment, an implanted
photoplethysmography sensor, remote from the patient's heart, is
used to obtain a photoplethysmography (PPG) signal indicative of
changes in arterial blood volume remote from the patient's heart.
In an alternative embodiment, implanted electrodes, remote from the
patient's heart, are used to obtain an impedance plethysmography
signal (IPG) indicative of changes in arterial blood volume remote
from the patient's heart. Other sensors remote from the patient's
heart can alternatively be used to obtain other signals indicative
of changes in arterial blood volume remote from the patient's
heart.
[0015] In certain embodiments, one or more predetermined features
of the CI signal are detected, as are one or more predetermined
features of the signal indicative of changes in arterial blood
volume remote from the patient's heart. Exemplary predetermined
features of the CI signal that can be detected include, but are not
limited to, minimum amplitude of the CI signal, maximum upward
slope of the CI signal, maximum amplitude of the CI signal and
maximum downward slope of the CI signal. Presuming the signal
indicative of changes in arterial blood volume is a PPG or IPG
signal, exemplary predetermined features that can be detected
include, but are not limited to, minimum amplitude of the PPG or
IPG signal, maximum upward slope of the PPG or IPG signal, maximum
amplitude of the PPG or IPG signal, dicrotic notch of the PPG
signal or IPG, maximum downward slope of the PPG or IPG signal
prior to the dicrotic notch, and maximum downward slope of the PPG
or IPG signal following the dicrotic notch.
[0016] In certain embodiments, one or more metrics indicative of
pulse arrival time (PAT) are determined, where each metric
indicative of PAT is determined by determining a time from one of
the detected features of the CI signal to one of the detected
features of the signal indicative of changes in arterial blood
volume. Based on at least one of the metric(s) indicative of PAT,
the patient's arterial blood pressure is estimated. This can
include determining values indicative of systolic blood pressure
(SBP), diastolic blood pressure (DBP), pulse pressure (PP) and/or
mean arterial blood pressure (MAP), and/or changes in such
values.
[0017] The above described techniques for monitoring a patient's
arterial blood pressure can be performed while a patient's heart is
beating intrinsically, or while the patient's heart is being paced
with a voltage sufficient to cause capture.
[0018] In accordance with further embodiments of the present
invention, where the patient's arterial blood pressure is being
monitored while the patient's heart is being paced, one or more
metrics indicative of pulse arrival time (PAT) can alternatively be
determined by determining a time from a paced cardiac event to one
or more predetermined features of the signal (e.g., PPG or IPG
signal) indicative of changes in arterial blood volume. In such
embodiments, these alternative metrics indicative of PAT can be
used to estimate the patient's arterial blood pressure.
[0019] In certain embodiments, a patient's electromechanical delay
(EMD) can be monitored. More specifically, one or more values
indicative EMD, between delivery of pacing and a mechanical cardiac
contraction resulting from the pacing, can be determined. In such
embodiments, metric(s) indicative of PAT can also be determined
based on the value(s) indicative of EMD. For example, a metric
indicative of PAT can be determined by determining a time from a
paced cardiac event to a predetermined feature of the signal
indicative of changes in arterial blood volume, minus the
determined value indicative of EMD. Determined values indicative of
EMD may also be used as feedback to adjust pacing, e.g., to
minimize variance of a value indicative of electromechanical delay
(EMD).
[0020] In specific embodiments, paced cardiac events are caused by
delivering sufficient pacing stimulation to cause capture of the
patient's heart. Using one or more electrodes implanting within
and/or on the patient's heart, a CI signal indicative of cardiac
contractile activity while the patient's heart is being paced is
obtained. One or more predetermined features of the obtained CI
signal is/are detected. One or more values indicative of the
patient's EMD can be determined by determining a time between a
delivered pacing stimulation and at least one of the one or more
detected features of the CI signal.
[0021] Additional and alternative embodiments, features and
advantages of the invention will appear from the following
description in which the preferred embodiments have been set forth
in detail, in conjunction with the accompanying drawings and
claims.
BRIEF DESCRIPTION OF THE FIGURES
[0022] FIG. 1A includes exemplary signal waveforms that are used to
show the relative timing of various signals, and how an exemplary
pulse arrival time (PAT) metric can be determined in accordance
with an embodiment of the present invention. The waveforms include
an IEGM/ECG signal, a left ventricular pressure signal, a CI signal
and a PPG/IPG signal.
[0023] FIG. 1B includes the same exemplary signal waveforms shown
in FIG. 1A, but shows how another exemplary PAT metric can be
determined in accordance with an embodiment of the present
invention.
[0024] FIG. 1C is similar to FIG. 1A, but the IEGM/ECG signal is
replaced with a ventricular pacing signal, and shows how still
another exemplary PAT metric can be determined in accordance with
an embodiment of the present invention.
[0025] FIG. 2A is a high level flow diagram that is used to explain
various embodiments of the present invention that can be used to
estimate a patient's blood pressure.
[0026] FIG. 2B is a high level flow diagram that is used to explain
alternative embodiments of the present invention that can be used
to estimate a patient's blood pressure.
[0027] FIG. 2C is a high level flow diagram that is used to explain
embodiments of the present invention that can be used to monitor an
electro mechanical delay (EMD) of a patient's heart.
[0028] FIG. 3 illustrates an exemplary implantable cardiac
stimulation device that includes a PPG sensor, and which can be
used to perform various embodiments of the present invention.
[0029] FIG. 4 is a simplified block diagram that illustrates
possible components of the implantable device shown in FIG. 3.
[0030] FIG. 5 is a block diagram of an exemplary impedance
measuring circuit architecture that can be used to obtain CI
signals and/or IPG signals that can be used in various embodiments
of the present invention.
[0031] FIG. 6 is a flow diagram that is used to describe how
features of a PPG or IPG signal can be detected in accordance with
specific embodiments of the present invention.
[0032] FIG. 7A illustrates an exemplary raw PPG signal over 20
seconds.
[0033] FIG. 7B illustrates the PPG signal of FIG. 7A after it has
been band-passed filtered, which caused a reduction in noise due to
respiration, high frequency noise, and motion artifacts.
[0034] FIG. 7C is the same as FIG. 7B, but with R-wave markers
added as vertical dashed lines.
[0035] FIG. 7D is similar to FIG. 7C, but shows the removal of
three outlier beats.
[0036] FIG. 7E illustrates an averaged PPG signal resulting from
ensemble averaging the remaining cycles of FIG. 7D, and illustrates
various feature of the PPG signal that can be determined and used
with embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The following description is of the best modes presently
contemplated for practicing various embodiments of the present
invention. The description is not to be taken in a limiting sense
but is made merely for the purpose of describing the general
principles of the invention. The scope of the invention should be
ascertained with reference to the claims. In the description of the
invention that follows, like numerals or reference designators will
be used to refer to like parts or elements throughout. In addition,
the first digit of a reference number identifies the drawing in
which the reference number first appears.
[0038] It would be apparent to one of skill in the art that the
present invention, as described below, may be implemented in many
different embodiments of hardware, software, firmware, and/or the
entities illustrated in the figures. Any actual software, firmware
and/or hardware described herein is not limiting of the present
invention. Thus, the operation and behavior of the present
invention will be described with the understanding that
modifications and variations of the embodiments are possible, given
the level of detail presented herein.
[0039] Referring to FIG. 1A, the representative signal waveforms
therein are used to show the relative timing of electrical and
mechanical cardiac events that occur during cardiac cycles. The
upper most waveform is representative of an electrocardiogram (ECG)
or intracardiac electrogram (IEGM) signal 102 (collectively
referred to as ECG/IEGM signal 102), which is indicative of
electrical activity of the patient' heart. The following waveform
is representative of a left ventricular (LV) pressure signal 112.
The next waveform is representative of a cardiogenic impedance (CI)
signal 122, which is an impedance measurement that has been
inverted to reflect blood volume in the heart and aorta. The last
waveform is representative of a photoplethysmography (PPG) signal
or impedance plethysmography signal (IPG) 132, both of which are
indicative of changes in arterial blood volume remote from the
patient's heart. Signals 112, 122 and 132 are all indicative of
mechanical activity of a patient's heart. For example, the PPG or
IPG signal 132 (collectively referred to as PPG/IPG signal 132) is
indicative of mechanical activity of the patient's heart because
the PPG/IPG signal 132 represents changes in the flow of blood
through the vessels probed by the PPG/IPG sensor (or stated another
way, changes in arterial blood volume), which is dependent on the
mechanical activity of the heart.
[0040] Referring to the ECG/IEGM signal 102, each cycle of the
signal 102 is shown as including a P wave, a QRS complex (including
Q, R and S waves) and a T wave. The P wave is caused by
depolarization of the atria. This is followed by atrial
contraction, during which expulsion of blood from the atrium
results in further filling of the ventricle. Ventricular
depolarization, indicated by the QRS complex, initiates contraction
of the ventricles resulting in a rise in ventricular pressure until
it exceeds the pulmonary and aortic diastolic blood pressures to
result in forward flow as the blood is ejected from the ventricles.
Ventricular repolarization occurs thereafter, as indicated by the T
wave and this is associated with the onset of ventricular
relaxation in which forward flow stops from the ventricles into the
aorta and pulmonary arteries. Thereafter, the pressure in the
ventricles falls below that in the atria at which time the mitral
and tricuspid valves open to begin to passively fill the ventricles
during diastole.
[0041] An exemplary metric indicative of pulse arrival time, which
can be used to determine to estimate a patient's blood pressure, is
also shown in FIG. 1A. In general, a metric indicative of pulse
arrival time (PAT) can be determined, in accordance with
embodiments of the present invention, by determining a time from a
detected predetermined feature of a CI signal (e.g., 122) to a
detected predetermined feature of the signal indicative of changes
in arterial volume, which can be a PPG or IPG signal (e.g., 132),
but is not limited thereto. In FIG. 1A, the predetermined feature
of the CI signal is the maximum amplitude, and the predetermined
feature of the PPG/IPG signal is the maximum upward slope. In other
words, the metric indicative of PAT can be determined by
determining a time from the maximum amplitude of the CI signal 122
to the maximum upward slope of the PPG/IPG signal 132, as
illustrated in FIG. 1A. Alternatively, as illustrated in FIG. 1B,
the metric indicative of PAT can be determined by determining a
time from the maximum upward slope of the CI signal 122 to the
maximum downward slope of the PPG/IPG signal 132. These are just a
few examples, which are not meant to be limiting. Alternative
predetermined features of the CI signal can be used, as can
alternative predetermined features of the PPG/IPG signal. Examples
of other features of the CI signal and the PPG/IPG signal are
discussed below.
