U.S. patent application number 09/751431 was filed with the patent office on 2002-07-04 for apparatus and method for ventricular rate regularization.
Invention is credited to Kramer, Andrew P., Stahmann, Jeffrey E..
Application Number | 20020087198 09/751431 |
Document ID | / |
Family ID | 25021947 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020087198 |
Kind Code |
A1 |
Kramer, Andrew P. ; et
al. |
July 4, 2002 |
Apparatus and method for ventricular rate regularization
Abstract
A method and system for operating a cardiac rhythm management
device which employs pacing therapy to regularize the ventricular
rhythm. Such ventricular rate regularization may be employed with
conventional bradycardia pacing, ventricular resynchronization
therapy, or anti-tachyarrhythmia therapy.
Inventors: |
Kramer, Andrew P.;
(Stillwater, MN) ; Stahmann, Jeffrey E.; (Ramsey,
MN) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. Box 2938
Minneapolis
MN
55402
US
|
Family ID: |
25021947 |
Appl. No.: |
09/751431 |
Filed: |
December 29, 2000 |
Current U.S.
Class: |
607/9 |
Current CPC
Class: |
A61N 1/3622
20130101 |
Class at
Publication: |
607/9 |
International
Class: |
A61N 001/362 |
Claims
What is claimed is:
1. A method for operating a cardiac pacemaker, comprising: sensing
ventricular depolarizations through a ventricular sensing channel
and generating ventricular sense signals in accordance therewith;
pacing a ventricle upon expiration of a ventricular escape interval
without receiving a ventricular sense from the ventricular sensing
channel, wherein the ventricular escape interval starts with either
a ventricular pace or a ventricular sense, the reciprocal of the
ventricular escape interval being the lower rate limit of the
pacemaker; measuring an R-R interval associated with each
ventricular sense, wherein an R-R interval is the time between a
ventricular sense and the preceding ventricular sense or
ventricular pace, the reciprocal of the R-R interval thus being the
measured intrinsic ventricular rate; and, adjusting the ventricular
escape interval in accordance with a measured R-R interval so that
the lower rate limit adjusts in accordance with changes in the
measured intrinsic ventricular rate.
2. The method of claim 1 wherein the ventricular escape interval is
adjusted to move toward the measured R-R interval multiplied by a
scaling factor.
3. The method of claim 2 wherein the ventricular escape interval is
adjusted by computing a weighted average of the measured R-R
interval multiplied by a scaling factor and the value of the
ventricular escape interval.
4. The method of claim 1 wherein the ventricular escape interval is
adjusted in accordance with a measured R-R interval after each
ventricular sense.
5. The method of claim 1 further comprising adjusting the
ventricular escape interval toward a value corresponding to a base
lower rate limit after a ventricular pace.
6. The method of claim 5 wherein the ventricular escape interval is
adjusted after a ventricular pace by multiplying the escape
interval by a decay coefficient.
7. The method of claim 1 wherein the ventricular escape interval is
adjusted by computing a weighted average of the measured R-R
interval multiplied by a scaling factor and the previous value of
the ventricular escape interval after each ventricular sense, and
wherein the ventricular escape interval is adjusted by multiplying
the escape interval by a decay coefficient after a ventricular
pace.
8. The method of claim 7 wherein the scaling factor is selected to
be greater than one such that the lower rate limit of the pacemaker
is moved toward a fraction of the intrinsic ventricular rate after
each ventricular sense.
9. The method of claim 7 wherein the scaling factor is selected to
be less than one such that the lower rate limit of the pacemaker is
moved toward a value that is above the intrinsic ventricular rate
after each ventricular sense.
10. The method of claim 1 further comprising: sensing ventricular
rate and synchronized heart chambers through separate channels and
generating sense signals upon detection of depolarization occurring
in a chamber; pacing the rate ventricle upon expiration of a
ventricular escape interval without receiving a ventricular sense
from the rate ventricular sensing channel, wherein the ventricular
escape interval starts with either a ventricular pace or a
ventricular sense, the reciprocal of the ventricular escape
interval being the lower rate limit of the pacemaker; measuring an
R-R interval associated with each ventricular rate chamber sense,
wherein an R-R interval is the time between a ventricular sense and
the preceding ventricular sense or ventricular pace, the reciprocal
of the R-R interval thus being the measured intrinsic ventricular
rate; adjusting the ventricular escape interval in accordance with
the measured R-R interval so that the lower rate limit adjusts in
accordance with changes in the measured intrinsic ventricular rate;
and, pacing the synchronized ventricle in accordance with a
synchronized pacing mode wherein a synchronized ventricle pace is
delivered at a specified pacing instant defined with respect to
expiration of a rate ventricle escape interval.
11. A cardiac rhythm management device, comprising: a ventricular
sensing channel for sensing ventricular depolarizations and
generating ventricular sense signals in accordance therewith; a
ventricular pacing channel for pacing a ventricle; a controller for
controlling operation of the device such that a ventricular pace is
delivered upon expiration of a ventricular escape interval without
receiving a ventricular sense from either ventricular sensing
channel, wherein the ventricular escape interval starts with either
a ventricular pace or a ventricular sense, the reciprocal of the
ventricular escape interval being the lower rate limit of the
pacemaker; wherein the controller is configured to measure an R-R
interval associated with each ventricular sense, wherein an R-R
interval is the time between a ventricular sense and the preceding
ventricular sense or ventricular pace, the reciprocal of the R-R
interval thus being the measured intrinsic ventricular rate; and,
wherein the controller is configured to be capable of adjusting the
ventricular escape interval in accordance with a measured R-R
interval so that the lower rate limit adjusts in accordance with
changes in the measured intrinsic ventricular rate.
12. The device of claim 11 wherein the ventricular escape interval
is adjusted to move toward the measured R-R interval multiplied by
a scaling factor.
13. The device of claim 12 wherein the ventricular escape interval
is adjusted by computing a weighted average of the measured R-R
interval multiplied by a scaling factor and the previous value of
the ventricular escape interval.
14. The device of claim 11 wherein the ventricular escape interval
is adjusted in accordance with a measured R-R interval after each
ventricular sense.
