Patent Publication Number: US-2023136887-A1

Title: Post-ventricular atrial blanking in a cardiac device

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional U.S. Patent Application No. 63/274,323, filed on Nov. 1, 2021, the entire contents of which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a medical device and method for setting a post-ventricular atrial blanking period. 
     BACKGROUND 
     Implantable cardiac pacemakers are often placed in a subcutaneous pocket and coupled to one or more transvenous medical electrical leads carrying pacing and sensing electrodes positioned in the heart. A cardiac pacemaker implanted subcutaneously may be a single chamber pacemaker coupled to one transvenous medical lead for positioning electrodes in one heart chamber, atrial or ventricular, or a dual chamber pacemaker coupled to two intracardiac leads for positioning electrodes in both an atrial and a ventricular chamber. Multi-chamber pacemakers are also available that may be coupled to three leads, for example, for positioning electrodes for pacing and sensing in one atrial chamber and both the right and left ventricles. 
     Intracardiac pacemakers have recently been introduced that are implantable within a ventricular chamber of a patient&#39;s heart for delivering ventricular pacing pulses. Such a pacemaker may sense R-wave signals attendant to intrinsic ventricular depolarizations and deliver ventricular pacing pulses in the absence of sensed R-waves. While single chamber ventricular sensing and pacing by an intracardiac ventricular pacemaker may adequately address some patient conditions, some patients may benefit from atrial and ventricular (dual chamber) sensing for providing atrial-synchronized ventricular pacing in order to maintain a regular heart rhythm. 
     SUMMARY 
     The techniques of this disclosure generally relate to setting a post-ventricular atrial blanking (PVAB) period in a cardiac device, such as a ventricular pacemaker capable of atrial synchronous ventricular pacing. The device may sense atrial event signals corresponding to the contraction of the atria for triggering the delivery of a ventricular pacing pulse synchronized to the atrial event. In some examples, the medical device may have a motion sensor configured to sense a cardiac motion signal. The medical device may be configured to sense atrial event signals from the motion signal. A medical device operating according to the techniques disclosed herein sets a PVAB period that is applied to the motion signal following ventricular events during which atrial event signals are not sensed from the motion signal. The medical device may be configured to adjust the duration of the PVAB period. In some examples, the medical device may adjust the duration of the PVAB period in response to changes in heart rate and/or changes in the frequency or timing of sensed atrial event signals. In some examples, the medical device may be configured to adjust the duration of the PVAB period following a ventricular event based on an analysis of the amplitude of the motion signal to promote reliable sensing of atrial event signals outside of the PVAB period and promote proper tracking of atrial events by the ventricular pacing pulses that may be generated by the medical device. 
     In one example, the disclosure provides a medical device including a motion sensor configured to sense a motion signal and a control circuit in communication with the motion signal. The control circuit can be configured to identify a first group of cardiac events, determine a first cardiac event interval based on the first group of cardiac events and determine whether the first cardiac event interval is less than a threshold interval or greater than the threshold interval. The control circuit can be configured to select a first blanking period duration if the first cardiac event interval is less than the threshold interval or a second blanking period duration if the first cardiac event interval is greater than the threshold interval. The second blanking period duration is greater than the first blanking period duration. The control circuit may further be configured to identify a second group of cardiac events occurring after the first group of cardiac events and start a PVAB period that is applied to the motion signal during the second group of cardiac events. The PVAB period may be applied to the motion signal in response to each ventricular event during the second group of cardiac events. The PVAB period that is applied to the motion signal during the second group of cardiac events is set to the selected one of the first blanking period duration or the second blanking period duration. 