[0042] Referring now to FIG. 1C, the top most waveform illustrates
a ventricular pacing signal 142 that is used to pace a patient's
heart. As illustrated in FIG. 1C, where ventricular pacing pulses
are being used to pace a patient's heart, the metric indicative of
PAT can be determined by determining a time from a ventricular
pacing pulse to a maximum upward slope (or some other predetermined
feature, e.g., the maximum amplitude, the dicrotic notch) of the
PPG/IPG signal 132.
[0043] The high level flow diagram of FIG. 2A will now be used to
explain various embodiments of the present invention that can be
used to estimate a patient's arterial blood pressure. Such
embodiments can be implemented by an implantable system, examples
of which are discussed below with reference to FIGS. 3 and 4. In
FIG. 2A and the other flow diagrams described herein, the various
algorithmic steps are summarized in individual `blocks`. Such
blocks describe specific actions or decisions that are made or
carried out as the algorithm proceeds. Where a microcontroller (or
equivalent) is employed, the flow diagram presented herein provides
the basis for a `control program` that may be used by such a
microcontroller (or equivalent) to effectuate the desired control
of the implantable system. Those skilled in the art may readily
write such a control program based on the flow diagram and other
descriptions presented herein.
[0044] Referring to FIG. 2A, at steps 202 and 204, one or more
electrodes implanted within and/or on the patient's heart is/are
used to obtain a cardiogenic impedance (CI) signal indicative of
cardiac contractile activity, and an implanted sensor (e.g.,
optical sensor) or implanted electrodes remote from the patient's
heart is used to obtain a signal indicative of changes in arterial
blood volume. The signal indicative of changes in arterial blood
volume obtained at step 204 can be a PPG signal, an IPG signal, or
some other plethysmography signal. An optical sensor can be used to
obtain a PPG signal, or implanted electrodes can be used to obtain
an IPG signal.
[0045] Examples of electrodes and circuitry that can be used to
obtain a CI signal are discussed below with reference to FIG. 3-5.
In certain embodiments, multiple CI vectors can be recorded
simultaneously, and such multiple CI vectors can be combined to
provide the CI signal obtained as step 202. Such embodiments may
increase the accuracy of the cardiac cycle reference point
determined at step 206, discussed below.
[0046] Exemplary sensors that can be used to obtain a PPG signal
are discussed below with reference to FIGS. 3 and 4. Exemplary
sensors (which can include electrodes and circuitry) that can be
used to obtain an IPG signal are also discussed below. In still
other embodiments, the plethysmography signal indicative of changes
in arterial blood volume can be a signal output by a sensor
including a piezo-electric diaphragm. Alternative sensors that can
be used to produce the plethysmography signal indicative of changes
in arterial blood volume, include, but are not limited to, a close
range microphone, a sensor including a small mass on the end of a
piezo bending beam with the mass located on the surface of a small
artery, a transmission mode infrared motion sensor sensing across
the surface of a small artery, or a MEMS accelerometer located on
the surface of a small artery. Such alternative sensors can be
located, e.g., on the tip of a short lead connected to a device
that is subcutaneously implanted. The implanted sensor is
preferably extravascular, and preferably a sufficient distance from
the patient's heart such that meaningful changes in the amount of
time it takes a pulse wave originating in the heart to reach the
implanted sensor can be detected, thereby enabling changes in
arterial blood pressure to be detected. For example, it is
preferred that the implanted sensor (used to obtain the signal
indicative of changes in arterial blood volume) is at least 10 mm
from the patient's aortic root. Such a sensor can be implanted,
e.g., in the pectoral region of a patient. An alternative location
for implantation of the sensor includes, but is not limited to, the
patient's abdominal region. For the remainder of this discussion,
it will be assumed that the signal obtained at step 204 is a PPG or
IPG signal, which are collectively referred to as a PPG/IPG signal.
However, as just explained above, alternative plethysmography
signals can be used.
[0047] Still referring to FIG. 2A, at steps 206 and 208, one or
more predetermined features of the CI signal is/are detected, and
one or more predetermined features of the signal indicative of
changes in arterial blood volume (e.g., the PPG/IPG signal) is/are
detected. The predetermined feature(s) of the CI signal, detected
at step 206, can be the minimum amplitude of the CI signal, the
maximum upward slope of the CI signal, the maximum amplitude of the
CI signal, and/or the maximum downward slope of the CI signal, but
is not limited thereto. The predetermined feature(s) of the PPG/IPG
signal, detected at step 208, can be the minimum amplitude of the
PPG/IPG signal, the maximum upward slope of the PPG/IPG signal, the
maximum amplitude of the PPG/IPG signal, the dicrotic notch of the
PPG/IPG signal, the maximum downward slope of the PPG/IPG signal
prior to the dicrotic notch, and/or the maximum downward slope of
the PPG/IPG signal after the dicrotic notch, but is not limited
thereto.
[0048] At step 210, one or more metrics indicative of pulse arrival
time (PAT) is/are determined, where each metric indicative of PAT
is determined by determining a time from one of the detected
features of the CI signal to one of the detected features of the
signal (e.g., PPG/IPG signal) indicative of changes in arterial
blood volume remote from the patient's heart. Exemplary metrics
indicative of PAT that can be determined at step 210 were discussed
above with reference to FIGS. 1A and 1B, but embodiments of the
present invention are not limited to such examples.
[0049] At step 212, the patient's arterial blood pressure is
estimated based on at least one determined metric indicative of
PAT. Various arterial blood pressure measurements can be estimated,
including systolic blood pressure (SBP), diastolic blood pressure
(DBP), pulse pressure (PP) and/or mean arterial blood pressure
(MAP). The SBP is the peak pressure in the arteries, which occurs
near the beginning of a cardiac cycle. The DBP is the lowest
pressure in the arteries, which occurs at the end of the diastolic
phase of the arterial circulation. This corresponds to the end of
the filling phase of the cardiac cycle with respect to ventricular
function. The PP is the difference between the systolic and
diastolic blood pressures. The MAP is a weighted average of
arterial blood pressure throughout the cardiac cycle.
[0050] Because implanted electrodes, and in certain embodiments an
implanted sensor, are used to obtain the various arterial blood
pressure estimates, a patient's arterial blood pressure can be
monitored on a chronic basis. Thus, arterial blood pressure can be
tracked to monitor a patient's evolving cardiac disease state, and
to trigger alerts (e.g., in response to which a patient may take
blood pressure medication). Additionally, arterial blood pressure
measurements can be used as a measure of a patient's hemodynamic
function.
[0051] Embodiments of the present invention use the concept of
pulse arrival time (PAT), also known as pulse transmit time (PTT),
or pulse wave velocity (PWV) to monitor arterial blood pressure.
However, embodiments of the present invention differ from prior art
non-implanted systems that rely on pulse arrival time. For example,
most such prior are systems are not practical for chronic use.
Further, unlike prior art systems, specific embodiments of the
present invention utilize a CI signal to determine PATs, which is
believed to be advantageous because the CI signal is not affected
by variations in pre-ejection periods that result from changes in
electromechanical coupling, heart failure and/or mitral valve
regurgitation. Thus, use of a CI signal to determine metrics
indicative of PAT may be superior to techniques that rely on an
IEGM or ECG signal to determine PATs, since during periods of heart
failure and mitral valve regurgitation the variations in
pre-ejection periods can affect IEGM and ECG signals in a manner
that may reduce that accuracy of blood pressure estimates based
thereon.
[0052] In accordance with certain embodiments, at step 212, one or
more values indicative of SBP, DBP, PP and/or MAP is/are determined
based on one or more metrics indicative of PAT (each also referred
to as a "PAT metric"), using an equation, model, lookup table, or
the like. Some exemplary equations and models that can be used to
estimate such arterial blood pressure measurements are discussed
below.
[0053] PAT generally has a negative correlation with SBP, in that
the greater the PAT the lower the SBP, and the lower the PAT the
greater the SBP. In a simplest embodiment based on a linear
approximation, the equation SBP.apprxeq.1/(PAT metric) can be used
to estimate SBP. In another embodiment, one or more patient
specific correlation factor (e.g., a constant K) is used when
estimating SBP at step 212. For example, the equation
SBP.apprxeq.K/(PAT metric) can be used to estimate SBP at step 212,
where K is a patient specific correlation factor determined during
a calibration procedure, an example of which is discussed below.
Non-linear approximations can also be used to estimate SBP, e.g.,
SBP.apprxeq.K/(PAT metric).sup.2, or SBP.apprxeq.K1/(PAT
metric).sup.2+K2/(PAT metric), with K, or K1 and K2 being patient
specific correlation factor(s) determined during a calibration
procedure. Alternative equations could be SBP.apprxeq.K/(PAT
metric)+.beta., SBP.apprxeq.K/(PAT metric).sup.2+.beta., or
SBP.apprxeq.K1/(PAT metric).sup.2+K2/(PAT metric+.beta.. These are
just a few exemplary equations that can be used to estimate SBP,
which are not meant to be limiting. Similar equations can be
provided for estimating DBP, PP and/or MAP based on PAT metrics. In
certain embodiments, one or more look-up table, which may be
calibrated for a patient, can be used to estimate arterial blood
pressure based on one or more PAT metric.
[0054] Multiple different metrics indicative of PAT can be
determined, and used in an equation to estimate one or more values
indicative of SBP, DBP, PP and/or MAP. Metrics indicative of
morphological features of the PPG or IPG signal indicative of the
changes in arterial blood volume, such as area under the curve,
peak-to-peak amplitude (a.sub.1 shown in FIGS. 1A and 1B) and/or
full width at half max (FWHM) of a PPG or IPG signal can also be
used in such equations. For example, the equation
SBP.apprxeq.K/(PAT metric)+.beta.*(peak-to-peak amplitude of
PPG/IPG curve) can be used, with K and .beta. being patient
specific correlation factors determined during a calibration
procedure. One of ordinary skill in the art reading this disclosure
would realize that various alternative linear or non-linear
equations can be used to estimate SBP, DBP, PP and/or MAP that are
within the scope of the present invention. Such equations can be
based on one or more metrics indicative of PAT, and optionally also
based on one or more metrics indicative of morphological features
of the signal indicative of changes in arterial blood volume.
[0055] In specific embodiments, a regression model and/or other
mathematical models and equations can be used to estimate SBP, DBP,
PP and/or MAP, using one or more metrics indicative of PAT,
optionally one or more metrics indicative of morphological features
of the signal indicative of changes in arterial blood volume, as
well as actual measures of SBP, DBP, PP and/or MAP obtained during
a calibration procedure. For example, a blood pressure estimation
model can combine one or more PAT metrics and one or more
morphological features. Such models can include nonlinear terms,
such as polynomial or exponential factors. In one embodiment, a
model uses multiple linear regression to estimate blood pressure. A
model may also use partial least squares or principal components
analysis. Such models can be calibrated using one or more reference
blood pressure measurements, some examples of which are discussed
below. Thereafter, as a patient's arterial blood pressure evolves
over time, changes in PAT metrics and optionally also morphological
features (e.g., peak-to-peak amplitude) derived from a
plethysmography signal are used to update the estimates of arterial
blood pressure.