15. The device of claim 11 wherein the controller is configured to
adjust the ventricular escape interval toward a value corresponding
to a base lower rate limit after a ventricular pace.
16. The device of claim 11 wherein the controller is configured to
adjust the ventricular escape interval after a ventricular pace by
multiplying the escape interval by a decay coefficient.
17. The device of claim 11 wherein the controller is configured to
adjust the ventricular escape interval by computing a weighted
average of the measured R-R interval multiplied by a scaling factor
and the previous value of the ventricular escape interval after
each ventricular sense, and wherein the ventricular escape interval
is adjusted by multiplying the escape interval by a decay
coefficient after a ventricular pace.
18. The device of claim 12 wherein the scaling factor is selected
to be greater than one such that the lower rate limit of the device
is moved toward a fraction of the intrinsic ventricular rate after
each ventricular sense.
19. The device of claim 12 wherein the scaling factor is selected
to be less than one such that the lower rate limit of the device is
moved toward a value that is above the intrinsic ventricular rate
after each ventricular sense.
20. The device of claim 11 further comprising: sensing channels for
sensing depolarizations from ventricles designated as a rate
ventricle and a synchronized ventricle; wherein the ventricular
escape interval and R-R intervals are defined with respect to rate
ventricle events; and, wherein the controller is programmed to pace
the synchronized chamber in accordance with a synchronized pacing
mode such that a synchronized ventricle pace is delivered at a
specified pacing instant defined with respect to expiration of the
rate ventricle escape interval.
21. The device of claim 11 further comprising a shock pulse
generator and shock electrodes for delivering a defibrillation
shock to a heart chamber upon detection of a tachyarrhythmia.
22. The device of claim 21 wherein the device is configured to
deliver defibrillation shocks to the atria.
23. The device of claim 21 wherein the controller is programmed
such that the ventricular escape interval is adjusted in accordance
with a measured R-R interval upon detection of a tachyarrhythmia.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to cardiac pacemakers and methods
for operating such devices. In particular, the invention relates to
methods for employing pacing therapy to maintain hemodynamic
stability.
CROSS REFERENCE TO RELATED APPLICATIONS
[0002] This application is related to the following co-pending,
commonly assigned patent application: "System Providing Ventricular
Pacing and Biventricular Coordination," U.S. Serial No. 09/316,588,
filed on May 21, 1999, which disclosure is herein incorporated by
reference in its entirety.
Background
[0003] Ventricular tachyarrhythmias, in which the ventricles beat
more rapidly and irregularly than normal, can be due to a variety
of etiologies. Certain patients, for example, are prone to
premature ventricular contractions due to ectopic excitatory foci
in the ventricular myocardium. Another cause of ventricular
tachyarrhythmia is atrial fibrillation where the atria depolarize
in a chaotic fashion with no effective pumping action. The
intrinsic ventricular rhythm that occurs during an episode of
atrial fibrillation is a result of the chaotically occurring
depolarizations occurring in the atria being passed through the AV
node to the ventricles. The intrinsic ventricular rate is thus
governed by the cycle length of the atrial fibrillation and the
refractory period of the AV node. Although the intrinsic
ventricular rate is less than the atrial rate, due to the
refractory period of the AV node, it is still rapid and
irregular.
[0004] When the ventricles contract at irregular intervals, the
contraction can occur prematurely before diastolic filling is
complete which decreases the stroke volume for that contraction.
This can be especially significant in, for example, congestive
heart failure patients who are already hemodynamically compromised.
Concomitant atrial fibrillation where the atria no longer act as
effective priming pumps can also contribute to the problem. An
irregular ventricular rate can thus depress cardiac output and
cause such symptoms as dyspnea, fatigue, vertigo, and angina. An
objective of the present invention is to use pacing therapy to
maintain hemodynamic stability in the presence of an irregular
intrinsic ventricular rhythm.
SUMMARY OF THE INVENTION
[0005] The present invention is a system and method for
regularizing the ventricular rate by adjusting the lower rate limit
of a pacemaker in accordance with changes in the measured intrinsic
ventricular rate. By making the ventricular escape interval track a
mean interval between intrinsic beats, less variability in the
overall ventricular rhythm is allowed by the pacemaker. Ventricular
rate regularization improves cardiac output when used with
conventional bradycardia pacing. Ventricular rate regularization
may also be used to improve the efficacy of ventricular
resynchronization therapy and anti-tachyarrhythmia therapy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a system diagram of a microprocessor-based cardiac
rhythm management device.
[0007] FIG. 2 shows an exemplary filter implementation of a
ventricular rate regularization system.
DETAILED DESCRIPTION OF THE INVENTION
[0008] As will be described below, ventricular rate regularization
may be advantageously applied together with a number of different
cardiac rhythm management therapies. These include conventional
bradycardia pacing, ventricular resynchronization therapy, and
antitachyarrhythmia therapy.
1. System Description
[0009] FIG. 1 shows a system diagram of a microprocessor-based
cardiac rhythm management device suitable for delivering
ventricular rate regularization therapy as well as various cardiac
rhythm management therapies with which ventricular rate
regularization can be advantageously combined. In the particular
embodiments to be described below, a device incorporating the
present invention may possess all of the components shown or only
those necessary to perform the functions described.
[0010] The device in FIG. 1 is a pacemaker that is physically
configured with sensing and pacing channels for both atria and both
ventricles. The controller 10 of the device is a microprocessor
which communicates with a memory 12 via a bidirectional data bus.
The memory 12 typically comprises a ROM (read-only memory) for
program storage and a RAM (random-access memory) for data storage.
The pacemaker has atrial sensing and pacing channels comprising
electrode 34a-b, leads 33a-b, sensing amplifiers 31a-b, pulse
generators 32a-b, and atrial channel interfaces 30a-b which
communicate bidirectionally with microprocessor 10. The device also
has ventricular sensing and pacing channels for both ventricles
comprising electrodes 24a-b, leads 23a-b, sensing amplifiers 21a-b,
pulse generators 22a-b, and ventricular channel interfaces 20a-b.
