U.S. patent application number 09/748797 was filed with the patent office on 2002-06-27 for mode transition timing for synchronized pacing.
Invention is credited to Kalgren, James, Quiles, Sylvia, Stahmann, Jeffrey E., Vanderlinde, Scott, Wentkowski, Rene H..
Application Number | 20020082657 09/748797 |
Document ID | / |
Family ID | 25010965 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020082657 |
Kind Code |
A1 |
Stahmann, Jeffrey E. ; et
al. |
June 27, 2002 |
MODE TRANSITION TIMING FOR SYNCHRONIZED PACING
Abstract
A system and method for avoiding short-interval paces during
pacing mode transitions. The method may be particularly useful in
switching to or from a biventricular pacing mode.
Inventors: |
Stahmann, Jeffrey E.;
(Ramsey, MN) ; Wentkowski, Rene H.; (Overijse,
BE) ; Kalgren, James; (Lino Lakes, MN) ;
Quiles, Sylvia; (Coon Rapids, MN) ; Vanderlinde,
Scott; (Plymouth, MN) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Family ID: |
25010965 |
Appl. No.: |
09/748797 |
Filed: |
December 26, 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 rhythm management device,
comprising: pacing a synchronized pacing site in accordance with a
first pacing mode in which the synchronized site is paced at a
specified pacing instant defined with respect to expiration of an
escape interval which is reset by senses or paces occurring at a
rate site; transitioning to a second pacing mode in which the
synchronized site is paced at an earlier specified pacing instant
defined with respect to expiration of an escape interval than in
the first pacing mode; and, delaying a pace to the synchronized
site during a cardiac cycle in which the transition occurs if the
pacing instant occurs at a pacing interval less than a specified
minimum pacing interval.
2. The method of claim 1 wherein the pace to the synchronized site
is delayed so as to increase the pacing interval to the specified
minimum pacing interval.
3. The method of claim 2 wherein the specified minimum pacing
interval is equal to the escape interval.
4. The method of claim 1 wherein expiration of the escape interval
is also delayed if the pacing instant for the synchronized site
occurs at a pacing interval less than the specified minimum pacing
interval.
5. The method of claim 1 wherein the synchronized site is in the
left ventricle and the rate site is in the right ventricle such
that and left ventricular paces are delivered upon expiration of a
ventricular escape interval based upon right ventricular
events.
6. The method of claim 5 wherein either the first or second pacing
mode is a biventricular pacing mode with a left ventricular
offset.
7. The method of claim 5 wherein the first pacing mode is a
biventricular pacing mode with a positive left ventricular offset
interval, and the second pacing mode is a left ventricular-only
pacing mode.
8. The method of claim 7 wherein a right ventricular safety pace is
delivered if the left ventricular pace is inhibited.
9. The method of claim 5 wherein the first pacing mode is a left
ventricular-only pacing mode, and the second pacing mode is a
biventricular pacing mode with a negative left ventricular offset
interval.
10. The method of claim 5 wherein the first pacing mode is a
biventricular pacing mode with a positive left ventricular offset
interval, and the second pacing mode is a biventricular pacing mode
with a negative left ventricular offset interval.
11. The method of claim 5 wherein the first pacing mode is a
biventricular pacing mode with a positive left ventricular offset
interval, and the second pacing mode is a biventricular pacing mode
with no left ventricular offset interval.
12. The method of claim 5 wherein the first pacing mode is a
biventricular pacing mode with no left ventricular offset interval,
and the second pacing mode is a biventricular pacing mode with a
negative left ventricular offset interval.
13. A cardiac rhythm management device, comprising: sensing/pacing
channels for sensing and pacing a plurality of sites; a controller
for controlling the delivery of paces to a pacing site in
accordance with a programmed pacing mode, wherein the controller is
programmed to: pace a selected site designated as a synchronized
site in accordance with a first pacing mode in which the
synchronized site is paced at a specified pacing instant defined
with respect to expiration of an escape interval which is reset by
a sense or pace occurring at another site designated as the rate
site; transition to a second pacing mode in which the synchronized
site is paced at an earlier specified pacing instant defined with
respect to expiration of an escape interval than in the first
pacing mode; and, delay a pace to the synchronized site during a
cardiac cycle in which the transition occurs if the pacing instant
occurs at a pacing interval less than a specified minimum pacing
interval.