[0056] Where multiple metrics indicative of PAT are determined,
such metrics can be combined, e.g., by determining a simple or
weighted average of the metrics. Alternatively, multiple metrics
indicative of PAT can be used in the equations or models used to
estimate SBP, DBP, PP and/or MAP. For example, DBP can be estimated
using the equation DBP.apprxeq.K1/(time from max amplitude of CI
signal to max upward slope of PPG or IPG signal)-K2/(time from max
amplitude of CI signal to max downward slope of PPG or IPG
signal)+.beta.. In this equation, one metric indicative of PAT is
the time from max amplitude of CI signal to max upward slope of PPG
or IPG signal, and another metric indicative of PAT is the time
from max amplitude of CI signal to max downward slope of PPG or IPG
signal.
[0057] PP can be can be determined by determining the difference
between SBP and DBP, and MAP can be determined by determining a
weighted average of SBP and DBP (e.g., MAP=1/3 SBP+2/3 DBP).
Alternatively, one or more equations, models or tables can be used
to estimate PP and/or MAP based on one or more of the various
metrics indicative of PAT described above, without first estimating
SBP and/or DBP.
[0058] An exemplary calibration procedure (performed at implant
and/or thereafter) will now be explained. During the calibration
procedure, actual measures of arterial blood pressure (including
SBP, DBP, PP and/or MAP) are measured using any known accurate
acute technique, and one or more metric indicative of PAT (i.e.,
PAT metric(s)) are measured in a manner described above using an
implanted device. Optionally, metrics indicative of morphological
features of the signal indicative of changes in arterial blood
volume, such as a peak-to-peak amplitude a.sub.1 of the PPG/IPG
signal, can also be determined by the implanted device. The actual
measure(s) of the patient's SBP, DBP, PP and/or MAP can be
obtained, e.g., using a non-invasive auscultatory or oscillometric
techniques, or an invasive intravascular cannula method, or any
other acute technique. For a more specific example, actual arterial
blood pressure measurements (e.g., SBP and DBP) can be measured
using a high fidelity micronometer-tipped pressure catheter (e.g.,
model 4F, SPC-120, available from Millar Instruments, Texas), which
is placed in the ascending aorta via a carotid arteriotomy.
[0059] Based on the actual measures of arterial blood pressure, and
the metrics indicative of PAT determined using the techniques
described above (and optionally also metrics indicative of
morphological features of the signal indicative of changes in
arterial blood volume, such as a peak-to-peak amplitude a.sub.1 of
the PPG/IPG signal), various patient specific correlation factors
(e.g., K and .beta.) can be calculated by an external programmer,
or the like, e.g., using regression models and/or linear or
non-linear equations. The patient could also be asked to exercise,
or could be appropriately paced, to change the patient's arterial
blood pressure, to thereby check the accuracy of the patient
correlation factor(s) over a range of SBPs, DBPs, PPs and/or MAPs
and PAT metrics. If appropriate, the patient correlation factor(s)
can be adjusted so that such factor(s) is/are accurate over a range
of systolic blood pressures. Presuming a metric indicative of PAT
is measured in msec, the units of the patient specific correlation
factor(s) can be, e.g., mmHgmsec or mmHgmsec.sup.2, so that when
multiplied by 1/(PAT metric) or 1/(PAT metric).sup.2, the resulting
estimate of arterial blood pressure has units of mmHg. Use of
alternative linear and non-linear equations, look up tables and
interpolation are also within the scope of the present invention.
After appropriate equations, models and/or look-up tables and one
or more patient specific correlations factors (determined during
calibration) are programmed into an implanted device, the implanted
device can determine estimates of arterial blood pressure (e.g.,
SBP, DBP, PP and/or MAP) in real time based on one or more PAT
metrics as determined by the implanted device in real time.
[0060] In some embodiments, an IEGM signal can also be obtained,
along the CI signal obtained at step 202 and the signal indicative
of changes in arterial blood volume obtained at step 204. In such
embodiments, by detecting R-waves of the IEGM signal (in any well
known manner), R-wave markers can be used to increase the accuracy
of detecting the pre-determined feature(s) of the CI signal at step
206. For example, if it is known that the maximum of the CI signal
should occur within a certain window that will typically occur
between X and Y msec following an R-wave, then the maximum of the
CI signal may only be looked for during that window. Such use of an
IEGM signal may increase the accuracy of arterial blood pressure
estimates.
[0061] Alternative embodiments of the present invention, for
monitoring a patient's arterial blood pressure, will now be
described with reference to the high level flow diagram of FIG. 2B.
The embodiments described with reference to FIG. 2B are
specifically for use when a patient's heart is being paced.
Referring to FIG. 2B, at a step 201, paced cardiac events are
caused by pacing the patient's heart, using at least one electrode
implanted within or on the patient's heart, with a voltage
sufficient to cause capture. An exemplary pacemaker and leads that
can be used to perform step 201 are described below with reference
to FIGS. 3 and 4.
[0062] At step 204', an implanted sensor or implanted electrodes,
remote from the patient's heart, is/are used to obtain a signal
indicative of changes in arterial blood volume remote from the
patient's heart while the patient's heart is being paced with the
voltage sufficient to cause capture. Step 204' is similar to step
204 described above, except that at step 204' the obtained signal
can be indicative of changes in arterial blood volume while the
patient's heart is being paced, and at step 204 the obtained signal
can be indicative of changes in arterial blood volume during
intrinsic and/or paced beating of the patient's heart. Accordingly,
step 204' need not be explained in further detail.
[0063] At step 208', one or more predetermined features of the
signal indicative of changes in arterial blood volume obtained at
step 204' is/are determined. At step 210', one or more metrics
indicative of pulse arrival time (PAT) is/are determined by
determining a time from a paced cardiac event caused at step 201 to
one or more of the predetermined features of the signal indicative
of changes in arterial blood volume detected at step 208'. At step
212', one or more estimate of the patient's arterial blood pressure
is determined based on at least one of the one or more metrics
indicative of PAT. Steps 208', 210' and 212' are similar to step
208, 210 and 212 discussed above, and thus additional details of
these steps can be understood from the discussion above.
[0064] In accordance with certain embodiments, there is a
determination of a value indicative of electromechanical delay
(EMD) between delivery of pacing caused at step 201 and a
mechanical cardiac contraction resulting from the pacing. In such
embodiments, the metric(s) indicative of PAT determined at step
210' can also be determined based on the value indicative of EMD.
For example, step 210' can be accomplished by determining a time
from a paced cardiac event, caused at step 201, to a predetermined
feature of the signal indicative of changes in arterial blood
volume, detected at step 208', minus the determined value
indicative of EMD. Additional details of how to determine a value
indicative of a patient's EMD are discussed below with reference to
FIG. 2C. It may be desirable to minimize the variance of EMD to
improve a patient's HF condition. Accordingly, in certain
embodiments, the pacing rate caused at step 201 can be adjusted to
minimize variance of the value indicative of electromechanical
delay (EMD).
[0065] In accordance with an embodiment, values indicative of SBP,
DBP, PP and/or MAP, and potentially other information, are stored
within memory of the implantable system for later analysis within
the device and/or for later transmission to an external device.
Such an external device (e.g., an external programmer or external
monitor) can then be used to analyze such data.
[0066] Embodiments of the present invention are not limited to the
exact order and/or boundaries of the steps shown in FIGS. 2A and
2B. In fact, many of the steps can be performed in a different
order than shown, and many steps can be combined, or separated into
multiple steps. For another example, certain steps shown in the
FIGS. can be separated into two or more steps. The only time order
is important is where a step acts on the results of a previous
step.
[0067] In accordance with specific embodiments of the present
invention, an alarm can be triggered based on comparisons of the
values indicative of SBP, DBP, PP and/or MAP to corresponding
thresholds, and/or based on comparisons of changes in values
indicative of SBP, DBP, PP and/or MAP to corresponding thresholds.
Such an alarm can be part of an implanted system. Alternatively, an
implanted system can trigger a non-implanted alarm of a
non-implanted system. In still other embodiments, where arterial
pulse pressure information is transmitted (e.g., via telemetry) to
an external device, a non-implanted alarm can be triggered.
[0068] In accordance with specific embodiments of the present
invention, the method described with reference to FIG. 2A or 2B can
be repeated from time-to-time, to thereby track changes in SBP,
DBP, PP and/or MAP. For example, steps 202-212 can be performed
periodically (e.g., once a minute, hour, day, week, or the like).
The values indicative of SBP, DBP, PP and/or MAP can be compared in
real time to corresponding thresholds. Alternatively, or
additionally, values indicative of SBP, DBP, PP and/or MAP can be
stored in memory of the implanted system. Such stored values can be
analyzed by the implanted system and/or transmitted (e.g., via
telemetry) to an external system (e.g., external programmer and
external monitor) and analyzed by the external system. Use of
various thresholds can be used to trigger alarms and/or therapy, as
will be described below.
[0069] Depending on the frequency, periodic monitoring of arterial
blood pressure may be costly in terms of energy, memory and/or
processing resources. Accordingly, it may be more efficient to
trigger the performance of certain steps upon detection of an
event, such as a specific activity, or lack thereof, and/or a
specific posture of the patient. For example, an activity sensor
and/or posture sensor (e.g., sensor 415 in FIG. 4) can be used to
trigger the performance of steps of FIG. 2A or 2B. For example, the
steps of FIG. 2A or 2B can be triggered when it is detected that a
patient is inactive and lying down. Additionally, or alternatively,
such steps can be triggered when a patient is upright and walking.
In still other embodiments, such steps can be triggered to occur,
at specific intervals following a patient changing their posture
(e.g., assuming an upright posture, or lying down) and/or activity
level. For example, following a triggering event, values of
arterial blood pressure can be determined once a minute for 10
minutes, or at 1 minute, 2 minutes, 5 minutes and 10 minutes after
the triggering event. Of course, other variations are also
possible, and within the scope of the present invention. It may
also be that one or more specific step is performed substantially
continually, but other steps are only performed in response to a
triggering event or on demand.
[0070] It is normal for there to be a normal circadian variation in
arterial blood pressure values, including SBP, DBP, PP and MAP
values. For example, a drop in such values when a patient is
sleeping, at rest and/or supine is normal. However, a drop in such
values when a patient is active, or upright, or within a short
period of a patient assuming an upright posture, is abnormal.