     In another example, the disclosure provides a method that may be performed by a medical device. The method may include sensing a motion signal, identifying a first group of cardiac events, determining a first cardiac event interval based on the first group of cardiac events and determining whether the first cardiac event interval is less than a threshold interval or greater than the threshold interval. The method may further include selecting one of a first blanking period duration if the first cardiac event interval is less than the threshold interval or a second blanking period duration if the first cardiac event interval is greater than the threshold interval. The second blanking period duration can be greater than the first blanking period duration. The method may further include identifying a second group of cardiac events occurring after the first group of cardiac events and applying a PVAB period to the motion signal during the second group of cardiac events. The PVAB period may be applied to the motion signal in response to each ventricular event during the second group of cardiac events. The PVAB period that is applied to the motion signal during the second group of cardiac events is set to the selected one of the first blanking period duration or the second blanking period duration. 
     In another example, the disclosure provides a non-transitory, computer-readable storage medium storing a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to sense a motion signal, identify a first group of cardiac events, determine a first cardiac event interval based on the first group of cardiac events, and determine whether the first cardiac event interval is less than a threshold interval or greater than the threshold interval. The instructions may further cause the medical device to select one of a first blanking period duration if the first cardiac event interval is less than the threshold interval or a second blanking period duration if the first cardiac event interval is greater than the threshold interval. The second blanking period duration can be greater than the first blanking period duration. The instructions may further cause the medical device to identify a second group of cardiac events occurring after the first group of cardiac events and apply a PVAB period to the motion signal during the second group of cardiac events. The PVAB period may be applied to the motion signal in response to each ventricular event during the second group of cardiac events. The PVAB period that is applied to the motion signal during the second group of cardiac events is set to the selected one of the first blanking period duration or the second blanking period duration. 
     Further disclosed herein is the subject matter of the following clauses:
     1. A medical device comprising: a motion sensor configured to sense a motion signal; a pulse generator configured to generate ventricular pacing pulses; and a control circuit in communication with the motion sensor and the pulse generator, the control circuit configured to: identify a first plurality of cardiac events; determine a first cardiac event interval based on the first plurality of cardiac events; determine whether the first cardiac event interval is less than a threshold interval or greater than the threshold interval; select one of a first blanking period duration if the first cardiac event interval is less than the threshold interval or a second blanking period duration if the first cardiac event interval is greater than the threshold interval, the second blanking period duration being greater than the first blanking period duration; identify a second plurality of cardiac events after the first plurality of cardiac events; and apply a post-ventricular atrial blanking period to the motion signal during the second plurality of cardiac events, the post-ventricular atrial blanking period being set to the selected one of the first blanking period duration or the second blanking period duration.   2. The medical device of clause 1, wherein the control circuit is further configured to: determine a second cardiac event interval from the second plurality of cardiac events; compare the second cardiac event interval to the threshold interval; based on the comparison of the second cardiac event interval to the threshold interval, select one of the first blanking period duration in response to the second cardiac event interval being less than the threshold interval or the second blanking period duration in response to the second cardiac event interval being greater than the threshold interval; identify a third plurality of cardiac events after the second plurality of cardiac events; and apply the post-ventricular atrial blanking period to the motion signal during the third plurality of cardiac events, where the post-ventricular atrial blanking period is set to one of the first blanking period duration or the second blanking period duration that is selected based on the comparison of the second cardiac event interval to the threshold interval.   3. The medical device of any of clauses 1-2, wherein the control circuit is further configured to identify the first plurality of cardiac events by identifying at least one ventricular pacing pulse generated by the pulse generator.   4. The medical device of any of clauses 1-3, further comprising a sensing circuit configured to receive a cardiac electrical signal and sense ventricular events from the cardiac electrical signal wherein the control circuit is configured to identify the first plurality of cardiac events by identifying at least one ventricular event sensed by the sensing circuit.   