# Patent

## Title
Methods for promoting intrinsic activation in single chamber implantable cardiac pacing systems

## Abstract

Cardiac pacing methods for an implantable single chamber pacing system, establish an offset rate for pacing at a predetermined decrement from either a baseline rate (i.e. dictated by a rate response sensor), or an intrinsic rate. Pacing maintains the offset rate until x of y successive events are paced events, at which time the offset rate is switched to the baseline rate for pacing over a predetermined period of time. Following the period, if an intrinsic event is not immediately detected, within the interval of the offset rate, the rate is switched back to baseline for pacing over an increased period of time. Some methods establish a preference rate, between the offset and baseline rates, wherein an additional criterion, for switching from the offset rate to the baseline rate, is established with respect to the preference rate.

## Background
TECHNICAL FIELD
The present invention pertains to cardiac pacing methods and more particularly to pacing methods that promote intrinsic activation of ventricular depolarization to preserve natural conduction and increase system efficiency in single chamber implantable cardiac pacing systems.
BACKGROUND
The traditional implantable cardiac pacemaker includes a pulse generator device to which one or more flexible elongate lead wires are coupled. The device is typically implanted in a subcutaneous pocket, remote from the heart, and each of the one or more lead wires extends therefrom to a corresponding electrode, coupled thereto and positioned at a pacing site, either endocardial or epicardial. Mechanical complications and/or MRI compatibility issues, which are sometimes associated with elongate lead wires and well known to those skilled in the art, have motivated the development of cardiac pacing systems that are wholly contained within a relatively compact package for implant in close proximity to the pacing site, for example, within the right ventricle (RV) of the heart. With reference to FIGS. 1A-B, such a system 100 is illustrated, wherein pace/sense electrodes 111, 112 are formed on an exterior surface of a capsule 101 that hermetically contains a pulse generator 103 (shown in FIG. 1B via a block diagram). FIG. 1A further illustrates tine members 115 mounted to an end of capsule 101, in proximity to electrode 111, in order to secure electrode 111 against the endocardial surface of RV, and electrode 112 offset distally from electrode 111. Capsule 101 is preferably formed from a biocompatible and biostable metal such as titanium overlaid with an insulative layer, for example, medical grade polyurethane or silicone, except where electrode 112 is formed as an exposed portion of capsule 101. An hermetic feedthrough assembly (not shown), such as any known to those skilled in the art, couples electrode 111 to pulse generator 103 contained within capsule 103.
With further reference to FIGS. 1A-B, those skilled in the art will appreciate that system 100, via electrodes 111, 112, has the capability to sense intrinsic ventricular depolarization (i.e. R-waves) and, in the absence of the intrinsic depolarization, to apply stimulation pulses to the RV in order to create paced ventricular depolarization. Pulse generator 103 of system 100 further includes rate response sensor 135 that monitors a patient's general level of physical activity to determine an appropriate pacing rate for the patient. Examples of suitable rate response sensors include, without limitation, a force transducing sensor, such as a piezoelectric crystal like that described in commonly assigned U.S. Pat. No. 4,428,378 Anderson et al.; an AC or DC accelerometer like those described in commonly assigned U.S. Pat. No. 5,957,957 to Sheldon; and any type of physiological sensor known in the art, such as those that measure minute ventilation, QT intervals, blood pressure, blood pH, blood temperature, blood oxygen saturation etc. Numerous cardiac pacing methods that employ such RV pacing and sensing and physical activity monitoring are known in the art, for example, as disclosed in commonly assigned U.S. Pat. Nos. 4,428,378 (to Anderson et al.), 6,772,005 (to Casavant et al.), and 5,522,859 (to Stroebel et al.), as well as U.S. Pat. Nos. 5,374,281 (to Kristall et al.) and 6,122,546 (to Sholder et al.). Many of the aforementioned disclosures address the desire to limit the amount of pacing stimulation delivered from implantable pacemakers, particularly right ventricular stimulation in patients that have intact AV conduction (through the AV node, from the sinus node in the right atrial wall to the right and left bundle branches in the ventricular septum), in order to preserve the patient's natural conduction and increase pacemaker efficiency. However, the relatively more sophisticated pacing methods that are geared toward preserving the patient's natural conduction rely upon dual chamber sensing as these methods were developed in concert with the evolution of pacemaker systems from single chamber to dual chamber. Thus, there is a need for new cardiac pacing methods that preserve natural conduction and increase system efficiency for single chamber implantable pacing systems, of either the traditional type or the relatively compact type, like that shown in FIGS. 1A-B.
SUMMARY
Embodiments of the present invention include single chamber pacing systems that employ the methods disclosed. According to some methods of the present invention, an offset rate for pacing is established according to a predetermined decrement of either a baseline rate or a greater of the baseline rate and an intrinsic rate, wherein the baseline rate is established according to input from one or more rate response sensors. Pacing stimulation is applied when necessary to maintain the offset rate (for example, as determined via sensing for intrinsic ventricular depolarization), until x of y successive events (x>1 and y≧x) are paced events, at which time the offset rate is switched to the baseline rate and pacing stimulation at the baseline rate is applied over a predetermined period of time. According to some methods, the predetermined period of time may be shortened in response to the detection of intrinsic events occurring at a rate greater than the baseline rate. At the end of the predetermined period of time, sensing for intrinsic events resumes, and, according to some preferred methods, if an intrinsic event is not immediately detected, within the time interval necessary to at least maintain the offset rate, the rate is switched back to the baseline rate for pacing over an increased period of time.
According to some preferred methods, the predetermined decrement may be increased to establish an even lower offset rate, when a preference rate is established in between the baseline and offset rates. The establishment of the preference rate is associated with inclusion of another switching criterion in addition to the aforementioned x of y criterion. When the preference rate is employed, a switch from the offset rate to the baseline rate occurs, even if the aforementioned x of y criterion is not met, when successive intrinsic events meet another predetermined criterion with respect to the preference rate, for example, when a detected measure of central tendency for successive intrinsic events, over a predetermined interval, falls below the preference rate.



BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings are illustrative of particular embodiments and methods of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Methods of the present invention will hereinafter be described in conjunction with the appended drawings wherein like numerals denote like elements, and
FIGS. 1A-B are schematics providing context for methods of the present invention;
FIG. 2 is a flowchart outlining some methods of the present invention;
FIGS. 3A-B are plots illustrating examples that correspond to the flow chart of FIG. 2;
FIG. 4 is a flowchart outlining some alternate methods of the present invention; and
FIG. 5 is a plot illustrating an example that corresponds to the flow chart of FIG. 4.



DETAILED DESCRIPTION
The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical examples, and those skilled in the art will recognize that some of the examples may have suitable alternatives.
With reference to FIG. 1A, pacing system 100 is shown wholly implanted against an endocardial surface within the right ventricle RV for right ventricular pacing and stimulation. However, methods of the present invention may be employed by any single chamber pacing system (either the traditional type or the relatively compact type) when implanted endocardially or epicardially, for either ventricular pacing and sensing, or atrial pacing and sensing (left or right chambers). Thus the term “intrinsic events” used in the following description can designate either ventricular or atrial depolarization signals. FIG. 1B is a block diagram of the electrical components of pulse generator 103 including a microcomputer circuit 148, known to those skilled in art, wherein a microprocessor element thereof may be preprogrammed to direct pulse generator 103 to execute any of methods disclosed herein. Although not shown, an appropriate implantable battery power source is preferably included within capsule 101 to power the electrical components of pulse generator 103.
FIG. 2 is a flowchart outlining some methods of the present invention, whereby baseline and offset rates, established via dynamic input from rate response sensor 135 and electrodes 111, 112, are utilized by pacing system 100 to provide adequate pacing support to a patient while minimizing any unnecessary pacing stimulation. In a first step 200 of the outlined method, the baseline rate is dynamically established by means of input from rate response sensor 135; that is, the rate response sensor, which may include one or more activity or physiologic sensors, such as those described above, tracks a level of patient activity and provides input concerning an appropriate pacing rate to support the patient at any given time, which appropriate rate is designated as the baseline rate. Step 200 further includes establishment of the corresponding offset rate, which is lower than the baseline rate and generally tracks the baseline rate at a predetermined decrement thereof, according to some methods. However, according to some alternate methods, if an intrinsic rate, as sensed by electrodes 111, 112, exceeds the baseline rate, the offset rate is established by a predetermined decrement of the sensed intrinsic rate. According to some methods, the predetermined decrement may be absolute or percentage based, according to rate (bpm) or interval (ms), or any combination thereof; some examples are presented in Table 1. In Table 1, the term ‘tracked rate’ is used to generically designate either the baseline rate or the intrinsic rate, depending upon the method employed−baseline rate, if the first of the aforementioned methods is employed or if the baseline rate exceeds the intrinsic rate, when the latter, alternative method is employed; and intrinsic rate, if the latter method is employed and the intrinsic exceeds the baseline.










