Leadless pacemaker system

1. A leadless cardiac device comprising

a signal generator module configured to deliver pacing pulses to an atrium;

a processing module configured to:

detect a ventricular activation event,

determine a length of an interval between the ventricular activation event and a previous atrial event that preceded the ventricular activation event,

schedule a time at which to deliver a pacing pulse to the atrium based on the length of the interval, and

control the signal generator module to deliver the pacing pulse at the scheduled time;

a housing configured for implantation within the atrium, wherein the housing encloses the signal generator and the processing module; and

a memory that stores a baseline atrioventricular (AV) value that is an expected value of the interval between the ventricular activation event and the previous atrial event during normal AV conduction of the heart;

wherein the processing module is configured to:

compare the determined length of the interval to the baseline AV value, and

schedule the time at which to deliver the pacing pulse to the atrium in order to maintain a baseline atrial pacing rate based on the comparison, and

wherein the processing module is configured to continue to compare the stored baseline atrioventricular (AV) value, unchanged, to subsequent intervals.

2. The device of claim 1 , further comprising a sensing module configured to sense ventricular electrical activity, wherein the processing module is configured to detect the ventricular activation event by detecting the ventricular electrical activity.

3. The device of claim 2 , wherein the ventricular electrical activity is a far-field R-wave.

4. The device of claim 3 , wherein the processing module is configured to:

detect the ventricular activation event by detecting a far-field R-wave (FFRW);

determine the length of the interval between the ventricular activation event and the previous atrial event by determining a length of an interval between the detected FFRW and the previous atrial event; and

schedule the time at which to deliver the pacing pulse to the atrium based on the length of the interval between the detected FFRW and the previous atrial event.

5. The device of claim 1 , further comprising a sensor that generates signals that indicate mechanical cardiac characteristics, wherein the processing module is configured to detect contraction of a ventricle based on the generated signals.

6. The device of claim 5 , wherein the mechanical cardiac characteristics include heart sounds.

7. The device of claim 6 , wherein the processing module is configured to:

detect the ventricular activation event by detecting an S1 heart sound;

determine the length of the interval between the ventricular activation event and the previous atrial event by determining a length of an interval between the detected S1 heart sound and the previous atrial event; and

schedule the time at which to deliver the pacing pulse to the atrium based on the length of the interval between the detected S1 heart sound and the previous atrial event.

8. The device of claim 1 , wherein the processing module is configured to determine that the ventricular activation event went undetected, and wherein the processing module is configured to schedule a pacing pulse based on the determination that the ventricular activation event went undetected.

9. The device of claim 1 , wherein the processing module is configured to determine when multiple ventricular activation events have occurred after the previous atrial event but before the scheduled time at which the pacing pulse is to be delivered, and wherein the processing module is configured to schedule a new time at which to deliver a pacing pulse to the atrium based on the determination that multiple ventricular activation events have occurred.

10. The system of claim 1 , wherein the processing module is configured to schedule the time at which to deliver the pacing pulse to the atrium based on the length of the interval by at least selecting a delay between an atrial event and ventricular activation, wherein the processing module is configured to schedule the time at which to deliver the pacing pulse to the atrium in order to maintain a baseline atrial pacing rate based on the comparison by at least:

in response to determining the interval is shorter than the baseline AV value, maintaining the delay for at least one cardiac cycle, and subsequently increasing the delay; and

in response to determining the interval is longer than the baseline AV value, maintaining the delay for at least one cardiac cycle, and subsequently shortening the delay.

11. A leadless cardiac device comprising:

a signal generator module configured to deliver pacing pulses to an atrium;

a processing module configured to:

detect a ventricular activation event,

determine a length of an interval between the ventricular activation event and a previous atrial event that preceded the ventricular activation event,

schedule a time at which to deliver a pacing pulse to the atrium based on the length of the interval, and

control the signal generator module to deliver the pacing pulse at the scheduled time;

a housing configured for implantation within the atrium, wherein the housing encloses the signal generator and the processing module; and

a memory that stores a baseline atrioventricular (AV) value that is an expected value of the interval between the ventricular activation event and the previous atrial event during normal AV conduction of the heart;

wherein the processing module is configured to:

compare the determined length of the interval to the baseline AV value,

schedule the time at which to deliver the pacing pulse to the atrium based on the comparison,

identify the interval as having one of a normal duration, a short duration, or a long duration based on the comparison, and

schedule the time at which to deliver the pacing pulse to the atrium in order to maintain a baseline atrial pacing rate based on whether the interval is identified as having a normal duration, a short duration, or a long duration.