5. The medical device of any of clauses 1-4, wherein the control circuit is further configured to sense an atrial event signal from the motion signal outside the post-ventricular atrial blanking period; and generate an output in response to sensing the atrial event signal; the medical device further comprising a memory in communication with the control circuit, the memory configured to store the output generated by the control circuit in response to sensing the atrial event signal.   6. The medical device of any of clauses 1-5, wherein the control circuit is further configured to set a power conservation time period during the post-ventricular atrial blanking period; and disable at least a portion of the motion sensor during the power conservation time interval.   7. The medical device of clause  6 , wherein the control circuit is further configured to set the power conservation time period to expire a predetermined time interval less than (e.g., earlier than) the post-ventricular atrial blanking period that is set to the selected one of the first blanking period duration or the second blanking period duration.   8. The medical device of any of clauses 1-7, wherein the control circuit is further configured to select the first blanking period duration by selecting a minimum blanking period duration in response to the first cardiac event interval being less than the threshold interval.   9. The medical device of any of clauses 1-8, wherein the control circuit is further configured to select the second blanking period by selecting a maximum blanking period duration in response to the first cardiac event interval being greater than the threshold interval.   10. The medical device of any of clauses 1-9, wherein the control circuit is further configured to determine when the first cardiac event interval is less than the threshold interval; in response to the first cardiac event interval being less than the threshold interval, determine an amplitude of the motion signal during at least one post-ventricular atrial blanking period; determine that the amplitude is greater than a threshold amplitude;   

     in response to the amplitude being greater than the threshold amplitude, withhold selecting the first blanking period duration in response to the first cardiac event interval being less than the threshold interval; and select the second blanking period in response to determining that the amplitude of the motion signal is greater than the threshold amplitude when the first cardiac event interval is less than the threshold interval.
     11. The medical device of any of clauses 1-10, wherein the control circuit is further configured to determine an amplitude of the motion signal; and set the first blanking period duration based on the amplitude of the motion signal.   12. The medical device of any of clauses 1-11, wherein the control circuit is further configured to determine a time of one of a peak amplitude or a threshold amplitude crossing of the motion signal; and set the first blanking period duration based on the determined time.   13. The medical device of any of clauses 1-12, wherein the control circuit is further configured to wait for one of a predetermined time interval or a predetermined number of cardiac event intervals; identify a second plurality of cardiac events; determine a second cardiac event interval from the second plurality of cardiac events; and adjust the post-ventricular atrial blanking period based on the second cardiac event interval after waiting for the one of the predetermined time interval or the predetermined number of cardiac event intervals.   14. The medical device of any of clauses 1-13, wherein the control circuit is further configured to select the threshold interval from one of a first longer threshold interval and a second shorter threshold interval based on a current duration of the post-ventricular atrial blanking period.   15. The medical device of any of clauses 1-14, wherein the control circuit is further configured to sense an atrial event signal from the motion signal after an expiration of the post-ventricular atrial blanking period; and the pulse generator is configured to generate a ventricular pacing pulse in response to the control circuit sensing the atrial event signal.   16. A method, comprising: sensing a motion signal; identifying a first plurality of cardiac events; determining a first cardiac event interval based on the first plurality of cardiac events; determining whether the first cardiac event interval is less than a threshold interval or greater than the threshold interval; selecting one of a first blanking period duration if the first cardiac event interval is less than the threshold interval or a second blanking period duration if the first cardiac event interval is greater than the threshold interval, the second blanking period duration being greater than the first blanking period duration; identifying a second plurality of cardiac events after the first plurality of cardiac events; and applying a post-ventricular atrial blanking period to the motion signal during the second plurality of cardiac events, the post-ventricular atrial blanking period being set to the selected one of the first blanking period duration or the second blanking period duration.   