Implanted activity and/or posture sensors (e.g., sensor 415 in FIG.
4) can thus be used to assist in defining when an alarm or the like
should be triggered. For example, a posture sensor can be used to
trigger the monitoring of arterial blood pressure values when a
patient assumes an upright posture. In this manner, such monitoring
can be used to determine whether a drop in blood pressure within a
specific amount of time (e.g., 10 minutes), following the patient
assuming of an upright position, exceeds a specified threshold.
Such a threshold can be, e.g., an absolute value or a percentage.
In specific embodiments, the SBP, DBP, PP and/or MAP thresholds to
which determined SBP, DBP, PP and/or MAP values are compared can be
based on the activity and/or posture of the patient.
[0071] Where at least some of steps of FIG. 2A or 2B are triggered
in response to detection of various different activity and/or
posture states, information about the patient's activity and/or
posture can also be stored along with the arterial blood pressure
information, so that such information can be correlated. In other
words, there could be a cross-correlation of arterial blood
pressure values with levels of activity and/or posture.
[0072] Accordingly, embodiments of the present invention can be
used to determine, or assist with the determination of, whether
there is a correlation between levels of arterial blood pressure,
levels of activity and/or posture, and myocardial ischemic episodes
experienced by a patient. Such information will enable a medical
practitioner to analyze whether ischemic episodes that the patient
experienced may have precipitated changes in arterial blood
pressure, posture and/or activity.
[0073] In accordance with specific embodiments of the present
invention, measures of arterial blood pressure, including values
indicative of SBP, DBP, PP and/or MAP can be stored so that a
physician or clinician can upload such measurements when visiting
the physician or clinician.
[0074] More generally, measures of arterial blood pressure,
obtained in accordance with embodiments of the present invention
can be used to assess the hemodynamic status of a patient. This can
include tracking a patient's cardiac disease state, including but
not limited to, heart failure. For example, deviations from a
baseline beyond a threshold in measures of arterial blood pressure
over time can be interpreted as a worsening of a heart failure
condition.
[0075] FIG. 2C will now be used to describe a method for monitoring
a patient's electromechanical delay (EMD). At step 201' (which is
similar to step 201 discussed with reference to FIG. 2B), paced
cardiac events are cause by pacing the patient's heart, using
implanted electrodes, with a voltage sufficient to cause capture.
At step 202', implanted electrodes are used to obtain a cardiogenic
impedance (CI) signal indicative of cardiac contractile activity
while the patient's heart is being paced at step 201'. Step 202' is
similar to step 202 described above with reference to FIG. 2A,
except that the patient's heart is definitely being paced at step
202'. At step 206', in a similar manner as was discussed above with
reference to step 206 in FIG. 2A, one or more predetermined
features of the CI signal is detected.
[0076] Still referring to FIG. 2C, at step 214, one or more values
indicative of the patient's EMD is/are determined by determining a
time between pacing stimulation delivered at step 201' and one or
more features of the CI signal detected at step 206'. As indicated
by line 215, steps 201', 202', 206' and 214 are repeated from time
to time (e.g., periodically, aperiodically, in response to a
triggering event, etc.), with one or more values indicative of the
patient's EMD determined each time. As indicated at step 216,
changes in the patient's EMD can be monitored based on changes in
at least one of the one or more values indicative EMD determined at
step 214. For example, increases in a value indicative of EMD can
be indicative of increases in EMD, and vise versa. This technique
can be used, e.g., to monitor a patient's HF condition based on
changes in the patient's EMD. For example, it is expected that a
patient's EMD will increase as the patient's HF condition worsens,
and will decrease if the patient's HF condition improves.
[0077] Additionally, or alternatively, the pacing can be adjusted
in an attempt to reduce (and preferably minimize) variance of one
or more values indicative of EMD, which is believed to improve an
HF condition. Examples of pacing parameters that can be adjusted in
an attempt to reduce the variance of one or more values indicative
of EMD include, but are not limited to, pacing rate,
atrio-ventricular delay, interventricular delay and interatrial
delay.
[0078] EMD is believed to be affected by conduction abnormalities,
myocardial contractility and cardiac diseases, including, but not
limited to mitral stenosis, hypertension, and as mentioned above,
HF. Accordingly, monitoring changes in EMD can also be useful for
monitoring changes in conduction abnormalities, myocardial
contractility and cardiac diseases.
[0079] As the term is being used herein, EMD is synonymous with
pre-ejection period (PEP). Thus, a patient's PEP can be monitored
using the embodiments of the present invention described above.
Exemplary Implantable System
[0080] FIGS. 3 and 4 will now be used to describe an exemplary
implantable system that can be used to implement embodiments of the
present invention including but not limited to monitoring a
patient's arterial blood pressure, monitoring a patient's EMD
and/or monitoring a patient's HF condition. Referring to FIG. 3,
the implantable system is shown as including an implantable
stimulation device 310, which can be a pacing device and/or an
implantable cardioverter defibrillator. The device 310 is shown as
being in electrical communication with a patient's heart 312 by way
of three leads, 320, 324 and 330, which can be suitable for
delivering multi-chamber stimulation and shock therapy. The leads
can also be used to obtain CI, IEGM and/or IPG signals, for use in
embodiments of the present invention. As described below, it is
also possible that one of these leads (or another lead) can include
an optical sensor (also referred to as a PPG sensor) that is useful
for obtaining a PPG signal, similar to signal 122 shown in FIG.
1.
[0081] In FIG. 3, the implantable device 310 is shown as having a
PPG sensor 303 (also referred to as an optical sensor) attached to
its housing 340. The PPG sensor 303, which can be used to obtain a
PPG signal similar to signal 122 shown in FIG. 1, includes a light
source 305 and a light detector 307. The light source 305 can
include, e.g., at least one light-emitting diode (LED),
incandescent lamp or laser diode, but is not limited thereto. The
light detector 307 can include, e.g., at least one photoresistor,
photodiode, phototransistor, photodarlington or avalanche
photodiode, but is not limited thereto. Light detectors are often
also referred to as photodetectors or photocells.
[0082] The light source 305 outputs light that is reflected or
backscattered by surrounding patient tissue, and
reflected/backscattered light is received by the light detector
307. In this manner, changes in reflected light intensity are
detected by the light detector, which outputs a signal indicative
of the changes in detected light. The output of the light detector
can be filtered and amplified. The signal can also be converted to
a digital signal using an analog to digital converter, if the PPG
signal is to be analyzed in the digital domain. A PPG sensor can
use a single wavelength of light, or a broad spectrum of many
wavelengths. Additional details of exemplary implantable PPG
sensors are disclosed in U.S. Pat. Nos. 6,409,675 and 6,491,639,
both entitled "Extravascular Hemodynamic Sensor" (both Turcott),
which are incorporated herein by reference.
[0083] It is generally the output of the photodetector that is used
to produce a PPG signal. However, there exist techniques where the
output of the photodetector is maintained relatively constant by
modulating the drive signal used to drive the light source, in
which case the PPG signal is produced using the drive signal, as
explained in U.S. Pat. No. 6,731,967, entitled "Methods and Devices
for Vascular Plethysmography via Modulation of Source Intensity,"
(Turcott), which is incorporated herein by reference.
[0084] The PPG sensor 302 can be attached to a housing 340 of an
implantable device, which as mentioned above can be, e.g., a
pacemaker and/or an implantable cardioverter-defibrillator (ICD),
or a simple monitoring device. Exemplary details of how to attach a
sensor module to an implantable cardiac stimulation device are
described in U.S. patent application Ser. No. 10/913,942, entitled
"Autonomous Sensor Modules for Patient Monitoring" (Turcott et
al.), filed Aug. 4, 2004 (Attorney Docket No. A04P3019-US1), which
is incorporated herein by reference. It is also possible that the
PPG sensor 302 be integrally part of the implantable cardiac
stimulation device 310. For example, the PPG sensor 302 can be
located within the housing 340 of an ICD (and/or pacemaker) that
has a window through which light can be transmitted and detected.
In a specific embodiment, the PPG sensor 302 has a titanium frame
with a light transparent quartz or sapphire window that can be
welded into a corresponding slot cut in the housing of the ICD.
This will insure that the ICD enclosure with the welded PPG sensor
will maintain a hermetic condition.
[0085] Where the PPG sensor is incorporated into or attached to a
chronically implantable device 310, the light source 305 and the
light detector 307 can be mounted adjacent to one another on the
housing or header of the implantable device, or on the bottom of
the device, or at any other location. The light source 305 and the
light detector 307 can be placed on the side of the implantable
device 310 that, following implantation, faces the chest wall, and
are configured such that light cannot pass directly from the source
to the detector. The placement on the side of the device 310 that
faces the chest wall maximizes the signal to noise ratio by
directing the signal toward the highly vascularized musculature,
and shielding the source and detector from ambient light that
enters the body through the skin. Alternatively, at the risk of
increasing susceptibility to ambient light, the light source 305
and the light detector 307 can be placed on the face of the device
310 that faces the skin of the patient. Other variations are also
possible.
[0086] In an alternative embodiment, the PPG sensor 303 (or other
plethysmography sensor) can be is remote from the housing 340 of
the device 310, but communicates with the electronics in the device
housing 340 via one or more wires, optical fibers, or wirelessly
(e.g., using telemetry, RF signals and/or using body fluid as a
communication bus medium). This embodiment enables an obtained PPG
signal to be indicative of changes in arterial blood volume at a
location remote from the patient's heart, where such location is
also remote from the device housing 340. If desired, multiple PPG
signals can be obtained, e.g., using multiple PPG sensors at
different locations.
[0087] In another embodiment, optical fibers can be used to
transmit light into and detect light from tissue that is remote
from the device housing, even though the light source and light
detector are located within or adjacent the device housing 140.
This embodiment enables an obtained PPG signal to be indicative of
changes in arterial blood volume at a location remote from the
patient's heart, where such location is remote from the device
housing 140, even though the light source 105 and light detector
107 are not remote from the housing. The distal end of the optical
fiber(s) associated with the light source can be generally parallel
to the distal end of the optical fiber(s) associated with the light
detector, so that the light detector detects the portion of light
reflected from tissue. Alternatively, the distal end of the optical
fiber(s) associated with the light source can generally face the
distal end of the optical fiber(s) associated with the light
detector, with tissue therebetween, so that the light detector
detects the portion of light transmitted through (as opposed to
reflected from) the tissue therebetween.