In the figure, "a" designates one ventricular or atrial channel and
"b" designates the channel for the contralateral chamber. In this
embodiment, a single electrode is used for sensing and pacing in
each channel, known as a unipolar lead. Other embodiments may
employ bipolar leads which include two electrodes for outputting a
pacing pulse and/or sensing intrinsic activity. The channel
interfaces 20a-b and 30a-b include analog-to-digital converters for
digitizing sensing signal inputs from the sensing amplifiers and
registers which can be written to by the microprocessor in order to
output pacing pulses, change the pacing pulse amplitude, and adjust
the gain and threshold values for the sensing amplifiers. An
exertion level sensor 330 (e.g., an accelerometer or a minute
ventilation sensor) enables the controller to adapt the pacing rate
in accordance with changes in the patient's physical activity. A
telemetry interface 40 is also provided for communicating with an
external programmer 500 which has an associated display 510.
[0011] The device of FIG. 1 is also configured to deliver
anti-tachyarrhythmia therapy by anti-tachycardia pacing and/or
cardioversion/defibrillation. Tachyarrhythmias are abnormal heart
rhythms characterized by a rapid heart rate. Examples of
tachyarrhythmias include supraventricular tachycardias such as
sinus tachycardia, atrial tachycardia, and atrial fibrillation
(AF), and ventricular tachyarrhythmias such as ventricular
tachycardia (VT) and ventricular fibrillation (VF). Both
ventricular tachycardia and ventricular fibrillation are
hemodynamically compromising, and both can be life-threatening.
Atrial fibrillation is not immediately life threatening, but since
atrial contraction is lost, the ventricles are not filled to
capacity before systole which reduces cardiac output. If atrial
fibrillation remains untreated for long periods of time, it can
also cause blood to clot in the left atrium, possibly forming
emboli and placing patients at risk for stroke.
[0012] Cardioversion (an electrical shock delivered to the heart
synchronously with an intrinsic depolarization) and defibrillation
(an electrical shock delivered without such synchronization) can be
used to terminate most tachyarrhythmias, including AF, VT, and VF.
As used herein, the term defibrillation should be taken to mean an
electrical shock delivered either synchronously or not in order to
terminate a fibrillation. In electrical defibrillation, a current
depolarizes a critical mass of myocardial cells so that the
remaining myocardial cells are not sufficient to sustain the
fibrillation. The electric shock may thus terminate the
tachyarrhythmia by depolarizing excitable myocardium, which thereby
prolongs refractoriness, interrupts reentrant circuits, and
discharges excitatory foci.
[0013] The device in FIG. 1 has a cardioversion/defibrillation
functionality as implemented by a shock pulse generator 50
interfaced to the microprocessor for delivering shock pulses via a
pair of shock electrodes 51a and 51b placed in proximity to regions
of the heart. The device may have one such shock pulse generator
and shock electrode pair for delivering defibrillation shocks to
either the atria or the ventricles or may be capable of delivering
shocks to both chambers. The sensing channels are used to both
control pacing and for measuring heart rate in order to detect
tachyarrhythmias such as fibrillation. The device detects an atrial
or ventricular tachyarrhythmia by measuring the atrial or
ventricular rate, respectively, as well as possibly performing
other processing on data received from the sensing channels.
2. Bradycardia Pacing Modes
[0014] Bradycardia pacing modes refer to pacing algorithms used to
pace the atria and/or ventricles when the intrinsic ventricular
rate is inadequate either due to AV conduction blocks or sinus node
dysfunction. Such modes may either be single-chamber pacing, where
either an atrium or a ventricle is paced, or dual-chamber pacing in
which both an atrium and a ventricle are paced. The modes are
generally designated by a letter code of three positions where each
letter in the code refers to a specific function of the pacemaker.
The first letter refers to which heart chambers are paced and which
may be an A (for atrium), a V (for ventricle), D (for both
chambers), or 0 (for none). The second letter refers to which
chambers are sensed by the pacemaker's sensing channels and uses
the same letter designations as used for pacing. The third letter
refers to the pacemaker's response to a sensed P wave from the
atrium or an R wave from the ventricle and may be an I (for
inhibited), T (for triggered), D (for dual in which both triggering
and inhibition are used), and O (for no response). Modem pacemakers
are typically programmable so that they can operate in any mode
which the physical configuration of the device will allow.
Additional sensing of physiological data allows some pacemakers to
change the rate at which they pace the heart in accordance with
some parameter correlated to metabolic demand. Such pacemakers are
called rate-adaptive pacemakers and are designated by a fourth
letter added to the three-letter code, R.
[0015] Pacemakers can enforce a minimum heart rate either
asynchronously or synchronously. In asynchronous pacing, the heart
is paced at a fixed rate irrespective of intrinsic cardiac
activity. There is thus a risk with asynchronous pacing that a
pacing pulse will be delivered coincident with an intrinsic beat
and during the heart's vulnerable period which may cause
fibrillation. Most pacemakers for treating bradycardia today are
therefore programmed to operate synchronously in a so-called demand
mode where sensed cardiac events occurring within a defined
interval either trigger or inhibit a pacing pulse. Inhibited demand
pacing modes utilize escape intervals to control pacing in
accordance with sensed intrinsic activity. In an inhibited demand
mode, a pacing pulse is delivered to a heart chamber during a
cardiac cycle only after expiration of a defined escape interval
during which no intrinsic beat by the chamber is detected. If an
intrinsic beat occurs during this interval, the heart is thus
allowed to "escape" from pacing by the pacemaker. Such an escape
interval can be defined for each paced chamber. For example, a
ventricular escape interval can be defined between ventricular
events so as to be restarted with each ventricular sense or pace.
The inverse of this escape interval is the minimum rate at which
the pacemaker will allow the ventricles to beat, sometimes referred
to as the lower rate limit (LRL).
[0016] In atrial tracking pacemakers (i.e., VDD or DDD mode),
another ventricular escape interval is defined between atrial and
ventricular events, referred to as the atrio-ventricular interval
(AVI). The atrio-ventricular interval is triggered by an atrial
sense or pace and stopped by a ventricular sense or pace. A
ventricular pace is delivered upon expiration of the
atrio-ventricular interval if no ventricular sense occurs before.
Atrial-tracking ventricular pacing attempts to maintain the
atrio-ventricular synchrony occurring with physiological beats
whereby atrial contractions augment diastolic filling of the
ventricles. If a patient has a physiologically normal atrial
rhythm, atrial-tracking pacing also allows the ventricular pacing
rate to be responsive to the metabolic needs of the body.