14. The device of claim 13 wherein the pace to the synchronized
site is delayed so as to increase the pacing interval to the
specified minimum pacing interval.
15. The device of claim 14 wherein the specified minimum pacing
interval is equal to the escape interval.
16. The device of claim 13 wherein expiration of the escape
interval is also delayed if the pacing instant for the synchronized
site occurs at a pacing interval less than the specified minimum
pacing interval.
17. The device of claim 13 wherein the synchronized site is in the
left ventricle and the rate site is in the right ventricle such
that and left ventricular paces are delivered upon expiration of a
ventricular escape interval based upon right ventricular
events.
18. A method for operating a cardiac rhythm management device,
comprising: pacing a synchronized pacing site in accordance with a
first pacing mode in which the synchronized site is paced at a
specified pacing instant defined with respect to expiration of an
escape interval which is reset by senses or paces occurring at a
rate site; transitioning to a second pacing mode in which the
synchronized site is paced at an earlier specified pacing instant
defined with respect to expiration of an escape interval than in
the first pacing mode; and, inhibiting a pace to the synchronized
site during a cardiac cycle in which the transition occurs if the
pacing instant occurs at a pacing interval less than a specified
minimum pacing interval.
19. A cardiac rhythm management device, comprising: sensing/pacing
channels for sensing and pacing a plurality of sites; a controller
for controlling the delivery of paces to a pacing site in
accordance with a programmed pacing mode, wherein the controller is
programmed to: pace a selected site designated as a synchronized
site in accordance with a first pacing mode in which the
synchronized site is paced at a specified pacing instant defined
with respect to expiration of an escape interval which is reset by
a sense or pace occurring at another site designated as the rate
site; transition to a second pacing mode in which the synchronized
site is paced at an earlier specified pacing instant defined with
respect to expiration of an escape interval than in the first
pacing mode; and, inhibit a pace to the synchronized site during a
cardiac cycle in which the transition occurs if the pacing instant
occurs at a pacing interval less than a specified minimum pacing
interval.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to methods and apparatus for cardiac
rhythm management. In particular, the invention relates to methods
and apparatus for providing ventricular resynchronization
therapy.
BACKGROUND
[0002] Cardiac rhythm management devices are implantable devices
that provide electrical stimulation to selected chambers of the
heart in order to treat disorders of cardiac rhythm. A pacemaker,
for example, is a cardiac rhythm management device that paces the
heart with timed pacing pulses. (As the term is used herein, a
pacemaker is any cardiac rhythm management device with a pacing
functionality regardless of any other functions it may perform such
as cardioversion or defibrillation.) The most common condition for
which pacemakers are used is in the treatment of bradycardia, where
the ventricular rate is too slow. Atrio-ventricular conduction
defects (i.e., AV block) that are permanent or intermittent and
sick sinus syndrome represent the most common causes of bradycardia
for which permanent pacing may be indicated. If functioning
properly, the pacemaker makes up for the heart's inability to pace
itself at an appropriate rhythm in order to meet metabolic demand
by enforcing a minimum heart rate.
[0003] Pacing therapy can also be used in the treatment of
congestive heart failure (CHF), which is a clinical syndrome in
which an abnormality of cardiac function causes cardiac output to
fall below a level adequate to meet metabolic demand. CHF can be
due to a variety of etiologies with that due to 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. It has also been shown, however, that some CHF patients
suffer from intraventricular and/or interventricular conduction
defects (e.g., bundle branch blocks) such that their cardiac
outputs can be increased by improving the synchronization of right
and left ventricular contractions with electrical stimulation.
Cardiac rhythm management devices have therefore been developed
which provide electrical stimulation to one or both ventricles in
an attempt to improve the coordination of ventricular contractions,
termed ventricular resynchronization therapy. Such electrical
stimulation will be referred to herein as "pacing" even if the
stimulation is not delivered so as to enforce a particular heart
rate.