TABLE 1






Offset Rate when
Offset Rate when
Offset Rate when



tracked rate =
tracked rate =
tracked rate =


Predeter-
60 beats
100 beats
175 beats


mined rate
per minute
per minute
per minute


decrement
(1000 ms intervals)
(600 ms intervals)
(342 ms intervals)







subtract 10
50 bpm
90 bpm
165 bpm


bpm


subtract 12
48 bpm
88 bpm
163 bpm


bpm


subtract 15
45 bpm
85 bpm
160 bpm


bpm


subtract 20
40 bpm
80 bpm
155 bpm


bpm


subtract 25%
45 bpm
75 bpm
131 bpm


bpm


add 25% ms
48 bpm
80 bpm
140 bpm



(1250 ms intervals)
(750 ms intervals)
(427 ms interval)


add 25%
50 bpm
85 bpm
160 bpm


ms-50 ms
(1200 ms intervals)
(700 ms intervals)
(377 ms intervals)


add 30% ms
46 bpm
76 bpm
134 bpm



(1300 ms intervals)
(780 ms intervals)
(445 ms intervals)





Note:


the last three rate decrements are presented in terms of beat-to-beat intervals in units of milliseconds.






In steps 202 and 204 of FIG. 2, sensing intrinsic events and only applying pacing stimulation to create paced events that maintain the lower offset rate allows the patient's natural conduction to persist until such time that the method determines pacing stimulation is necessary to support the patient. According to decision point 206 of FIG. 2, this time is reached when x of y successive events are paced events, wherein x is always greater than one and y may be equal to x or greater than x. When this x of y criterion is met, the pacing rate switches from the offset rate to the baseline rate for pacing over a predetermined period of time, according to step 231. Two examples of the x of y switching criterion are shown in FIGS. 3A-B.
FIGS. 3A-B are plots of rate vs. time wherein open boxes represent intrinsic events, closed circles represent paced events, and dashed lines denote the established baseline and offset rates. (Note that, according to the aforementioned alternate methods, the baseline rate in the Figures could alternately be the intrinsic rate, if and when the intrinsic rate is greater that the baseline rate.) FIG. 3A illustrates a switching criterion of x=2 and y=3 and FIG. 3B illustrates a switching criterion of x=8 and y=8. FIGS. 3A-B further illustrate a predetermined period of time of approximately one minute, over which pacing stimulation is applied at the baseline rate. However, according to some methods, if, before the end of the predetermined period of time, intrinsic events occurring at a greater rate are detected, the pacing stimulation is aborted, thereby effectively shortening the predetermined period of time. In either case, at the end of the predetermined period, if intrinsic events are immediately detected at a rate that at least corresponds to the offset rate, sensing continues per step 202 of FIG. 2, as shown in FIGS. 3A-B. Alternately, with reference to decision point 233 of FIG. 2, if, at the end of the predetermined period of time of pacing, an intrinsic event is not immediately detected within the time interval necessary to at least maintain the offset rate, so that a paced event is created for maintenance of the offset rate, the method switches the rate back to the baseline rate for pacing over an increased period of time, per step 239. Although not shown in FIG. 2, the method preferably includes a maximum limit on this increased period of time, for example, sixteen hours, so that subsequent increased periods of time, if necessary, do not exceed maximum.
FIG. 4 is a flowchart modified from that shown in FIG. 2 to outline yet further methods of the present invention. The methods outlined in FIG. 4 differ from those outlined in FIG. 2 in that an additional rate, called the preference rate, is established, per step 400, and an additional switching criterion, corresponding to the preference rate is included at decision point 406. According to methods outlined in FIG. 4, even if x of y paced events are not detected, the switch, per step 231, may be made, if the criterion at decision point 406 is met, for example as illustrated in the plot FIG. 5.