[0017] A pacemaker can also be configured to pace the atria on an
inhibited demand basis. An atrial escape interval is then defined
as the maximum time interval in which an atrial sense must be
detected after a ventricular sense or pace before an atrial pace
will be delivered. When atrial inhibited demand pacing is combined
with atrial-triggered ventricular demand pacing (i.e., DDD mode),
the lower rate interval is then the sum of the atrial escape
interval and the atrio-ventricular interval.
[0018] Another type of synchronous pacing is atrial-triggered or
ventricular-triggered pacing. In this mode, an atrium or ventricle
is paced immediately after an intrinsic beat is detected in the
respective chamber. Triggered pacing of a heart chamber is normally
combined with inhibited demand pacing so that a pace is also
delivered upon expiration of an escape interval in which no
intrinsic beat occurs. Such triggered pacing may be employed as a
safer alternative to asynchronous pacing when, due to far-field
sensing of electromagnetic interference from external sources or
skeletal muscle, false inhibition of pacing pulses is a problem. If
a sense in the chamber's sensing channel is an actual
depolarization and not a far-field sense, the triggered pace is
delivered during the chamber's physiological refractory period and
is of no consequence.
[0019] Finally, rate-adaptive algorithms may be used in conjunction
with bradycardia pacing modes. Rate-adaptive pacemakers modulate
the ventricular and/or atrial escape intervals based upon
measurements corresponding to physical activity. Such pacemakers
are applicable to situations in which atrial tracking modes cannot
be use. In a rate-adaptive pacemaker operating in a ventricular
pacing mode, the LRL is adjusted in accordance with exertion level
measurements such as from an accelerometer or minute ventilation
sensor in order for the heart rate to more nearly match metabolic
demand. The adjusted LRL is then termed the sensor-indicated
rate.
3. Ventricular Rate Regularization
[0020] Ventricular rate regularization (VRR) is a ventricular
pacing mode in which the LRL of the pacemaker is dynamically
adjusted in accordance with a detected intrinsic ventricular rate.
When a pacemaker is operating in a ventricular demand pacing mode
(e.g., VVI), intrinsic ventricular beats occur when the
instantaneous intrinsic rate rises above the LRL of the pacemaker.
Thus, paces can be interspersed with intrinsic beats, and the
overall ventricular rhythm as a result of both paces and intrinsic
beats is determined by the LRL and the mean value and variability
of the intrinsic ventricular rate. VRR regularizes the overall
ventricular rhythm by adjusting the LRL of the pacemaker in
accordance with changes in the measured intrinsic rate.
[0021] The intrinsic ventricular rate is the rate at which
intrinsic ventricular beats occur and can be defined both
instantaneously and as being at some mean value with a certain
variability about that mean. The instantaneous intrinsic rate can
be determined by measuring an R-R interval, where an R-R interval
is the time between a present ventricular sense (i.e., an R-wave or
intrinsic ventricular depolarization) and the preceding ventricular
sense or ventricular pace, with the instantaneous rate being the
reciprocal of the measured interval. The mean intrinsic rate can be
determined by averaging the instantaneous R-R intervals over a
period of time. The LRL of a pacemaker is initially set to a
programmed base value and defines the ventricular escape interval,
which is the maximum time between ventricular beats allowed by the
pacemaker and is the reciprocal of the LRL. At any particular mean
intrinsic rate above the LRL, a ventricular pace is delivered only
when, due to the variability in the intrinsic rate, an R-R interval
would be longer than the ventricular escape interval were it
allowed to occur. As the mean intrinsic ventricular rate increases
above the LRL, fewer paces are delivered and more variability in
the overall ventricular rhythm is allowed. The VRR pacing mode
counteracts this by increasing the LRL as the mean intrinsic
ventricular rate increases to thereby increase the frequency of
paced beats which decreases the incidence of long intrinsic R-R
intervals and thus lessens the variability in the overall
ventricular rate. The VRR mode then decreases the LRL toward its
base value as the number of paces delivered increases due to a
decrease in either the mean intrinsic ventricular rate or its
variability. The LRL adjusted in this manner is also referred to
herein as the VRR-indicated rate.
[0022] In one embodiment of VRR, the LRL is adjusted to increase
toward a programmed maximum value by measuring an R-R interval when
a ventricular sense occurs and then computing an updated
ventricular escape interval based upon the measured R-R interval.
When a ventricular pace is delivered, on the other hand, the LRL is
made to decay toward the programmed base value. FIG. 2 shows an
exemplary implementation of a VRR system made up of a pair of
filters 515 and 516 which may be implemented as software executed
by the controller 10 and/or with discrete components. Filter 515 is
employed to compute the updated ventricular escape interval when a
ventricular sense occurs, and filter 516 is used when a ventricular
pace is delivered.
[0023] When a ventricular sense occurs, the measured R-R interval
is input to a recursive digital filter 515 whose output is the
updated ventricular escape interval. The filter 515 multiplies the
measured R-R interval by a filter coefficient A and then adds the
result to the previous value of the output (i.e., the present
ventricular escape interval) multiplied by a filter coefficient B.
The operation of the filter is thus described by
VEI.sub.N=A(RR.sub.n)+B(VEI.sub.n-1), where A and B are selected
coefficients, RR.sub.n is the most recent R-R interval duration,
and VEI.sub.n-1 is the previous value of the ventricular escape
interval. A useful way to conceptualize the filter 515 is to
decompose the coefficients A and B into a scaling factor a and a
weighting coefficient w such that A=a.multidot.w and B=(1-w), where
w is between 0 and 1. Viewed this way, the filter is seen as
computing a weighted average of the present R-R interval multiplied
by the scaling factor a and the present ventricular escape
interval. The filter thus causes the value of the ventricular
escape interval to move toward the present R-R interval multiplied
by the scaling factor at a rate determined by the weighting
coefficient. This corresponds to the filter moving the pacemaker's
LRL toward a fraction 1/a of the instantaneous intrinsic
ventricular rate, up to a maximum pacing rate MPR, as determined by
the measured R-R interval. If a ventricular sense has occurred, the
current LRL is necessarily less than the measured instantaneous
intrinsic ventricular rate. If it is also less than 1/a of the
intrinsic rate, the LRL is increased by the filter up to a value
that is 1/a of the intrinsic rate (as limited by the MPR) to result
in more pacing and less variability in the overall ventricular
rhythm.