[0088] In an embodiment, a PPG sensor can be within or attached to
a lead that may extend from a main device housing 140. Accordingly,
in this embodiment, a housing of the sensor module is sized to fit
within the implantable lead. For example, the PPG can be located
proximal from the distal tip of the lead so that the PPG sensor is
sufficiently remote from the heart that variations in pulse
transmission time are detectable and meaningful. The portion of the
lead that is adjacent to a window of the PPG sensor module, where
light is to exit and enter, should allow the light to pass in and
out of the sensor. Thus, the lead may be transparent, or include
its own window, opening, or the like. The lead can including tines
for attaching the lead in its desired position, but may include any
other type of fixation means (e.g., a pigtail shaped fixation
means), or none at all. The lead can also have a suture sleeve,
that enables the lead to be sutured to patient tissue. Additional
details of a lead that includes an optical sensor that can be used
to produce a PPG signal are provided in U.S. patent application
Ser. No. 11/231,555, entitled "Improved Multi-Wavelength
Implantable Oximeter Sensor" (Poore), filed Sep. 20, 2005 (Attorney
Docket No. A05P1078), and U.S. patent application Ser. No.
11/282,198, entitled "Implantable Device with a Calibration
Photodetector" (Poore), filed Nov. 17, 2005 (Attorney Docket No.
AO5P1078US01).
[0089] The implantable PPG sensor 303 obtains a PPG signal that
after filtering is similar to signal 122 shown in FIG. 1, that
pulsates over the cardiac cycle. Modulation of the signal occurs
because arteries distend as the pressure wave created by the
heart's pumping mechanism reaches the sensor site. Such a signal
can be filtered and/or amplified as appropriate, e.g., to remove
respiratory affects on the signal, and the like. Additionally, the
signal can be digitized using an analog to digital converter.
Exemplary techniques for performing filtering and other processing
of a PPG signal (or other plethysmography signal) are explained
with reference to FIGS. 6 and 7A-7E.
[0090] For much of above description, it has been assumed that the
plethysmography sensor used to produce a plethysmography signal is
a PPG sensor. Thus, the plethysmography signal has often been
referred to as a PPG signal. However, it should be noted that other
types of plethysmography sensors can alternatively be used. Thus,
embodiments of the present invention should not be limited to use
with PPG sensors and PPG signals. Further, as mentioned above,
electrodes of the various leads can be used to obtain an IPG
signal, and the IPG signal can be used in place of the PPG
signal.
[0091] In specific embodiments, the plethysmography signal can be
produced using non-radiant methods and devices, including, but not
limited to mechanical strain, electrical impedance, or pressure.
More specifically, rather than using a PPG sensor that includes a
light source and detector, the implanted plethysmography sensor can
include a strain gauge, a linear displacement sensor, or an
ultrasound transducer, each of which is known in the art.
Alternatively, an impedance plethysmography sensor, which is also
known in the art, can be used. Details of exemplary implantable
sensors that produce an impedance plethysmography signals are
disclosed, e.g., in U.S. Pat. Nos. 4,674,518, 4,686,987 and
5,334,222 (all to Salo), which are incorporated herein by
reference.
[0092] Still referring to FIG. 3, to sense atrial cardiac signals
and to provide right atrial chamber stimulation therapy, the device
310 is coupled to an implantable right atrial lead 320 having at
least an atrial tip electrode 322, which typically is implanted in
the patient's right atrial appendage. To sense left atrial and
ventricular cardiac signals and to provide left-chamber pacing
therapy, the device 310 is coupled to a "coronary sinus" lead 324
designed for placement in the "coronary sinus region" via the
coronary sinus for positioning a distal electrode adjacent to the
left ventricle and/or additional electrode(s) adjacent to the left
atrium. As used herein, the phrase "coronary sinus region" refers
to the vasculature of the left ventricle, including any portion of
the coronary sinus, great cardiac vein, left marginal vein, left
posterior ventricular vein, middle cardiac vein, and/or small
cardiac vein or any other cardiac vein accessible by the coronary
sinus.
[0093] Accordingly, an exemplary coronary sinus lead 324 is
designed to receive atrial and ventricular cardiac signals and to
deliver left ventricular pacing therapy using at least a left
ventricular tip electrode 326, left atrial pacing therapy using at
least a left atrial ring electrode 327, and shocking therapy using
at least a left atrial coil electrode 328.
[0094] The device 310 is also shown in electrical communication
with the patient's heart 312 by way of an implantable right
ventricular lead 330 having, in this embodiment, a right
ventricular tip electrode 332, a right ventricular ring electrode
334, a right ventricular (RV) coil electrode 336, and an SVC coil
electrode 338. Typically, the right ventricular lead 330 is
transvenously inserted into the heart 312 so as to place the right
ventricular tip electrode 332 in the right ventricular apex so that
the RV coil electrode 336 will be positioned in the right ventricle
and the SVC coil electrode 338 will be positioned in the superior
vena cava. Accordingly, the right ventricular lead 330 is capable
of receiving cardiac signals and delivering stimulation in the form
of pacing and shock therapy to the right ventricle.
[0095] FIG. 4 will now be used to provide some exemplary details of
the components of the implantable devices 310. Referring now to
FIG. 4, the implantable devices 310, and alternative versions
thereof, can include a microcontroller 460. As is well known in the
art, the microcontroller 460 typically includes a microprocessor,
or equivalent control circuitry, and can further include RAM and/or
ROM memory, logic and timing circuitry, state machine circuitry
and/or I/O circuitry. Typically, the microcontroller 460 includes
the ability to process or monitor input signals (data) as
controlled by a program code stored in a designated block of
memory. The details of the design of the microcontroller 460 are
not critical to the present invention. Rather, any suitable
microcontroller 460 can be used to carry out the functions
described herein. The use of microprocessor-based control circuits
for performing timing and data analysis functions are well known in
the art. In specific embodiments of the present invention, the
microcontroller 460 performs some or all of the steps associated
with determining estimates of SBP, DBP, PP, MAP, EMD and/or HF.
Additionally, the microcontroller 460 may detect arrhythmias, and
select and control delivery of anti-arrhythmia therapy.
[0096] Representative types of control circuitry that may be used
with embodiments of the present invention include the
microprocessor-based control system of U.S. Pat. No. 4,940,052
(Mann et. al.) and the state-machines of U.S. Pat. No. 4,712,555
(Sholder) and U.S. Pat. No. 4,944,298 (Sholder). For a more
detailed description of the various timing intervals used within
the pacing device and their inter-relationship, see U.S. Pat. No.
4,788,980 (Mann et. al.). The '052, '555, '298 and '980 patents are
incorporated herein by reference.
[0097] Depending on implementation, the device 310 can be capable
of treating both fast and slow arrhythmias with stimulation
therapy, including pacing, cardioversion and defibrillation
stimulation. While a particular multi-chamber device is shown, this
is for illustration purposes only, and one of skill in the art
could readily duplicate, eliminate or disable the appropriate
circuitry in any desired combination to provide a device capable of
treating the appropriate chamber(s) with pacing, cardioversion and
defibrillation stimulation. For example, if the implantable device
is a monitor that does not provide any therapy, it is clear that
many of the blocks shown may be eliminated.
[0098] The housing 340, shown schematically in FIG. 4, is often
referred to as the "can", "case" or "case electrode" and may be
programmably selected to act as the return electrode for all
"unipolar" modes. The housing 340 may further be used as a return
electrode alone or in combination with one or more of the coil
electrodes, 128, 136 and 138, for shocking purposes. The housing
340 can further include a connector (not shown) having a plurality
of terminals, 442, 444, 446, 448, 452, 454, 456, and 458 (shown
schematically and, for convenience, the names of the electrodes to
which they are connected are shown next to the terminals). As such,
to achieve right atrial sensing and pacing, the connector includes
at least a right atrial tip terminal (A.sub.R TIP) 442 adapted for
connection to the atrial tip electrode 322.
[0099] To achieve left atrial and ventricular sensing, pacing and
shocking, the connector includes at least a left ventricular tip
terminal (V.sub.L TIP) 444, a left atrial ring terminal (A.sub.L
RING) 446, and a left atrial shocking terminal (A.sub.L COIL) 448,
which are adapted for connection to the left ventricular tip
electrode 326, the left atrial ring electrode 327, and the left
atrial coil electrode 328, respectively.
[0100] To support right ventricle sensing, pacing and shocking, the
connector further includes a right ventricular tip terminal
(V.sub.R TIP) 452, a right ventricular ring terminal (V.sub.R RING)
454, a right ventricular shocking terminal (R.sub.V COIL) 456, and
an SVC shocking terminal (SVC COIL) 458, which are adapted for
connection to the right ventricular tip electrode 332, right
ventricular ring electrode 334, the RV coil electrode 336, and the
SVC coil electrode 338, respectively.
[0101] An atrial pulse generator 470 and a ventricular pulse
generator 472 generate pacing stimulation pulses for delivery by
the right atrial lead 320, the right ventricular lead 330, and/or
the coronary sinus lead 324 via an electrode configuration switch
474. It is understood that in order to provide stimulation therapy
in each of the four chambers of the heart, the atrial and
ventricular pulse generators, 470 and 472, may include dedicated,
independent pulse generators, multiplexed pulse generators, or
shared pulse generators. The pulse generators, 470 and 472, are
controlled by the microcontroller 460 via appropriate control
signals, 476 and 478, respectively, to trigger or inhibit the
stimulation pulses.
[0102] The microcontroller 460 further includes timing control
circuitry 479 which is used to control pacing parameters (e.g., the
timing of stimulation pulses) as well as to keep track of the
timing of refractory periods, noise detection windows, evoked
response windows, alert intervals, marker channel timing, etc.,
which is well known in the art. Examples of pacing parameters
include, but are not limited to, atrio-ventricular delay,
interventricular delay and interatrial delay.
[0103] The switch bank 474 includes a plurality of switches for
connecting the desired electrodes to the appropriate I/O circuits,
thereby providing complete electrode programmability. Accordingly,
the switch 474, in response to a control signal 480 from the
microcontroller 460, determines the polarity of the stimulation
pulses (e.g., unipolar, bipolar, etc.) by selectively closing the
appropriate combination of switches (not shown) as is known in the
art.
[0104] Atrial sensing circuits 482 and ventricular sensing circuits
484 may also be selectively coupled to the right atrial lead 320,
coronary sinus lead 324, and the right ventricular lead 330,
through the switch 474 for detecting the presence of cardiac
activity in each of the four chambers of the heart. Accordingly,
the atrial (ATR. SENSE) and ventricular (VTR. SENSE) sensing
circuits, 482 and 484, may include dedicated sense amplifiers,
multiplexed amplifiers, or shared amplifiers. The switch 474
determines the "sensing polarity" of the cardiac signal by
selectively closing the appropriate switches, as is also known in
the art. In this way, the clinician may program the sensing
polarity independent of the stimulation polarity.