SUMMARY OF THE INVENTION
[0004] Cardiac resynchronization pacing modes involve the delivery
of paces to a pacing site based upon events occurring at another
site. In such modes, one chamber is designated the rate chamber or
rate site, and one or more pacing sites in the same or
contralateral chamber are designated synchronized sites. The
synchronized sites are paced upon expiration of escape intervals
which are reset by senses or paces occurring at the rate site. The
present invention is concerned with the effect of transitions
between pacing modes in which a pacing site is paced at different
pacing instants with respect to expiration of a programmed escape
interval that is reset by a sense or pace from another site. During
the cardiac cycle in which the pacing mode transition takes place,
a synchronized site may receive a pace at an abnormally short
pacing interval if it is paced at an earlier pacing instant with
respect to the escape interval expiration in the mode that is
switched to, where a pacing interval is the time between successive
paces delivered to the site in the absence of intrinsic activity.
This is because the pacing mode itself does not directly control
the interval at which a pace occurs until after the transition is
completed. Such a shortened pacing interval may have adverse
consequences. In accordance with the present invention, if a pacing
interval for a pacing site would be below a specified minimum
pacing interval value during a pacing mode transition, the pace may
be inhibited or the pacing interval lengthened for the cardiac
cycle in which the transition takes place.
[0005] Exemplary embodiments of the invention are applied to
situations where the left or both ventricles are paced upon
expiration of escape intervals reset by right ventricular senses or
paces, and where there is a transition to or from a biventricular
pacing mode with a ventricular offset interval. In those
situations, the left ventricle is paced in accordance with a first
pacing mode such that a pace is delivered at a specified pacing
instant defined with respect to expiration of a ventricular escape
interval, and a transition is made to a second pacing mode in which
the left ventricle is paced at an earlier specified pacing instant
defined with respect to expiration of a ventricular escape interval
than in the first pacing mode. In accordance with the invention, a
left ventricular pace is delayed during the cardiac cycle in which
the transition occurs if the left ventricular pacing instant occurs
at an interval less than a specified minimum pacing interval. In
another embodiment, the pace is inhibited for that cardiac cycle if
the pacing interval would be below the specified minimum value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a system diagram of a pacemaker configured for
biventricular pacing and sensing.
[0007] FIG. 2 illustrates a biventricular synchronized pacing
mode.
[0008] FIGS. 3A through 3E show examples of pacing timing diagrams
during mode transitions.
DESCRIPTION OF THE INVENTION
[0009] As is described more fully below, a pacing mode defines how
pacing pulses are delivered to the heart by a pacemaker in response
to sensed events and expiration of specified time intervals, the
latter sometimes referred to as escape intervals. Certain pacing
modes that pace multiple sites, such as resynchronization pacing,
may define different pacing instants for different pacing sites
with respect to the expiration of an escape interval. A pacemaker
may be capable of operating in a number of different pacing modes
and be capable of transitioning between modes either upon command
from an external programmer or automatically under certain
circumstances. When transitioning to or from such a pacing mode, a
pacing site may be paced at a shorter interval than would otherwise
be allowed by the pacemaker. This may have the effect of triggering
the fault-protection circuitry in the pacemaker which is designed
to protect against overpacing. There is also the risk that pacing a
ventricle at an abnormally short pacing interval may induce an
arrhythmia. The present invention is directed toward providing a
system and method for dealing with this problem.
[0010] 1. Hardware Platform
[0011] Pacemakers are typically implanted subcutaneously or
submuscularly in a patient's chest and have leads threaded
intravenously into the heart to connect the device to electrodes
used for sensing and pacing. Leads may also be positioned on the
epicardium by various means. A programmable electronic controller
causes the pacing pulses to be output in response to lapsed time
intervals and sensed electrical activity (i.e., intrinsic heart
beats not as a result of a pacing pulse). Pacemakers sense
intrinsic cardiac electrical activity by means of internal
electrodes disposed near the chamber to be sensed. A depolarization
wave associated with an intrinsic contraction of the atria or
ventricles that is detected by the pacemaker is referred to as an
atrial sense or ventricular sense, respectively. In order to cause
such a contraction in the absence of an intrinsic beat, a pacing
pulse (either an atrial pace or a ventricular pace) with energy
above a certain pacing threshold is delivered to the chamber.