17. The method of clause 16, further comprising: determining a second cardiac event interval from the second plurality of cardiac events; comparing the second cardiac event interval to the threshold interval; based on the comparing of the second cardiac event interval to the threshold interval, selecting one of the first blanking period duration in response to the second cardiac event interval being less than the threshold interval or the second blanking period duration in response to the second cardiac event interval being greater than the threshold interval; identifying a third plurality of cardiac events after the second plurality of cardiac events; and applying the post-ventricular atrial blanking period to the motion signal during the third plurality of cardiac events, where the post-ventricular atrial blanking period is set to the selected one of the first blanking period duration or the second blanking period duration that is selected based on the comparison of the second cardiac event interval to the threshold interval.   18. The method of any of clauses 16-17, further comprising generating ventricular pacing pulses and identifying the first plurality of cardiac events by identifying at least one ventricular pacing pulse.   19. The method of any of clauses 16-18, further comprising receiving a cardiac electrical signal; sensing ventricular events from the cardiac electrical signal; and identifying the first plurality of cardiac events by identifying at least one ventricular event sensed by the sensing circuit.   20. The method of any of clauses 16-19, further comprising sensing an atrial event signal from the motion signal outside the post-ventricular atrial blanking period; generating an output in response to sensing the atrial event signal; and storing in a memory the output generated in response to sensing the atrial event signal.   21. The method of any of clauses 16-20, further comprising setting a power conservation time period during the post-ventricular atrial blanking period; and disabling at least a portion of the motion sensor during the power conservation time interval.   22. The method of clause  21 , further comprising setting the power conservation time period to expire a predetermined time interval earlier than the post-ventricular atrial blanking period.   23. The method of any of clauses 16-22, wherein selecting the first blanking period duration comprises selecting a minimum blanking period duration in response to the first cardiac event interval being less than the threshold interval.   24. The method of any of clauses 16-23, wherein selecting the second blanking period duration comprises selecting a maximum blanking period duration in response to the first cardiac event interval being greater than the threshold interval.   25. The method of any of clauses 16-24, further comprising determining when the first cardiac event interval is less than the threshold interval; in response to the first cardiac event interval being less than the threshold interval, determining an amplitude of the motion signal during at least one post-ventricular atrial blanking period; determining that the amplitude is greater than a threshold amplitude; in response to the amplitude being greater than the threshold amplitude, withholding selecting the first blanking period duration in response to the first cardiac event interval being less than the threshold interval; and selecting the second blanking period in response to determining that the amplitude of the motion signal is greater than the threshold amplitude when the first cardiac event interval is less than the threshold interval.   26. The method of any of clauses 16-25, further comprising determining an amplitude of the motion signal and setting the first blanking period duration based on the amplitude of the motion signal.   27. The method of any of clauses 16-26, further comprising determining a time of one of a peak amplitude or a threshold amplitude crossing of the motion signal and setting the first blanking period duration based on the determined time.   28. The method of any of clauses 16-27, further comprising waiting for one of a predetermined time interval or a predetermined number of cardiac event intervals; identifying a second plurality of cardiac events; determining a second cardiac event interval from the second plurality of cardiac events; and adjusting the post-ventricular atrial blanking period based on the second cardiac event interval after waiting for the one of the predetermined time interval or the predetermined number of cardiac event intervals.   29. The method of any of clauses 16-28, further comprising selecting the threshold interval from one of a first threshold interval and a second threshold interval that is shorter than the first threshold interval based on a current duration of the post-ventricular atrial blanking period.   30. The method of any of clauses 16-29, further comprising sensing an atrial event signal from the motion signal after an expiration of the post-ventricular atrial blanking period and generating a ventricular pacing pulse in response to the control circuit sensing the atrial event signal.   31. A non-transitory, computer-readable medium storing a set of instructions that, when executed by a control circuit of a medical device, cause the medical device to: sense a motion signal; identify a first plurality of cardiac events; determine a first cardiac event interval based on the first plurality of cardiac events; determine whether the first cardiac event interval is less than a threshold interval or greater than the threshold interval; select one of a first blanking period duration if the first cardiac event interval is less than the threshold interval or a second blanking period duration if the first cardiac event interval is greater than the threshold interval, the second blanking period duration being greater than the first blanking period duration; identify a second plurality of cardiac events after the first plurality of cardiac events; and apply a post-ventricular atrial blanking period to the motion signal during the second plurality of cardiac events, the post-ventricular atrial blanking period being set to the selected one of the first blanking period duration or the second blanking period duration.   