[0105] Each sensing circuit, 482 and 484, preferably employs one or
more low power, precision amplifiers with programmable gain and/or
automatic gain control, band-pass filtering, and a threshold
detection circuit, as known in the art, to selectively sense the
cardiac signal of interest. The automatic gain control enables the
device 310 to deal effectively with the difficult problem of
sensing the low amplitude signal characteristics of atrial or
ventricular fibrillation. Such sensing circuits, 482 and 484, can
be used to determine cardiac performance values used in the present
invention. Alternatively, an automatic sensitivity control circuit
may be used to effectively deal with signals of varying
amplitude.
[0106] The outputs of the atrial and ventricular sensing circuits,
482 and 484, are connected to the microcontroller 460 which, in
turn, are able to trigger or inhibit the atrial and ventricular
pulse generators, 470 and 472, respectively, in a demand fashion in
response to the absence or presence of cardiac activity, in the
appropriate chambers of the heart. The sensing circuits, 482 and
484, in turn, receive control signals over signal lines, 486 and
488, from the microcontroller 460 for purposes of measuring cardiac
performance at appropriate times, and for controlling the gain,
threshold, polarization charge removal circuitry (not shown), and
timing of any blocking circuitry (not shown) coupled to the inputs
of the sensing circuits, 482 and 486.
[0107] For arrhythmia detection, the device 310 includes an
arrhythmia detector 462 that utilizes the atrial and ventricular
sensing circuits, 482 and 484, to sense cardiac signals to
determine whether a rhythm is physiologic or pathologic. The timing
intervals between sensed events (e.g., P-waves, R-waves, and
depolarization signals associated with fibrillation) can be
classified by the microcontroller 460 by comparing them to a
predefined rate zone limit (i.e., bradycardia, normal, low rate VT,
high rate VT, and fibrillation rate zones) and various other
characteristics (e.g., sudden onset, stability, physiologic
sensors, and morphology, etc.) in order to assist with determining
the type of remedial therapy that is needed (e.g., bradycardia
pacing, anti-tachycardia pacing, cardioversion shocks or
defibrillation shocks, collectively referred to as "tiered
therapy"). Additionally, the arrhythmia detector 462 can perform
arrhythmia discrimination, e.g., using measures of arterial blood
pressure determined in accordance with embodiments of the present
invention. The arrhythmia detector 462 can be implemented within
the microcontroller 460, as shown in FIG. 4. Thus, this detector
462 can be implemented by software, firmware, or combinations
thereof. It is also possible that all, or portions, of the
arrhythmia detector 462 can be implemented using hardware. Further,
it is also possible that all, or portions, of the ischemia detector
462 can be implemented separate from the microcontroller 460.
[0108] In accordance with an embodiment of the present invention,
the implantable device 310 includes an arterial blood pressure
monitor 467, a heart failure monitor 468 and an electromechanical
delay monitor 469, which can be used to estimate SBP, DBP, PP, MAP,
EMD and/or HF (and/or changes therein), using the techniques
described above. The monitors 467, 468 and 469 can be implemented
within the microcontroller 460, as shown in FIG. 4, and can the be
implemented by software, firmware, or combinations thereof. It is
also possible that all, or portions, of the monitors 467, 468
and/or 469 to be implemented using hardware. Further, it is also
possible that all, or portions, of the monitors 467, 468 and/or 469
can be implemented separate from the microcontroller 460. The
monitors 467, 468 and/or 469 can be used in a closed loop control
system to provide an assessment of hemodynamic condition during
pacing parameter adjustments, and/or as an assessment of
hemodynamic condition during a detected arrhythmia. Such measures
of hemodynamic condition can be used when determining which
anti-arrhythmia therapy options are appropriate. It is also noted
that monitors 467, 468 and/or 469 can be combined into a single
monitor, or separated into further blocks.
[0109] The implantable device 310 can also include a pacing
controller 466, which can adjust a pacing rate and/or pacing
intervals based on estimates of SBP, DBP, PP, MAP, EMD and/or HF,
in accordance with embodiments of the present invention. The pacing
controller 466 can be implemented within the microcontroller 460,
as shown in FIG. 4. Thus, the pacing controller 466 can be
implemented by software, firmware, or combinations thereof. It is
also possible that all, or portions, of the pacing controller 466
can be implemented using hardware. Further, it is also possible
that all, or portions, of the pacing controller 466 can be
implemented separate from the microcontroller 460.
[0110] The implantable device can also include a medication pump
403, which can deliver medication to a patient if the patient's
SBP, DBP, PP, MAP, EMD and/or HF fall outside certain thresholds or
ranges. Information regarding implantable medication pumps may be
found in U.S. Pat. No. 4,731,051 (Fischell) and in U.S. Pat. No.
4,947,845 (Davis), both of which are incorporated by reference
herein.
[0111] Still referring to FIG. 4, cardiac signals are also applied
to the inputs of an analog-to-digital (A/D) data acquisition system
490. The data acquisition system 490 can be configured to acquire
various signal, including but not limited to, CI, IEGM, PPG and IPG
signals, convert the raw analog data into a digital signal, and
store the digital signals for later processing and/or telemetric
transmission to an external device 402. The data acquisition system
490 can be coupled to the right atrial lead 320, the coronary sinus
lead 324, and the right ventricular lead 330 through the switch 474
to sample cardiac signals across any pair of desired
electrodes.
[0112] The data acquisition system 490 can be coupled to the
microcontroller 460, or other detection circuitry, for detecting an
evoked response from the heart 312 in response to an applied
stimulus, thereby aiding in the detection of "capture". Capture
occurs when an electrical stimulus applied to the heart is of
sufficient energy to depolarize the cardiac tissue, thereby causing
the heart muscle to contract. The microcontroller 460 detects a
depolarization signal during a window following a stimulation
pulse, the presence of which indicates that capture has occurred.
The microcontroller 460 enables capture detection by triggering the
ventricular pulse generator 472 to generate a stimulation pulse,
starting a capture detection window using the timing control
circuitry 479 within the microcontroller 460, and enabling the data
acquisition system 490 via control signal 492 to sample the cardiac
signal that falls in the capture detection window and, based on the
amplitude, determines if capture has occurred.
[0113] The implementation of capture detection circuitry and
algorithms are well known. See for example, U.S. Pat. No. 4,729,376
(Decote, Jr.); U.S. Pat. No. 4,708,142 (Decote, Jr.); U.S. Pat. No.
4,686,988 (Sholder); U.S. Pat. No. 4,969,467 (Callaghan et. al.);
and U.S. Pat. No. 5,350,410 (Mann et. al.), which patents are
hereby incorporated herein by reference. The type of capture
detection system used is not critical to the present invention.
[0114] The microcontroller 460 is further coupled to the memory 494
by a suitable data/address bus 496, wherein the programmable
operating parameters used by the microcontroller 460 are stored and
modified, as required, in order to customize the operation of the
implantable device 310 to suit the needs of a particular patient.
Such operating parameters define, for example, pacing pulse
amplitude, pulse duration, electrode polarity, rate, sensitivity,
automatic features, arrhythmia detection criteria, and the
amplitude, waveshape and vector of each shocking pulse to be
delivered to the patient's heart 312 within each respective tier of
therapy. The memory 494 can also store data including information
about estimates of SBP, DBP, PP, MAP, EMD and/or HF.
[0115] The operating parameters of the implantable device 310 may
be non-invasively programmed into the memory 494 through a
telemetry circuit 401 in telemetric communication with an external
device 402, such as a programmer, transtelephonic transceiver, or a
diagnostic system analyzer. The telemetry circuit 401 can be
activated by the microcontroller 460 by a control signal 406. The
telemetry circuit 401 advantageously allows intracardiac
electrograms and status information relating to the operation of
the device 310 (as contained in the microcontroller 460 or memory
494) to be sent to the external device 402 through an established
communication link 404. The telemetry circuit can also be use to
transmit arterial blood pressure data to the external device
402.
[0116] For examples of telemetry devices, see U.S. Pat. No.
4,809,697, entitled "Interactive Programming and Diagnostic System
for use with Implantable Pacemaker" (Causey, III et al.); U.S. Pat.
No. 4,944,299, entitled "High Speed Digital Telemetry System for
Implantable Device" (Silvian); and U.S. Pat. No. 6,275,734 entitled
"Efficient Generation of Sensing Signals in an Implantable Medical
Device such as a Pacemaker or ICD" (McClure et al.), which patents
are hereby incorporated herein by reference.
[0117] The implantable device 310 additionally includes a battery
411 which provides operating power to all of the circuits shown in
FIG. 4. If the implantable device 310 also employs shocking
therapy, the battery 411 should be capable of operating at low
current drains for long periods of time, and then be capable of
providing high-current pulses (for capacitor charging) when the
patient requires a shock pulse. The battery 411 should also have a
predictable discharge characteristic so that elective replacement
time can be detected.
[0118] The implantable device 310 is also shown as including an
activity and/or posture sensor 415. Such a sensor 415 can be a
simple one dimensional sensor that converts mechanical motion into
a detectable electrical signal, such as a back electro magnetic
field (BEMF) current or voltage, without requiring any external
excitation. Alternatively, the sensor 415 can measure
multi-dimensional activity information, such as two or more of
acceleration, direction, posture and/or tilt. Examples of
multi-dimensional activity sensors include, but are not limited to:
the three dimensional accelerometer-based position sensor disclosed
in U.S. Pat. No. 6,658,292 to Kroll et al., which is incorporated
herein by reference; the AC/DC multi-axis accelerometer disclosed
in U.S. Pat. No. 6,466,821 to Pianca et al., which in incorporated
herein by reference; and the commercially available precision
dual-axis accelerometer model ADXL203 and three-axis accelerometer
model ADXL346, both available from Analog Devices of Norwood,
Mass.
[0119] The implantable device 310 can also include a magnet
detection circuitry (not shown), coupled to the microcontroller
460. It is the purpose of the magnet detection circuitry to detect
when a magnet is placed over the implantable device 310, which
magnet may be used by a clinician to perform various test functions
of the implantable device 310 and/or to signal the microcontroller
460 that the external programmer 402 is in place to receive or
transmit data to the microcontroller 460 through the telemetry
circuits 401.