[0024] When a ventricular pace is delivered due to expiration of
the ventricular escape interval without a ventricular sense, filter
516 multiplies the present ventricular escape interval by a filter
coefficient C so that VEI.sub.n=C(VEI.sub.n-1). To provide stable
operation, the coefficient C must be set to a value greater than 1.
Filter 516 then causes the ventricular escape interval to increase
in an exponential manner with each pace as successive values of the
escape interval are input to the filter up to a value corresponding
to the base LRL.
[0025] The updating of the ventricular escape interval may be
performed in various ways including on a beat-to-beat basis, at
periodic intervals, or with averages of successive R-R intervals.
In a presently preferred embodiment, however, the updating is
performed on a beat-to-beat basis with each ventricular sense or
pace causing adjustment of the LRL by filter 515 or 516,
respectively. The two filters operating together thus cause the LRL
to move closer to 1/a of the measured intrinsic rate (up to the
MPR) after a ventricular sense and to decay toward the base LRL
value after a ventricular pace.
[0026] The coefficients a and w (or A and B) and C are selected by
the user and may be made programmable so that the behavior of the
system can be adjusted to produce the clinically best result in an
individual patient. For example, as the scaling factor a is made
greater than 1, the filter 515 causes the LRL to move toward a
smaller fraction 1/a of the detected intrinsic rate which allows
more intrinsic beats to occur and greater variability in the
overall rhythm. As a is decreased back toward 1, the filter 515
tends to move the LRL of the pacemaker toward a larger fraction of
the detected instantaneous intrinsic rate, thus increasing the
amount of pacing and decreasing the amount of variability allowed
in the overall ventricular rhythm. If a is made smaller than 1, the
LRL is moved toward a rate higher than the intrinsic rate, further
increasing the amount of pacing to a point where most of the
ventricular rhythm is made up of paced beats. The larger the
weighting factor w, the faster the LRL is moved to the specified
fraction of the intrinsic rate, making the system more responsive
to increases in the variability of the intrinsic rhythm. The larger
the decay coefficient C, the more rapidly will filter 516 cause the
LRL to decrease toward its programmed base value when ventricular
paces are delivered due to no ventricular senses being detected
within the ventricular escape interval. The controller limits the
updated ventricular escape interval as a result of the operations
of filters 515 and 516 to minimum and maximum values in accordance
with the programmed maximum pacing rate MPR and base lower rate
limit LRL, respectively.
[0027] As noted, the coefficients of filters 515 and 516 can be
made programmable by the user, such as by using a remote
programmer. In another embodiment, the user selects a desired
performance parameter (e.g., desired degree of rate regularization,
desired amount of pacing, desired decay rate, etc.) from a
corresponding range of possible values. The appropriate
combinations of coefficients for filters 515 and 516 are then
automatically selected to provide filter settings that correspond
to the selected user-programmed performance parameter. The filter
coefficients can also be made functions of other parameters, such
as the measured R-R interval and current LRL setting, and
dynamically adjusted.
[0028] The VRR system in this embodiment uses the programmed base
LRL of the pacemaker as the lower limit to which the LRL is
permitted to decay when no ventricular senses are detected. The
base LRL can be changed periodically by the user with an external
programmer, and certain pacemakers also have the capability of
dynamically adjusting the LRL in order to adapt to exercise. In
such rate-adaptive pacemakers, the LRL is adjusted in accordance
with exertion level measurements such as from an accelerometer or
minute ventilation sensor in order for the heart rate to more
nearly match metabolic demand. The adjusted LRL is then termed the
sensor-indicated rate. If a rate-adaptive pacemaker is operated in
a VRR mode, the sensor-indicated rate can simply be regarded by the
pacemaker as the base LRL. The lower limit for the VRR-indicated
rate is then the sensor-indicated rate rather than the programmed
base LRL.
[0029] VRR can thus be employed to modify conventional bradycardia
pacing in order to improve the deleterious hemodynamic effects
brought about by an irregular intrinsic ventricular rhythm. As will
be described below, VRR also has some special advantages when used
in conjunction with other cardiac rhythm management therapies.
4. Ventricular Rate Regularization with Cardiac Resynchronization
Therapy
[0030] Heart failure is clinical syndrome in which an abnormality
of cardiac function causes cardiac output to fall below a level
adequate to meet the metabolic demand of peripheral tissues and is
usually referred to as congestive heart failure (CHF) due to the
accompanying venous and pulmonary congestion. CHF can be due to a
variety of etiologies with ischemic heart disease being the most
common. Some CHF patients suffer from some degree of AV block or
are chronotropically deficient such that their cardiac output can
be improved with conventional bradycardia pacing. Such pacing,
however, may result in some degree of uncoordination in atrial
and/or ventricular contractions due to the way in which pacing
excitation is typically spread throughout the myocardium without
the benefit of the heart's specialized conduction system. The
resulting diminishment in cardiac output may be significant in a
CHF patient whose cardiac output is already compromised.
Intraventricular and/or interventricular conduction defects (e.g.,
bundle branch blocks) are also commonly found in CHF patients. In
order to treat these problems, cardiac rhythm management devices
have been developed which provide pacing stimulation to one or more
heart chambers in an attempt to improve the coordination of atrial
and/or ventricular contractions, termed cardiac resynchronization
therapy.
[0031] Cardiac resynchronization therapy is pacing stimulation
applied to one or more heart chambers in a manner that restores or
maintains synchronized bilateral contractions of the atria and/or
ventricles and thereby improves pumping efficiency. Certain
patients with conduction abnormalities may experience improved
cardiac synchronization with conventional single-chamber or
dual-chamber pacing as described above. For example, a patient with
left bundle branch block may have a more coordinated contraction of
the ventricles with a pace than as a result of an intrinsic
contraction. In that sense, conventional bradycardia pacing of an
atrium and/or a ventricle may be considered as resynchronization
therapy. Resynchronization pacing, however, may also involve pacing
both ventricles and/or both atria in accordance with a synchronized
pacing mode as described below. A single chamber may also be
resynchronized to compensate for intra-atrial or intra-ventricular
conduction delays by delivering paces to multiple sites of the
chamber.