FIG. 5 is a plot of rate vs. time, wherein open boxes represent intrinsic events and closed circles paced events, like the plots of FIGS. 3A-B. The dashed lines in FIG. 5 denote the established baseline, preference and offset rates. The preference rate, which is between the baseline rate and the offset rate allows for a greater predetermined decrement, or a lower offset rate, without sacrificing pacing support in the event of chronotropic incompetence, which, according to decision point 406 of FIG. 4, corresponds with the successive intrinsic events meeting a predetermined criterion with respect to the preference rate over a predetermined interval, as designated by the bracket in FIG. 5. This criterion may be x of y intrinsic events (x>1 and y≧x) falling below the preference rate, although remaining above the offset rate, or some measure of central tendency, like mean, median or mode, of successive intrinsic events over a predetermined interval being less than the preference rate, although greater than the offset rate. Establishing the lower offset rate with the protection of the preference rate, according to the methods of FIG. 4, can allow normal/physiological micro rate changes that temporarily slow the intrinsic heart rate (i.e. those associated with respiration changes) to be tolerated without creating paced events. With reference back to Table 1, the same exemplary means used to establish the offset rate may be used to establish the preference rate, according to some methods.
Finally, with further reference to the plots of FIGS. 3A-B and 5, according to some methods, at least one pacing stimulation pulse at a rate between either the offset rate or the preference rate and the baseline rate (as designated by grey circles) is applied just prior to the switch to the baseline rate. Whether or not such transition, or “rate smoothing” pulses are applied, and the number of pulses applied depends upon the magnitude of the difference between the two rates at the time the switch is made.
In the foregoing detailed description, the invention has been described with reference to specific methods. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims.

## Claims

1. A method for rate adaptive cardiac pacing that employs implanted electrodes for single chamber pacing and sensing and one or more rate response sensors; and the method comprising the steps of:
establishing a baseline rate for pacing stimulation according to input from the one or more rate response sensors;
establishing an offset rate for pacing stimulation, the offset rate being lower than the baseline rate and generally tracking therewith such that a difference between the baseline rate and the offset rate remains substantially constant over time;
sensing for intrinsic events by means of the implanted electrodes;
applying a pacing stimulation pulse by means of the implanted electrodes, when an intrinsic event is not detected within a time interval necessary to at least maintain the offset rate, the applied pacing stimulation pulse creating a paced event to maintain the offset rate;
switching from the offset rate to the baseline rate and pacing at the baseline rate over a predetermined period of time, when x of y successive events are paced events, x being greater than one and y being greater than or equal to x; and
at the end of the predetermined period of time, again sensing for intrinsic events, and repeating the above last two steps;
wherein the offset rate is established according to a predetermined decrement of one of: the baseline rate; and a greater of the baseline rate and an intrinsic rate, the intrinsic rate being determined by the sensing for intrinsic events; and further comprising:
establishing a preference rate that falls between the baseline rate and the offset rate; and
switching from the offset rate to the baseline rate and pacing at the baseline rate over the predetermined period of time, when the rates of successive intrinsic events, over a predetermined interval, even if greater than the offset rate, meet a criterion with respect to the preference rate, the criterion being one of: x of y successive intrinsic events falling below the preference rate over the predetermined interval, x being greater than one and y being greater than or equal to x; and a measure of central tendency of the rates of successive intrinsic events falling below the preference rate over the predetermined interval.