12. The system of claim 11 , wherein the processing module is configured to schedule the time at which to deliver the pacing pulse to the atrium based on the length of the interval by at least selecting a delay between an atrial event and ventricular activation, wherein the processing module is configured to schedule the time at which to deliver the pacing pulse to the atrium in order to maintain a baseline atrial pacing rate based on the comparison by at least:

in response to determining the interval has the short duration, maintaining the delay for at least one cardiac cycle, and subsequently increasing the delay; and

in response to determining the interval has the long duration, maintaining the delay for at least one cardiac cycle, and subsequently shortening the delay.

13. A leadless cardiac device comprising:

a signal generator module configured to deliver pacing pulses to an atrium;

a processing module configured to:

detect a ventricular activation event,

determine a length of an interval between the ventricular activation event and a previous atrial event that preceded the ventricular activation event,

schedule a time at which to deliver a pacing pulse to the atrium based on the length of the interval, and

control the signal generator module to deliver the pacing pulse at the scheduled time;

a housing configured for implantation within the atrium, wherein the housing encloses the signal generator and the processing module; and

a memory that stores a baseline atrioventricular (AV) value that is an expected value of the interval between the ventricular activation event and the previous atrial event during normal AV conduction of the heart;

wherein the processing module is configured to:

compare the determined length of the interval to the baseline AV value,

identify the interval as having a normal duration when the determined length of the interval is approximately equal to the baseline AV value,

identify the interval as having a short duration when the determined length of the interval is less than the baseline AV value by a threshold amount, and

schedule the time at which to deliver the pacing pulse to the atrium at a delay following the ventricular activation event based on whether the interval is identified as having the normal duration or the short duration, wherein the delay is increased responsive to an identified short duration interval.

14. The system of claim 13 , wherein the processing module is configured to schedule the time at which to deliver the pacing pulse to the atrium at the delay following the ventricular activation event based on whether the interval is identified as having the normal duration or the short duration by at least, in response to determining the interval has the short duration, maintaining the delay for at least one cardiac cycle, and subsequently increasing the delay.

15. A leadless cardiac device comprising:

a signal generator module configured to deliver pacing pulses to an atrium;

a processing module configured to:

detect a ventricular activation event,

determine a length of an interval between the ventricular activation event and a previous atrial event that preceded the ventricular activation event,

schedule a time at which to deliver a pacing pulse to the atrium based on the length of the interval, and

control the signal generator module to deliver the pacing pulse at the scheduled time;

a housing configured for implantation within the atrium, wherein the housing encloses the signal generator and the processing module; and

a memory that stores a baseline atrioventricular (AV) value that is an expected value of the interval between the ventricular activation event and the previous atrial event during normal AV conduction of the heart;

wherein the processing module is configured to:

compare the determined length of the interval to the baseline AV value,

identify the interval as having a normal duration when the determined length of the interval is approximately equal to the baseline AV value,

identify the interval as having a long duration when the determined length of the interval is greater than the baseline AV value by a threshold amount, and

schedule the time at which to deliver the pacing pulse to the atrium at a delay following the ventricular activation event based on whether the interval is identified as having the normal duration or the long duration, wherein the delay is decreased responsive to an identified long duration interval.

16. The system of claim 15 , wherein the processing module is configured to schedule the time at which to deliver the pacing pulse to the atrium at the delay following the ventricular activation event based on whether the interval is identified as having the normal duration or the short duration by at least, in response to determining the interval has the short duration, maintaining the delay for at least one cardiac cycle, and subsequently increasing the delay.