[0032] It is advantageous to deliver resynchronization therapy in
conjunction with one or more synchronous bradycardia pacing modes,
such as are described above. One atrial and/or one ventricular
pacing sites are designated as rate sites, and paces are delivered
to the rate sites based upon pacing and sensed intrinsic activity
at the site in accordance with the bradycardia pacing mode. In a
single-chamber bradycardia pacing mode, for example, one of the
paired atria or one of the ventricles is designated as the rate
chamber. In a dual-chamber bradycardia pacing mode, either the
right or left atrium is selected as the atrial rate chamber and
either the right or left ventricle is selected as the ventricular
rate chamber. The heart rate and the escape intervals for the
pacing mode are defined by intervals between sensed and paced
events in the rate chambers only. Resynchronization therapy may
then be implemented by adding synchronized pacing to the
bradycardia pacing mode where paces are delivered to one or more
synchronized pacing sites in a defined time relation to one or more
selected sensing and pacing events that either reset escape
intervals or trigger paces in the bradycardia pacing mode. Multiple
synchronized sites may be paced through multiple synchronized
sensing/pacing channels, and the multiple synchronized sites may be
in the same or different chambers as the rate site.
[0033] In bilateral synchronized pacing, which may be either
biatrial or biventricular synchronized pacing, the heart chamber
contralateral to the rate chamber is designated as a synchronized
chamber. For example, the right ventricle may be designated as the
rate ventricle and the left ventricle designated as the
synchronized ventricle, and the paired atria may be similarly
designated. Each synchronized chamber is then paced in a timed
relation to a pace or sense occurring in the contralateral rate
chamber.
[0034] One synchronized pacing mode may be termed offset
synchronized pacing. In this mode, the synchronized chamber is
paced with a positive, negative, or zero timing offset as measured
from a pace delivered to its paired rate chamber, referred to as
the synchronized chamber offset interval. The offset interval may
be zero in order to pace both chambers simultaneously, positive in
order to pace the synchronized chamber after the rate chamber, or
negative to pace the synchronized chamber before the rate chamber.
One example of such pacing is biventricular offset synchronized
pacing where both ventricles are paced with a specified offset
interval. The rate ventricle is paced in accordance with a
synchronous bradycardia mode which may include atrial tracking, and
the ventricular escape interval is reset with either a pace or a
sense in the rate ventricle. (Resetting in this context refers to
restarting the interval in the case of an LRL ventricular escape
interval and to stopping the interval in the case of an AVI.) Thus,
a pair of ventricular paces are delivered after expiration of the
AVI escape interval or expiration of the LRL escape interval, with
ventricular pacing inhibited by a sense in the rate ventricle that
restarts the LRL escape interval and stops the AVI escape interval.
In this mode, the pumping efficiency of the heart will be increased
in some patients by simultaneous pacing of the ventricles with an
offset of zero. However, it may be desirable in certain patients to
pace one ventricle before the other in order to compensate for
different conduction velocities in the two ventricles, and this may
be accomplished by specifying a particular positive or negative
ventricular offset interval.
[0035] Another synchronized mode is triggered synchronized pacing.
In one type of triggered synchronized pacing, the synchronized
chamber is paced after a specified trigger interval following a
sense in the rate chamber, while in another type the rate chamber
is paced after a specified trigger interval following a sense in
the synchronized chamber. The two types may also be employed
simultaneously. For example, with a trigger interval of zero,
pacing of one chamber is triggered to occur in the shortest time
possible after a sense in the other chamber in order to produce a
coordinated contraction. (The shortest possible time for the
triggered pace is limited by a sense-to-pace latency period
dictated by the hardware.) This mode of pacing may be desirable
when the intra-chamber conduction time is long enough that the
pacemaker is able to reliably insert a pace before depolarization
from one chamber reaches the other. Triggered synchronized pacing
can also be combined with offset synchronized pacing such that both
chambers are paced with the specified offset interval if no
intrinsic activity is sensed in the rate chamber and a pace to the
rate chamber is not otherwise delivered as a result of a triggering
event. A specific example of this mode is ventricular triggered
synchronized pacing where the rate and synchronized chambers are
the right and left ventricles, respectively, and a sense in the
right ventricle triggers a pace to the left ventricle and/or a
sense in the left ventricle triggers a pace to the right
ventricle.
[0036] As with other synchronized pacing modes, the rate chamber in
a triggered synchronized pacing mode can be paced with one or more
synchronous bradycardia pacing modes. If the rate chamber is
controlled by a triggered bradycardia mode, a sense in the rate
chamber sensing channel, in addition to triggering a pace to the
synchronized chamber, also triggers an immediate rate chamber pace
and resets any rate chamber escape interval. The advantage of this
modal combination is that the sensed event in the rate chamber
sensing channel might actually be a far-field sense from the
synchronized chamber, in which case the rate chamber pace should
not be inhibited. In a specific example, the right and left
ventricles are the rate and synchronized chambers, respectively,
and a sense in the right ventricle triggers a pace to the left
ventricle. If right ventricular triggered pacing is also employed
as a bradycardia mode, both ventricles are paced after a right
ventricular sense has been received to allow for the possibility
that the right ventricular sense was actually a far-field sense of
left ventricular depolarization in the right ventricular channel.
If the right ventricular sense were actually from the right
ventricle, the right ventricular pace would occur during the right
ventricle's physiological refractory period and cause no harm.