2. The method of claim 1, wherein the predetermined period of time is shortened in response to detection of intrinsic events occurring at a rate that is greater than the baseline rate.

3. The method of claim 1, wherein the preference rate is established according to a predetermined decrement of the baseline rate, the predetermined decrement being one or a combination of: a fixed beats per minute decrement, a percentage beats per minute decrement, a fixed interval increment, and a percentage interval increment.

4. The method of claim 1, further comprising applying at least one pacing stimulation pulse at a rate between the preference rate and the baseline rate, when the rates of successive intrinsic events meet the criterion with respect to the preference rate, and just prior to the corresponding switch to the baseline rate.

5. The method of claim 1, wherein, at the end of the predetermined period of time, if an intrinsic event is not immediately detected within the time interval necessary to at least maintain the offset rate, switching back to the baseline rate and pacing at the baseline rate over a another predetermined period of time that is increased from the previous predetermined period.

6. The method of claim 1, wherein, at the end of the predetermined period of time, if an intrinsic event is not immediately detected within the time interval necessary to at least maintain the offset rate, switching back to the baseline rate and pacing at the baseline rate over a another predetermined period of time that is increased from the previous predetermined period.

7. The method of claim 1, wherein the offset rate is established according to a predetermined decrement of the baseline rate, the predetermined decrement being one or a combination of: a fixed beats per minute decrement, a percentage beats per minute decrement, a fixed interval increment, and a percentage interval increment.

8. The method of claim 1, further comprising applying at least one pacing stimulation pulse at a rate between the offset rate and the baseline rate, just prior to the switch to the baseline rate.

9. The method of claim 1, wherein x equals two and y equals three or x and y both equal eight.

10. The method of claim 1, wherein the predetermined period of time is equal to approximately one minute.

11. A single chamber cardiac pacing system configured to be wholly implanted against an endocardial or epicardial surface, the system including a pulse generator having one or more rate response sensors and a microprocessor element being preprogrammed to direct the pulse generator to execute a method comprising the following steps:
establishing a baseline rate for pacing stimulation according to input from the one or more rate response sensors;
establishing an offset rate for pacing stimulation, the offset rate being lower than the baseline rate and generally tracking therewith such that a difference between the baseline rate and the offset rate remains substantially constant over time;
sensing for intrinsic events by means of the implanted electrodes;
applying a pacing stimulation pulse by means of the implanted electrodes, if an intrinsic event is not detected within a time interval necessary to at least maintain the offset rate, the applied pacing stimulation pulse creating a paced event to maintain the offset rate;
switching from the offset rate to the baseline rate and pacing at the baseline rate over a predetermined period of time, if x of y successive events are paced events, x being greater than one and y being greater than or equal to x; and
at the end of the predetermined period of time, again sensing for intrinsic events, and repeating the above last two steps;
wherein the offset rate is established according to a predetermined decrement of one of: the baseline rate; and a greater of the baseline rate and an intrinsic rate, the intrinsic rate determined according to prior sensed intrinsic events; and wherein the method further comprises:
establishing a preference rate that falls between the baseline rate and the offset rate; and
switching from the offset rate to the baseline rate and pacing at the baseline rate over the predetermined period of time, if the rates of successive intrinsic events, over a predetermined interval, even if greater than the offset rate, meet a criterion with respect to the preference rate, the criterion being one of: x of y successive intrinsic events falling below the preference rate over the predetermined interval, x being greater than one and y being greater than or equal to x; and a measure of central tendency of the rates of successive intrinsic events falling below the preference rate over the predetermined interval.