32. The non-transitory, computer-readable medium of clause 31, further comprising instructions that cause the medical device to determine a second cardiac event interval from the second plurality of cardiac events; compare the second cardiac event interval to the threshold interval; based on the comparing of the second cardiac event interval to the threshold interval, select one of the first blanking period duration in response to the second cardiac event interval being less than the threshold interval or the second blanking period duration in response to the second cardiac event interval being greater than the threshold interval; identify a third plurality of cardiac events after the second plurality of cardiac events; and apply the post-ventricular atrial blanking period to the motion signal during the third plurality of cardiac events, where the post-ventricular atrial blanking period is set to the selected one of the first blanking period duration or the second blanking period duration that is selected based on the comparison of the second cardiac event interval to the threshold interval.   33. The non-transitory, computer-readable medium of any of clauses 31-32, further comprising instructions that cause the medical device to generate ventricular pacing pulses and identify the first plurality of cardiac events by identifying at least one ventricular pacing pulse.   34. The non-transitory, computer-readable medium of any of clauses 31-33, further comprising instructions that cause the medical device to receive a cardiac electrical signal; sense ventricular events from the cardiac electrical signal; and identify the first plurality of cardiac events by identifying at least one ventricular event sensed by the sensing circuit.   35. The non-transitory, computer-readable medium of any of clauses 31-34, further comprising instructions that cause the medical device to sense an atrial event signal from the motion signal outside the post-ventricular atrial blanking period; generate an output in response to sensing the atrial event signal; and store in a memory of the medical device the output generated in response to sensing the atrial event signal.   36. The non-transitory, computer-readable medium of any of clauses 30-35, further comprising instructions that cause the medical device to set a power conservation time period during the post-ventricular atrial blanking period; and disable at least a portion of the motion sensor during the power conservation time interval.   37. The non-transitory, computer-readable medium of clause 36, further comprising instructions that cause the medical device to set the power conservation time period to expire a predetermined time interval earlier than the post-ventricular atrial blanking period.   38. The non-transitory, computer-readable medium of any of clauses 30-37, further comprising instructions that cause the medical device to select the first blanking period duration by selecting a minimum blanking period duration in response to the first cardiac event interval being less than the threshold interval.   39. The non-transitory, computer-readable medium of any of clauses 30-38, further comprising instructions that cause the medical device to select the second blanking period duration by selecting a maximum blanking period duration in response to the first cardiac event interval being greater than the threshold interval.   40. The non-transitory, computer-readable medium of any of clauses 30-39, further comprising instructions that cause the medical device to determine when the first cardiac event interval is less than the threshold interval; in response to the first cardiac event interval being less than the threshold interval, determine an amplitude of the motion signal during at least one post-ventricular atrial blanking period; determine that the amplitude is greater than a threshold amplitude; in response to the amplitude being greater than the threshold amplitude, withhold selecting the first blanking period duration in response to the first cardiac event interval being less than the threshold interval; and select the second blanking period in response to determining that the amplitude of the motion signal is greater than the threshold amplitude when the first cardiac event interval is less than the threshold interval.   41. The non-transitory, computer-readable medium of any of clauses 30-40, further comprising instructions that cause the medical device to determine an amplitude of the motion signal; and set the first blanking period duration based on the amplitude of the motion signal.   42. The non-transitory, computer-readable medium of any of clauses 30-41, further comprising instructions that cause the medical device to determine a time of one of a peak amplitude or a threshold amplitude crossing of the motion signal; and set the first blanking period duration based on the determined time.   43. The non-transitory, computer-readable medium of any of clauses 31-42, further comprising instructions that cause the medical device to wait for one of a predetermined time interval or a predetermined number of cardiac event intervals; identify a second plurality of cardiac events; determine a second cardiac event interval from the second plurality of cardiac events; and adjust the post-ventricular atrial blanking period based on the second cardiac event interval after waiting for the one of the predetermined time interval or the predetermined number of cardiac event intervals.   