[0037] As mentioned above, certain patients may experience some
cardiac resynchronization from the pacing of only one ventricle
and/or one atrium with a conventional bradycardia pacing mode. It
may be desirable, however, to pace a single atrium or ventricle in
accordance with a pacing mode based upon senses from the
contralateral chamber. This mode, termed synchronized chamber-only
pacing, involves pacing only the synchronized chamber based upon
senses from the rate chamber. One way to implement synchronized
chamber-only pacing is to pseudo-pace the rate chamber whenever the
synchronized chamber is paced before the rate chamber is paced,
such that the pseudo-pace inhibits a rate chamber pace and resets
any rate chamber escape intervals. Such pseudo-pacing can be
combined with the offset synchronized pacing mode using a negative
offset to pace the synchronized chamber before the rate chamber and
thus pseudo-pace the rate chamber, which inhibits the real
scheduled rate chamber pace and resets the rate chamber pacing
escape intervals. One advantage of this combination is that sensed
events in the rate chamber will inhibit the synchronized
chamber-only pacing, which may benefit some patients by preventing
pacing that competes with intrinsic activation (i.e., fusion
beats). Another advantage of this combination is that rate chamber
pacing can provide backup pacing when in a synchronized
chamber-only pacing mode, such that when the synchronized chamber
pace is prevented, for example to avoid pacing during the chamber
vulnerable period following a prior contraction, the rate chamber
will not be pseudo-paced and thus will be paced upon expiration of
the rate chamber escape interval. Synchronized chamber-only pacing
can be combined also with a triggered synchronized pacing mode, in
particular with the type in which the synchronized chamber is
triggered by a sense in the rate chamber. One advantage of this
combination is that sensed events in the rate chamber will trigger
the synchronized chamber-only pacing, which may benefit some
patients by synchronizing the paced chamber contractions with
premature contralateral intrinsic contractions.
[0038] An example of synchronized chamber-only pacing is left
ventricle-only synchronized pacing where the rate and synchronized
chambers are the right and left ventricles, respectively. Left
ventricle-only synchronized pacing may be advantageous where the
conduction velocities within the ventricles are such that pacing
only the left ventricle results in a more coordinated contraction
by the ventricles than with conventional right ventricular pacing
or biventricular pacing. Left ventricle-only synchronized pacing
may be implemented in inhibited demand modes with or without atrial
tracking, similar to biventricular pacing. A left ventricular pace
is then delivered upon expiration of the AVI escape interval or
expiration of the LRL escape interval, with left ventricular pacing
inhibited by a right ventricular sense that restarts the LRL escape
interval and stops the AVI escape interval.
[0039] In the synchronized modes described above, the rate chamber
is synchronously paced with a mode based upon detected intrinsic
activity in the rate chamber, thus protecting the rate chamber
against paces being delivered during the vulnerable period. In
order to provide similar protection to a synchronized chamber or
synchronized pacing site, a synchronized chamber protection period
(SCPP) may be provided. (In the case of multi-site synchronized
pacing, a similar synchronized site protection period may be
provided for each synchronized site.) The SCPP is a programmed
interval which is initiated by sense or pace occurring in the
synchronized chamber during which paces to the synchronized chamber
are inhibited. For example, if the right ventricle is the rate
chamber and the left ventricle is the synchronized chamber, a left
ventricular protection period LVPP is triggered by a left
ventricular sense which inhibits a left ventricular pace which
would otherwise occur before the escape interval expires. The SCPP
may be adjusted dynamically as a function of heart rate and may be
different depending upon whether it was initiated by a sense or a
pace. The SCPP provides a means to inhibit pacing of the
synchronized chamber when a pace might be delivered during the
vulnerable period or when it might compromise pumping efficiency by
pacing the chamber too close to an intrinsic beat. In the case of a
triggered mode where a synchronized chamber sense triggers a pace
to the synchronized chamber, the pacing mode may be programmed to
ignore the SCPP during the triggered pace. Alternatively, the mode
may be programmed such that the SCPP starts only after a specified
delay from the triggering event, which allows triggered pacing but
prevents pacing during the vulnerable period.
[0040] In the case of synchronized chamber-only synchronized
pacing, a synchronized chamber pace may be inhibited if a
synchronized chamber sense occurs within a protection period prior
to expiration of the rate chamber escape interval. Since the
synchronized chamber pace is inhibited by the protection period,
the rate chamber is not pseudo-paced and, if no intrinsic activity
is sensed in the rate chamber, it will be paced upon expiration of
the rate chamber escape intervals. The rate chamber pace in this
situation may thus be termed a safety pace. For example, in left
ventricle-only synchronized pacing, a right ventricular safety pace
is delivered if the left ventricular pace is inhibited by the left
ventricular protection period and no right ventricular sense has
occurred.
[0041] As noted above, synchronized pacing may be applied to
multiple sites in the same or different chambers. The synchronized
pacing modes described above may be implemented in a multi-site
configuration by designating one sensing/pacing channel as the rate
channel for sensing/pacing a rate site, and designating the other
sensing/pacing channels in either the same or the contralateral
chamber as synchronized channels for sensing/pacing one or more
synchronized sites. Pacing and sensing in the rate channel then
follows rate chamber timing rules, while pacing and sensing in the
synchronized channels follows synchronized chamber timing rules as
described above. The same or different synchronized pacing modes
may be used in each synchronized channel.
[0042] In any of the resynchronization pacing modes discussed
above, the effectiveness of the therapy is increased to the extent
that the frequency of pacing is increased. Accordingly, VRR may be
employed to increase the pacing frequency in a ventricular
resynchronization pacing mode by adjusting the filter coefficients
in the manner described above to result in more paced beats. The
LRL adjusted by the VRR filter in this case then corresponds to the
ventricular rate chamber escape interval.
[0043] As described above, ventricular rate regularization involves
adjusting the ventricular escape interval in accordance with
measured R-R intervals in order to decrease the variability in the
overall ventricular rhythm. The closer the length of the escape
interval is to the intrinsic R-R interval, however, the greater the
probability that a pace will be delivered coincident with an
intrinsic beat. Even though ventricular pacing is inhibited by a
ventricular sense occurring before expiration of the ventricular
escape interval, a ventricular depolarization may begin some
distance away from the sensing/pacing electrode in a different
ventricle. The depolarization may then not be sensed in time to
inhibit the pacing pulse because of the conduction delay before the
depolarization wave reaches the sensing electrode. The result is a
hemodynamically inefficient fusion beat which counteracts the
otherwise beneficial effects of ventricular rate regularization in
maintaining hemodynamic stability. In order to minimize this
possibility, an intrinsic ventricular activation should be sensed
as soon as possible in order to inhibit pacing. In typical heart
failure patients with left bundle branch block, for example, the
site of earliest ventricular activation is the right ventricle, and
a right ventricular sensing channel is necessary to minimize the
possibility of a fusion beat even when only the left ventricle is
paced. The most flexible configuration is to use biventricular
sensing channels so that the earliest activation occurring in
either ventricle can be detected and used to inhibit pacing of one
or both ventricles.