[0012] FIG. 1 shows a system diagram of a microprocessor-based
pacemaker physically configured with sensing and pacing channels
for both atria and both ventricles. The controller 10 of the
pacemaker 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 electrodes 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. A pacemaker incorporating the
present invention may possess all of the components in FIG. 1 and
be programmable so as to operate in a number of different modes, or
it may have only those components necessary to operate in a
particular mode.
[0013] The controller 10 controls the overall operation of the
device in accordance with programmed instructions stored in memory.
The controller 10 interprets sense signals from the sensing
channels and controls the delivery of paces in accordance with a
pacing mode. The sensing circuitry of the pacemaker generates
atrial and ventricular sense signals when voltages sensed by the
electrodes exceed a specified threshold. The sense signals from
each channel, together with the paces delivered, represent an
electrogram that can either be transmitted via the telemetry link
to an external programmer or stored for later transmission. The
operation of the pacemaker and the patient's cardiac activity may
thus be observed in real-time or over a selected historical period.
In the latter case, the recording of an electrogram may be
triggered by the detection of certain events or conditions such as
an arrhythmia.
[0014] 2. Bradycardia Pacing Modes
[0015] Bradycardia pacing modes refer to pacing algorithms used to
pace the atria and/or ventricles when the intrinsic atrial and/or
ventricular rate is inadequate due to, for example, 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 O (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.
[0016] 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).
[0017] 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.
[0018] 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 limit interval is then the sum of the atrial escape
interval and the atrio-ventricular interval.
[0019] 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.
[0020] 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 used. In a rate-adaptive pacemaker operating in a ventricular
pacing mode, for example, 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.
[0021] 3. Cardiac Resynchronization Therapy
[0022] 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.
[0023] It is advantageous to deliver resynchronization therapy in
conjunction with one or more synchronous bradycardia pacing modes,
such as are described above. One atrium and/or one ventricle are
designated as rate chambers, and paces are delivered to the rate
chambers based upon pacing and sensed intrinsic activity in the
chamber 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. 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 in accordance
with a synchronized pacing mode as described below.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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. 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.
[0028] 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 the synchronized chamber, a
synchronized chamber protection period (SCPP) may be provided. 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.
[0029] 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.
[0030] Synchronized pacing may be applied to multiple sites of a
single chamber. In these synchronized modes, one sensing/pacing
channel is designated as the rate channel for sensing/pacing a rate
site, and the other sensing/pacing channels in either the same or
the contralateral chamber are designated as synchronized channels
for sensing one or more synchronized sites. Pacing and sensing in
the rate channel 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.
[0031] 4. Mode Transition Timing
[0032] In inhibited demand pacing modes, paces are always delivered
upon expiration of an escape interval. When a ventricle is paced
upon expiration of an escape interval that is restarted by a sense
or pace in that ventricle, for example, the pacing instant for the
ventricle is necessarily at the same time that the escape interval
expires. Certain pacing modes, however, particularly those used for
cardiac resynchronization, involve pacing a synchronized site at a
pacing instant defined with respect to expiration of an escape
interval that is reset by an event occurring at another site. In
these modes, the pacing instant for a pacing site does not
necessarily coincide with the time that the escape interval
expires.
[0033] For example, in biventricular pacing based upon right
ventricular events, the ventricular escape interval is reset by a
right ventricular sense or pace. The right ventricle in this mode
is always paced upon expiration of the escape interval, but the
pace for the left ventricle may either precede or follow the right
ventricular pace in accordance with the specified biventricular
offset interval. When transitioning to a pacing mode in which the
pacing instant for the left ventricle occurs earlier with respect
to the escape interval expiration than in the previous pacing mode,
the left ventricle will be paced at a pacing interval shorter than
the programmed escape interval during the cardiac cycle that the
transition takes place, where the pacing interval is the time
between successive left ventricular paces in the absence of
intrinsic activity. This may occur, for example, when a pacemaker
switches to or from a biventricular pacing mode in which paces to
the two ventricles are separated by an offset interval. The
examples that follow will deal with this specific situation.