[0120] As further shown in FIG. 4, the device 310 is also shown as
having an impedance measuring and processing circuit 413 which is
enabled by the microcontroller 460 via a control signal 414 and can
be used for obtaining many types of bodily and intracardiac
impedances, including a network of single- or multi-vector
impedance measurements. Such impedance measurements can be used,
e.g., for trending many kinds of physiological variables, and can
also be used for detection of air movement in and out of the lungs,
blockage of airways, lead impedance surveillance during acute and
chronic phases for proper lead positioning or dislodgement; lead
integrity by detecting insulation abrasion, operable electrodes,
and automatically switching to an operable pair if dislodgement
occurs; measuring respiration or minute ventilation; measuring
thoracic impedance for determining shock thresholds; detecting when
the device has been implanted; measuring cardiac stroke volume;
detecting the opening of heart valves; and so forth. The impedance
measuring circuit 413 may be coupled to the switch 474 so that any
desired electrodes may be used, and networks of vectors can be
selected. The impedance measuring circuit 413 can be used to obtain
cardiogenic impedance (CI) signals, which can be used with certain
embodiments of the present invention. Exemplary details of an
impedance measuring and processing circuit 413 are provided in, and
discussed with reference to FIG. 5. Additional exemplary details of
circuitry for obtaining CI signals are provided in U.S. patent
application Ser. No. 11/863,516, filed Sep. 28, 2007 and entitled
"Use of Cardiogenic Impedance Waveform Morphology to Analyze
Cardiac Conditions and to adjust Treatment Therapy," which is
incorporated herein by reference. The impedance measuring circuit
413, when measuring impedance using implanted electrodes that are
remote from the patient's heart, can be used to obtain impedance
plethysmography (IPG) signals, which can be used in certain
embodiments of the present invention.
[0121] In the case where the implantable device 310 is also
intended to operate as an implantable cardioverter/defibrillator
(ICD) device, it should detect the occurrence of an arrhythmia, and
automatically apply an appropriate electrical shock therapy to the
heart aimed at terminating the detected arrhythmia. To this end,
the microcontroller 460 further controls a shocking circuit 416 by
way of a control signal 418. The shocking circuit 416 generates
shocking pulses of low (up to 0.5 Joules), moderate (0.5-10
Joules), or high energy (11 to 40 Joules), as controlled by the
microcontroller 460. Such shocking pulses are applied to the
patient's heart 312 through at least two shocking electrodes, and
as shown in this embodiment, selected from the left atrial coil
electrode 328, the RV coil electrode 336, and/or the SVC coil
electrode 338. As noted above, the housing 340 may act as an active
electrode in combination with the RV electrode 336, or as part of a
split electrical vector using the SVC coil electrode 338 or the
left atrial coil electrode 328 (i.e., using the RV electrode as a
common electrode).
[0122] The above described implantable device 310 was described as
an exemplary pacing device. One or ordinary skill in the art would
understand that embodiments of the present invention can be used
with alternative types of implantable devices. Accordingly,
embodiments of the present invention should not be limited to use
only with the above described device.
Exemplary CI Circuit
[0123] For completeness, FIG. 5 shows an exemplary impedance
measurement circuit architecture 500 (e.g., which can be used to
implement block 278 on FIG. 2), including filter components to
obtain raw, cardiogenic, and respiratory impedances. The
illustrated architecture 500 is just one example configuration,
other configurations are also possible. In one implementation, the
exemplary impedance measurement architecture 500 includes a pulse
generator 502 for generating an exemplary pulse waveform, in this
case a current waveform 503, for application to the bodily tissue
of a patient 504 and a sensed signal processor 506 for processing
resulting waveforms detected in the tissue, in this case voltage
waveforms 507. The pulse generator 502 can be implemented by the
circuitry of blocks 470 or 472 in FIG. 4, or dedicated circuitry.
An injection (e.g., current pulse) multiplexor 508 implements the
single- or multi-vector aspect of signal application by determining
a first set of electrodes for injecting the exemplary waveform 503.
The selection of electrodes may be determined, e.g., by the
controller 460 (FIG. 4), or a dedicated vector engine (not shown).
Likewise, a sensing (voltage measurement) multiplexer 510
implements signal sensing by determining a second set of electrodes
for sensing the resulting voltage waveforms 507. The set of sensing
electrodes may also be determined, e.g., by the controller 460
(FIG. 4), or a dedicated vector engine (not shown). Both the
injection multiplexor 508 and the sensing multiplexor 510 may be
implemented in the implantable device 310 in the electrode
configuration switch 474 (FIG. 4).
[0124] A waveform 503 for application to bodily tissue that is
generated by the exemplary impedance measurement circuit
architecture 500 can possess special waveform features and
electrical characteristics that are well suited for probing and
measuring many types of physiological parameters in the body using
current modulated or voltage modulated pulses. Examples of such
waveforms are described in U.S. patent application Ser. No.
11/684,664, entitled "Tissue Characterization Using Intracardiac
Impedances with an Implantable Lead System", (Wong et al), filed
Mar. 12, 2007 (Attorney Docket No. A06P3006-US1), which are
incorporated herein by reference. Exemplary waveforms 503 are
multi-phasic, with negative phases (pulse segments below baseline)
that balance positive phases (pulse segments above baseline). The
illustrated waveform 503 is tri-phasic. Other versions of the
waveform 503 may have more than three phases, may be synchronous or
asynchronous, may be rectangular or sinusoidal, etc. In one
variation, the exemplary impedance measurement architecture applies
the waveform 503 as a voltage waveform instead of a current
waveform and senses the results as electrical current instead of
voltage.
[0125] Properties of the exemplary waveforms 503 include superior
penetration of some tissues than conventionally injected signals;
better differential penetration of tissues than conventionally
injected signals for improved differentiation and characterization
of tissues; broader frequency spectrum content than conventionally
injected signals in order to characterize tissue; greater
neutrality in the body than conventionally injected signals, i.e.,
the exemplary waveforms do not change the parameter they are trying
to measure, and moreover, do not create ionic imbalances or
imbalances of charge, voltage, etc., in the tissues or at
tissue-electrode interfaces.
[0126] Each waveform 503 preferably has a total duration less than
the charging time constant of the electrode-electrolyte interfaces
used to inject and sense the signals. These time constants are
typically in the range of a few milliseconds. In one
implementation, the duration of waveform 503 is less than 1
millisecond. This waveform feature is helpful for minimizing
polarization effects at these electrode-electrolyte interfaces.
Other features of the exemplary waveforms 503 include symmetric or
asymmetric phase duration, decreasing phase amplitudes, and
alternating phase signs. Each waveform 503 typically has null
durations in between phases to provide time to allow complete
processing of information caused by one phase before the next phase
of the waveform 503 begins. Implementations of the waveform 503
that have near perfect square wave pulses (or rectangular wave
pulses) contain a great deal of high-frequency content.
Near-sinusoidal implementations of the waveform 503 may contain
less high frequency content than the rectangular wave versions.
[0127] The features of exemplary waveforms 503 just enumerated
provide numerous advantages, including: eliminating the need for
fast digital sampling, minimizing artifacts introduced in the
measurement process, increased tolerance of small phase delays
between injected and sensed signals. The exemplary waveforms 503
also lend themselves to CMOS realization using low-value switched
capacitor solutions. Further, the wide frequency spectrum of the
injected signal can be used to implement algorithms that
differentiate tissues based on their frequency response, and/or
phase delay. The very low duty-cycle of the exemplary waveforms 503
make them safer for patients. The reduced duty-cycle brings the
injected charge and the root-mean-square value of the injected
signal well below levels that could be perceived by the patient or
that could induce adverse events.
[0128] It is noted that the net-zero voltage feature, also referred
to as the voltage-balanced feature, refers to the voltage formed on
blocking capacitors that appear in series with the load. The flow
of current through these capacitors builds up voltage across them.
Since these capacitors, such as capacitor 540 in FIG. 5, also
appear in circuits that are responsible for sensing cardiac
activity, it is important that the net voltage built up on them be
zero. As a result of the net-zero voltage feature, the influence of
an exemplary waveform 503 on the circuits that sense cardiac
activity is minimal.
[0129] Other features of the exemplary waveforms 503 derive from
the above-mentioned null segments--intra-waveform segments
containing no signal--that serve several purposes. First, the null
segments allow the electronics in processing circuits to settle
during measurement of phases and second, they allow multiple
instances of the waveform 503 to exist in the patient's tissue
simultaneously, being staggered by time multiplexing such that a
phase of one waveform can be measured during the time that there is
no signal between phases of another waveform.
[0130] In one implementation, the exemplary waveform 503 is used to
derive physiological measurements based on intracardiac impedances.
Based on such cardiogenic impedance measurements, many
physiological variables can be trended to detect changes in a
patient's condition, such as congestive heart failure (CHF) index,
pulmonary edema, systolic slope, contraction (e.g., dZ/dt(max)),
diastolic slope, relaxation (e.g., dZ/dt(min)), pre-ejection period
(in low resolution), ejection time, left ventricular ejection
fraction (LVEF), diastolic heart failure index (DHFI), cardiac
index, etc.
[0131] The exemplary waveform 503 provides an elegant and reliable
vehicle for measuring bodily impedances in a manner that gives
reliably reproducible results. Instead of a conventional technique
of trying to sense an instantaneous "snapshot" measurement of a
conventionally injected signal, the impedance measurement circuit
architecture 500 derives an impedance measurement by dividing the
area under the sensed voltage curve (waveform 507) by the area of
the injected current waveform 503. An exemplary implantable device
310 can perform this exemplary method by "integrating the curve" of
an absolute value of waveforms 503 or 507. Sometimes the exemplary
implantable device can closely approximate this integration without
having to perform an integration operation by directly measuring
and summing the area "under" the curve (e.g., under the rectangular
wave) of the waveform 503, that is, the area composed of the
absolute value of the three areas of the three phases of an
exemplary tri-phasic waveform 503.
[0132] Likewise, the exemplary implantable device can integrate, or
closely approximate the integration, by measuring and summing the
area "under" the curve (e.g., the rectangular wave) of the waveform
507, that is, the area composed of the absolute value of the three
areas of the three phases. In one implementation, the area of the
sensed voltage, waveform 507, is measured at the output of an
integrator circuit. The area of the injected current, waveform 503,
is computed by, or preset by, the micro-controller driving the
implantable device. An implantable device 310 may thus use this
area-based ("areal") approach to deriving a network of impedance
measurements over a multi-vector network 350.
[0133] Returning to description of the impedance measurement
circuit architecture 500 itself, the sensed signal processor 506
typically consists of pre-amplification circuitry, switched
capacitor filters, and an analog to digital converter 512. In one
implementation, the voltage signal from the voltage measurement
multiplexer 510 is processed by several voltage measurement lines
or paths. The illustrated sensed signal processor 506 is able to
obtain at least the three different impedance signals introduced
above with respect to FIG. 5, that is, low frequency raw impedance
Z.sub.o 513, respiration impedance Z.sub.r 515, and cardiogenic
impedance Z.sub.c 517. Each measurement can be activated separately
or simultaneously.