[0044] When the system is configured for VRR and ventricular
resynchronization pacing in a synchronized chamber-only mode, a
sensing refractory period can be initiated for the sensing channel
of the non-paced ventricle by a ventricular pace. During this
period, activity sensed by the channel are ignored for purposes of
inhibiting and triggering pacing pulses. This sensing refractory
period is of a duration sufficient to prevent detection of the
depolarization resulting from the pacing pulse and conducted by
cardiac tissue to the sensing electrode and can be
programmable.
[0045] As described above, the R-R interval can be defined as the
time between a present ventricular sense and the preceding
ventricular sense or ventricular pace, with the instantaneous rate
being the reciprocal of the measured interval. With biventricular
sensing, either ventricular sensing channel can be used for
defining the R-R intervals, but the first detected sense in a
cardiac cycle is preferably used. As an approximation to using the
first sense, advantage can be taken of the predominance of left
bundle branch blocks in the CHF patient population. In these
patients, the right ventricle depolarizes before the left
ventricle, and using a right ventricular sense to define the R-R
interval is a reasonable approximation for the first ventricular
sense. This approximation simplifies the VRR and pacing algorithms
when right ventricular senses are used to both define R-R intervals
for VRR implementation and to define the cardiac cycle for
bradycardia and anti-tachycardia pacing.
[0046] Alternatively, the R-R interval can be more particularly
defined, depending upon whether a uni-ventricular or biventricular
pacing mode is being used. In a uni-ventricular pacing mode, the
R-R interval can be defined as the time from either a first
ventricular sense or pace in the previous cardiac cycle to the
first ventricular sense in the current cycle. In a biventricular
pacing mode, the R-R interval can be defined as the time from a
first ventricular sense of the previous cycle to the first
ventricular sense of the current cycle, and as the time from a
ventricular pace in the previous cycle to the first ventricular
sense in the current cycle where the paced chamber of the previous
cycle is the same chamber as the first ventricular sense of the
current cycle.
[0047] The VRR algorithm adjusts the ventricular escape interval in
accordance with measurements of R-R intervals. In uni-ventricular
pacing with biventricular sensing, the ventricular escape interval
can be defined as the time between a first ventricular sense or
pace and a subsequent pace if no intrinsic activity is detected.
The ventricular escape interval may be more particularly defined
for biventricular pacing modes as the time from the first
ventricular sense of the previous cycle to a ventricular pace in
the current cycle where the paced chamber of the current cycle is
the same chamber as the first ventricular sense of the previous
cycle, and as the time from a ventricular pace in the previous
cycle to a ventricular pace in the same chamber during the current
cycle.
5. Ventricular Rate Regularization with Anti-tachyarrhythmia
Therapy
[0048] VRR may employed in ICDs implanted in patients who are prone
to ventricular arrhythmias but are not normally in need of either
bradycardia or resynchronization pacing. In these cases, the device
may detect a ventricular tachyarrythmia which does not warrant
either a defibrillation shock or anti-tachycardia pacing. VRR
pacing may then be initiated in order to improve the patient's
cardiac output and possibly lessen the chance of a more dangerous
tachyarrhythmia occurring. The device may be programmed to deliver
the VRR therapy for a specified length of time after each detection
of such a tachyarrhythmia.
[0049] VRR may also be employed in devices configured to deliver
atrial defibrillation shocks in order to both maintain hemodynamic
stability and to more safely deliver the shock. In order to avoid
the possible induction of ventricular fibrillation, atrial
defibrillation shocks should be delivered synchronously with a
sensed R wave and after a minimum pre-shock R-R interval. This is
done because the ventricle is especially vulnerable to induction of
fibrillation by a depolarizing shock delivered at a time too near
the end of the preceding ventricular contraction (i.e., close to
the T wave on an EKG). Delivering the shock synchronously with a
sensed R wave thus moves the shock away from the vulnerable period,
but at a very rapid ventricular rhythm, the ventricular beats may
be so close together that even synchronously delivered shocks may
induce ventricular fibrillation. Shocking should therefore be
delayed until the ventricular rhythm is slow enough to safely
deliver the defibrillation pulse as determined by measuring the R-R
interval. As noted above, however, the intrinsic ventricular rhythm
during atrial fibrillation tends to be both rapid and irregular. If
the intrinsic rhythm could be slowed and made more predictable, an
atrial defibrillation shock could be more safely delivered.
[0050] If AV conduction is intact in a patient, atrial fibrillation
results in a very rapid and intrinsic ventricular rhythm, and
regularizing the ventricular rate improves cardiac output directly
through its effect on diastolic filling. Ventricular rate
regularization may be applied in this instance with parameter
settings such that the ventricles are driven at a rate near the
intrinsic rate. The intrinsic ventricular rhythm that occurs during
an episode of atrial fibrillation is a result of the chaotically
occurring depolarizations occurring in the atria being passed
through the AV node to the ventricles. The intrinsic ventricular
rate is thus governed by the cycle length of the atrial
fibrillation and the refractory period of the AV node. If a
ventricular pacing pulse is delivered before the next intrinsic
beat occurs, the ventricular depolarization is conducted
retrogradely to the AV node causing late depolarization of the AV
node during the ventricular beat. The refractory period of the AV
node is also delayed, which delays the time before an atrial
depolarization can be conducted through the node to result in an
intrinsic beat. The effect of the pace is thus to lengthen the time
until the next intrinsic beat. Ventricular rate regularization at a
pacing rate near the intrinsic ventricular rate during atrial
fibrillation thus not only improves hemodynamics, but also
increases the probability that a shockable R-R interval will
occur.
[0051] Although the invention has been described in conjunction
with the foregoing specific embodiment, many alternatives,
variations, and modifications will be apparent to those of ordinary
skill in the art. Such alternatives, variations, and modifications
are intended to fall within the scope of the following appended
claims.
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