[0034] FIG. 2 is a diagram of right and left pacing channels
labeled RV and LV that shows how the pacing instants for the left
ventricle and right ventricle may differ in a biventricular pacing
mode based upon right ventricular events. The right ventricular
pace RVP is delivered upon expiration of a ventricular escape
interval. The left ventricular pace LVP may be delivered coincident
with the right ventricular pace or may occur before or after the
right ventricular pace as specified by a negative or positive
ventricular offset interval, respectively.
[0035] FIGS. 3A through 3E are timing diagrams showing the sequence
of right and left ventricular paces (designated as RVP and LVP,
respectively) during mode transitions in which the left ventricle
may be paced prematurely. The pace delivered upon expiration of the
escape interval is designated by an arrow. FIG. 3A shows the timing
of left and right ventricular paces when a pacemaker switches from
a left ventricular-only pacing mode to a biventricular mode with a
negative left ventricular offset interval. FIG. 3B shows the
transition from a biventricular pacing mode with a positive left
ventricular offset interval to a left ventricular-only mode, and
FIG. 3C shows the transition from a biventricular mode with a
positive left ventricular offset interval to a biventricular mode
with a negative left ventricular offset interval. FIG. 3D
illustrates the transition from a biventricular mode with a
positive left ventricular offset to a biventricular mode with no
offset, and FIG. 3E shows the transition from a biventricular mode
with no offset to a biventricular mode with a negative left
ventricular offset. In all of these cases, the pacing instant for
the left ventricle occurs at an interval shorter than the
programmed escape interval during the cardiac cycle in which the
mode transition takes place
[0036] The present invention provides a mechanism for avoiding
short-interval paces during mode transitions such as described
above. In synchronized pacing modes, one heart chamber or site is
paced through a rate sensing/pacing channel and another site is
paced through a synchronized sensing/pacing channel with the pacing
mode being based upon senses and paces in the rate channel. In
accordance with the present invention, if a pacing interval for a
synchronized pacing site would be below a specified minimum pacing
interval value during a pacing mode transition, the pace to the
synchronized site is inhibited for the cardiac cycle in which the
transition takes place. In another embodiment, the pacing interval
is lengthened by delaying the pace to the synchronized site.
[0037] In one embodiment of the invention, a minimum pacing
interval value is maintained such that if the pacing interval for a
pacing site falls below that value during a mode transition, the
pacing instant for the site is delayed by an amount necessary to
increase the pacing interval to the minimum value. For example, in
the examples illustrated by FIGS. 3A through 3E, the left
ventricular pacing instant occurs prematurely at a shorter pacing
interval than the programmed ventricular escape interval.
Accordingly, the left ventricular pace is delayed until the pace
occurs at a minimum pacing interval value. The paces to other sites
may be left undisturbed or also delayed to preserve a programmed
offset interval. The minimum pacing interval value may be made to
correspond with other programmable limits on the pacing rate such
as the maximum tracking rate MTR, which limits how fast the atria
are allowed to trigger ventricular paces in an atrial tracking
mode, or the maximum sensor-indicated rate, which limits how much a
rate-adaptive sensor may increase the pacing rate. The minimum
pacing interval in the case of left ventricular pacing may also be
made to correspond to the left ventricular protective period
described above. Alternatively, the minimum pacing interval may be
the maximum among a selected number of any other programmable
pacing rate limits or may be a parameter separately specified by
the user without regard to other programmable limits on the pacing
rate.
[0038] In another embodiment, if a left ventricular pacing instant
occurs at an interval shorter than the minimum pacing interval, the
left ventricular pace site is inhibited. When a pacemaker
transitions to a left ventricular-only pacing mode from a
biventricular mode with a positive offset interval as illustrated
in FIG. 3B, this embodiment may cause no pace at all to be
delivered during the cardiac cycle in which the transition occurs,
similar to the situation when a left ventricular pace is inhibited
by a left ventricular sense in a left ventricle-only synchronized
pacing mode. To avoid this no pace condition, a right ventricular
pace may be delivered when the left ventricular pace is inhibited
by the maximum pacing interval, referred to as a right ventricular
safety pace.
[0039] The examples above have dealt with biventricular pacing
modes that are based upon right ventricular events. It should be
appreciated that the invention could equally as well be applied to
biventricular pacing modes based upon left ventricular events,
biatrial pacing, and to multi-site pacing modes.
[0040] 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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