[0134] A digital form of raw impedance Z.sub.o 513 may be obtained.
First, the sensed signal, i.e., the tri-phasic voltage waveform 507
from the voltage measurement multiplexer 510, is sent to a
preamplifier 514. The next stage is embodied in a sign conversion
and integration module 516. At this stage, the signal is converted
into an absolute value and then integrated over time. Using the
integration process instead of conventional instantaneous
"snapshot" measurements of impedance components such as pure
resistance produces results that are more noise-free and more
accurate than the conventional techniques.
[0135] The signal is then applied to a discrete-to-continuous
signal conversion module 518. At this point in the architecture
500, the signals for low frequency impedance Z.sub.o 513,
respiration impedance Z.sub.r 515, and cardiogenic impedance
Z.sub.c 517 (also referred to as the CI signal) are extracted
separately by different filter paths, as summarized in FIG. 5. To
obtain the low frequency impedance Z.sub.o 513, the signal is sent
to a level shift and low pass filter module 520, and then to the
analog to digital converter 512.
[0136] A digital form of the respiration impedance Z.sub.r 515 may
be obtained by tapping the analog signal from the input of the
level shift and low pass filter module 520, and feeding the signal
to a line consisting of band-pass filters 522 and 524 and a low
pass filter 526. The signal is then fed to the analog to digital
converter 512 to obtain digital Z.sub.r 515.
[0137] A digital form of the cardiogenic impedance Z.sub.c 517
(also referred to as the CI signal) may likewise be obtained by
tapping the analog signal from the input of the level shift and low
pass filter module 520, and feeding the signal to a line consisting
of high pass filters 528 and 530 and a low pass filter 532. The
signal is then fed to the analog to digital converter 512 to obtain
digital Z.sub.c 517.
[0138] In one implementation, the pulse generator 502 consists of
two timing controlled current generators 534 and 536 with
programmable magnitude. The first current generator 534 sources
current, the other current generator 536 sinks the current. As part
of the charge and voltage balancing process, the switch
SW.sub.Balance 538 is used to discharge the external capacitor
Cap_Impulse 540 after each generated impulse. The pulse rate is
programmable.
[0139] Components of the impedance measurement architecture 500 may
be implemented in the impedance measuring and processing circuit
413 shown in FIG. 4, and may be implemented in hardware, software,
or combinations thereof. For example, the exemplary impedance
measurement architecture 500 may be implemented in hardware as part
of the microcontroller 460 and/or as hardware integrated into the
fabric of the exemplary implantable device 310; or as
software/firmware instructions programmed into an implementation of
the implantable device 310 and executed on the microcontroller 421
during certain modes of operation.
[0140] In one implementation, the preamplifier 514 is included in
the impedance measuring & processing circuits 478. The pulse
generator 502 can be implemented in the impedance processing module
440 as may some of the other components of the sensed signal
processor 506.
[0141] Although the illustrated version of the impedance
measurement circuit architecture 500 applies a current pulse
waveform 503 and senses a voltage pulse waveform 507, other
implementations can inject a voltage waveform and sense a current
waveform.
[0142] The "raw" impedance measurement, Z.sub.o 513, can be useful
for determining extra- or intra-cardiac impedances and examining
conditions such as pulmonary edema. The cardiogenic component of
impedance, Z.sub.c 517, can be used in the various embodiments of
the invention described in detail above.
Processing of Plethysmography Signals
[0143] Photoplethysmography (PPG) and Impedance Plethysmography
signals (collectively referred to as PPG/IPG signals), and other
plethysmography signals, show changes in a patient's arterial
system as a result of the patient's heart contracting, and such
signals are indicative of changes in arterial blood volume. A PPG
signal can be obtained using a PPG sensor, which as explained
above, can be an optical sensor including a light source and a
light detector. An IPG signal can be obtained using an IPG sensor,
which as explained above, can include electrodes and circuitry used
to measure the impedance between such electrodes. One or more such
electrodes can be located on one or more leads, and/or a mechanical
housing of an implanted device can act as one of the
electrodes.
[0144] When a PPG/IPG sensor is implanted at a location remote from
the patient's heart, an obtained pressure pulsation signal has been
shown to arrive from the heart to the PPG/IPG sensor after an
amount of time that is related to arterial blood pressure. The
velocity of the pressure pulsation traversing the arteries is
positively correlated with systolic blood pressure. Therefore, as
explained above, measures of pulse arrival time (PAT), and metrics
indicative of PAT, can be used to estimate arterial blood
pressure.
[0145] Better estimates of arterial blood pressure can be obtained
if the PPG/IPG signals used in the above described embodiments are
appropriately processed. Accordingly, certain embodiments of the
present invention relate to techniques for processing PPG/IPG
signals (or other plethysmography signals), as described below.
Further embodiments of the present invention, described below,
relate to how to extract features of PPG/IPG signals (or other
plethysmography signals), which features can be used to determine
metrics indicative of PAT, in the manners explained above.
[0146] FIGS. 6 and 7A-7E will now be used to describe exemplary
embodiments for obtaining a PPG signal and detecting predetermined
features of the PPG signal. Similar techniques can be used to
obtain an IPG signal (or other plethysmography signal) and detect
predetermined features of the IPG signal (or other plethysmography
signal). Referring to FIG. 6, at step 602 a PPG signal is recorded.
Recording of a PPG signal may be triggered, e.g., on an R wave,
based on respiratory cycle, based on activity levels, etc. An
exemplary raw PPG signal recorded over 20 second is shown in FIG.
7A.
[0147] At step 604, the PPG signal is filtered to remove
respiratory noise, motion artifact, baseline drift, etc. For
example, the signal can be band-pass filtered so that the pass-band
is from about 0.7 to 10 Hz, although other pass bands can be used.
FIG. 7B shows the raw PPG signal of FIG. 7A, after being
band-passed filtered using a pass-band of about 0.7 to 10 Hz. As
can be appreciated from FIG. 7B, most of the respiration signal and
high frequency noise is removed by the filtering.
[0148] At step 606, an outlier removal process is performed, to
remove "bad" heart beats. In an embodiment, the outlier removal can
be accomplished by grouping a plurality (e.g., 20) consecutive
heart beats, determining a mean of the filtered PPG signal for the
plurality of heart beats, and then comparing the determined mean to
individual cycles of the filtered PPG signal. Further, outlier
removal can be performed by removing each cardiac cycle of the
filtered PPG signal that deviates by at least a threshold amount
(e.g., 3 or some other number of standard deviations) from the mean
of the PPG signal for the plurality of consecutive beats. FIG. 7C
show the filtered signal of FIG. 7B with R-wave markers added
(shows as dashed vertical lines). FIG. 7D shows the filtered signal
of FIGS. 7B and 7C with 3 "bad" beats removed as a result of an
outlier removal process.
[0149] Still referring to FIG. 6, at step 608, the cycles of the
PPG signal remaining after the outlier removal step are then
ensemble averaged. The result is an average representation of the
PPG signal for the plurality of consecutive beats, with noise and
"bad" beats removed. FIG. 7E shows an exemplary ensemble averaged
PPG signal.
[0150] Thereafter, features of the PPG signal can be detected from
the ensemble-averaged PPG signal. For example, as indicated at
steps 610 and 612, the first derivative of the ensemble-averaged
PPG signal can be determined, and the location of the maximum
positive slope of the ensemble-averaged PPG signal can be detected
by determining the maximum of the first derivative. Further, since
it is believed that the maximum positive slope cannot be more than
70% of an R-R interval away from an R-wave, if the location of the
maximum positive slope is not within 70% of an R-R interval away
from an R wave, a maximum positive slope detection can be
determined to be bad, and not be used.
[0151] As indicated at steps 614 and 616, the second derivative of
the ensemble averaged PPG signal can be determined to find local
minima and maxima. The locations of a maximum and a minimum are
where the first derivative is equal to zero. The second derivative
can be used to determine if a specific location is a maximum or a
minimum. More specifically, if the second derivative is positive,
then the point is at a minimum. If the second derivative is
negative at a point, then the point is a maximum. The local minimum
and local maximum that are closest to the maximum positive slope
are the minimum and maximum amplitudes of the signal, which can be
used, e.g., to determine the peak-to-peak amplitude of the ensemble
averaged PPG signal. Further, as indicated at step 618, the maximum
negative slope can be determined by identifying, from the first
derivative, the local maximum that occurs after the maximum of the
averaged PPG signal, but before the subsequent R-wave. As indicated
at step 620, from the second derivative, the dicrotic notch can be
identified by identifying the local minimum following the maximum
of the averaged PPG signal, but before the subsequent R-wave. FIG.
7E shows examples of various predetermined features that can be
detected. As shown in FIG. 7E a maximum downward slope can be
detected prior to the dicrotic notch, as well as after the dicrotic
notch.
[0152] Alternative techniques for detecting predetermined features
of a PPG signal (or IPG signal) can be used, such as, but not
limited to, techniques that rely on template matching, wavelets,
neural networks, Fast Fourier Transform (FFT) and/or time warping.
Alternatively, or additionally, techniques for detecting
predetermined features of a PPG signal (or IPG signal) can utilize
respiratory cycles and R-R intervals.
[0153] In certain embodiments, since the presence of the dicrotic
notch comes and goes under different conditions, monitoring such
conditions can use the presence of the dicrotic notch as a binary
feature.
[0154] Metrics indicative of morphological features of the PPG
signal can also be determined based on the ensemble-averaged PPG
signal. Such metrics can include, but are not limited to, area
under the curve, full width at half max (FWHM), and as already
mentioned above, peak-to-peak amplitude (a.sub.1 shown in FIGS. 1A,
1B and 7E). As explained above, such morphological features may
also be used when determining estimates of arterial blood
pressure.
[0155] Embodiments of the present invention have been described
above with the aid of functional building blocks illustrating the
performance of specified functions and relationships thereof. The
boundaries of these functional building blocks have often been
arbitrarily defined herein for the convenience of the description.
Alternate boundaries can be defined so long as the specified
functions and relationships thereof are appropriately performed.
Any such alternate boundaries are thus within the scope and spirit
of the claimed invention. For example, it would be possible to
combine or separate some of the steps shown in FIGS. 2A-2C.
Further, it is possible to change the order of some of the steps
shown in FIGS. 2A-2C, without substantially changing the overall
events and results. For another example, it is possible to change
the boundaries of some of the blocks shown in FIG. 4.
[0156] The previous description of the preferred embodiments is
provided to enable any person skilled in the art to make or use the
embodiments of the present invention. While the invention has been
particularly shown and described with reference to preferred
embodiments thereof, it will be understood by those skilled in the
art that various changes in form and details may be made therein
without departing from the spirit and scope of the invention.
* * * * *