12. The system of claim 11, wherein the predetermined period of time is shortened in response to detection of intrinsic events occurring at a rate that is greater than the baseline rate.

13. The system of claim 11, wherein the preference rate is established according to a predetermined decrement of the baseline rate, the predetermined decrement being one or a combination of: a fixed beats per minute decrement, a percentage beats per minute decrement, a fixed interval increment, and a percentage interval increment.

14. The system of claim 11, wherein the method further comprises applying at least one pacing stimulation pulse at a rate between the preference rate and the baseline rate, when the rates of successive intrinsic events meet the criterion with respect to the preference rate, and just prior to the corresponding switch to the baseline rate.

15. The system of claim 11, wherein, at the end of the predetermined period of time, if an intrinsic event is not immediately detected within the time interval necessary to at least maintain the offset rate, switching back to the baseline rate and pacing at the baseline rate over a another predetermined period of time that is increased from the previous predetermined period.

16. The system of claim 11, wherein the offset rate is established according to a predetermined decrement of the baseline rate, the predetermined decrement being one or a combination of: a fixed beats per minute decrement, a percentage beats per minute decrement, a fixed interval increment, and a percentage interval increment.

17. The system of claim 11, wherein the method further comprises applying at least one pacing stimulation pulse at a rate between the offset rate and the baseline rate, just prior to the switch to the baseline rate.

18. The system of claim 11, wherein x equals two and y equals three or x and y both equal eight.

19. The system of claim 11, wherein the predetermined period of time is equal to approximately one minute.

20. A method for rate adaptive cardiac pacing that employs implanted electrodes for single chamber pacing and sensing and one or more rate response sensors; and the method comprising the steps of:
establishing a baseline rate for pacing stimulation according to input from the one or more rate response sensors;
establishing an offset rate for pacing stimulation, the offset rate being lower than the baseline rate;
sensing for intrinsic events by means of the implanted electrodes;
applying a pacing stimulation pulse by means of the implanted electrodes, when an intrinsic event is not detected within a time interval necessary to at least maintain the offset rate, the applied pacing stimulation pulse creating a paced event to maintain the offset rate;
establishing a preference rate that falls between the baseline rate and the offset rate; and
switching from the offset rate to the baseline rate and pacing at the baseline rate over the predetermined period of time, when the rates of successive intrinsic events, over a predetermined interval, even if greater than the offset rate, meet a criterion with respect to the preference rate, the criterion being one of: x of y successive intrinsic events falling below the preference rate over the predetermined interval; x being greater than one and y being greater than or equal to x; and a measure of central tendency of the rates of successive intrinsic events falling below the preference rate over the predetermined interval.

21. A single chamber cardiac pacing system configured to be wholly implanted against an endocardial or epicardial surface, the system including a pulse generator having one or more rate response sensors and a microprocessor element being preprogrammed to direct the pulse generator to execute a method comprising the following steps:
establishing a baseline rate for pacing stimulation according to input from the one or more rate response sensors;
establishing an offset rate for pacing stimulation, the offset rate being lower than the baseline rate;
sensing for intrinsic events by means of the implanted electrodes;
applying a pacing stimulation pulse by means of the implanted electrodes, if an intrinsic event is not detected within a time interval necessary to at least maintain the offset rate, the applied pacing stimulation pulse creating a paced event to maintain the offset rate;
establishing a preference rate that falls between the baseline rate and the offset rate; and
switching from the offset rate to the baseline rate and pacing at the baseline rate over the predetermined period of time, if the rates of successive intrinsic events, over a predetermined interval, even if greater than the offset rate, meet a criterion with respect to the preference rate, the criterion being one of: x of y successive intrinsic events falling below the preference rate over the predetermined interval, x being greater than one and y being greater than or equal to x; and a measure of central tendency of the rates of successive intrinsic events falling below the preference rate over the predetermined interval.