44. The non-transitory, computer-readable medium of any of clauses 30-43, further comprising instructions that cause the medical device to select the threshold interval from one of a first threshold interval and a second threshold interval that is shorter than the first threshold interval based on a current duration of the post-ventricular atrial blanking period.   45. The non-transitory, computer-readable medium of any of clauses 30-44, further comprising instructions that cause the medical device to sense an atrial event signal from the motion signal after an expiration of the post-ventricular atrial blanking period; and generate a ventricular pacing pulse in response to the control circuit sensing the atrial event signal.   46. A medical device comprising: a motion sensor configured to sense a motion signal; a pulse generator configured to generate ventricular pacing pulses; and a control circuit coupled to the motion sensor to receive the motion signal and configured to: identify a plurality of ventricular events; set a post-ventricular atrial blanking period following each of the plurality of ventricular events; determine an amplitude of the motion signal sensed by the motion sensor during at least one post-ventricular atrial blanking period of the post-ventricular atrial blanking periods; and adjust the post-ventricular atrial blanking period based on the determined amplitude of the motion signal.   47. The medical device of clause 46, wherein the control circuit is further configured to identify the plurality of ventricular events by identifying at least one ventricular pacing pulse generated by the pulse generator.   48. The medical device of any of clauses 46-47, further comprising a sensing circuit configured to receive a cardiac electrical signal and sense ventricular events from the cardiac electrical signal, wherein the control circuit is configured to identify the plurality of ventricular events by identifying at least one ventricular event sensed by the sensing circuit.   49. The medical device of any of clauses 46-48, wherein the control circuit is further configured to: determine the amplitude by determining a peak amplitude from the motion signal sensed during at least a portion of the at least one post-ventricular atrial blanking period; determine whether the peak amplitude is less than an amplitude threshold; and adjust the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the peak amplitude being less than the amplitude threshold.   50. The medical device of any of clauses 46-49, wherein the control circuit is further configured to: determine the amplitude by determining a peak amplitude from the motion signal sensed during at least a portion of the at least one post-ventricular atrial blanking period; determine whether the peak amplitude is greater than to an amplitude threshold; and adjust the post-ventricular atrial blanking period by increasing the post-ventricular atrial blanking period in response to the peak amplitude being greater than the amplitude threshold.   51. The medical device of any of clauses 56-50, wherein the control circuit is further configured to adjust the post-ventricular atrial blanking period by adjusting the duration of a post-ventricular atrial blanking period applicable to a future cardiac cycle.   52. The medical device of any of clauses 46-51, wherein the control circuit is further configured to: determine the amplitude by detecting a latest amplitude threshold crossing by the motion signal during the at least one post-ventricular atrial blanking period; determine that the amplitude threshold crossing is earlier than a threshold time interval from an expiration of the post-ventricular atrial blanking period; and adjust the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the amplitude threshold crossing being earlier than the threshold time interval from the expiration of the post-ventricular atrial blanking period.   53. The medical device of clause 52, wherein the control circuit is configured to set the threshold time interval based on a decrement interval used to adjust the post-ventricular atrial blanking period.   54. The medical device of any of clauses 46-53, wherein the control circuit is further configured to: detect a change in a heart rate; and determine the amplitude of the motion signal in response to detecting the change in the heart rate.   55. The medical device of any of clauses 46-54, wherein the control circuit is further configured to: set an amplitude analysis window during the at least one post-ventricular atrial blanking period; determine the amplitude by determining a peak amplitude of the motion signal sensed during the amplitude analysis window; determine that the peak amplitude is less than an amplitude threshold; and adjust the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the peak amplitude being less than the amplitude threshold.   56. The medical device of any of clauses 46-55, wherein the control circuit is further configured to: detect a decrease in a heart rate; increase the post-ventricular atrial blanking period in response to detecting the decrease in the heart rate; detect an increase in the heart rate; and based on the determined amplitude and the increase in the heart rate adjust the post-ventricular atrial blanking period by decreasing the post-ventricular atrial blanking period or hold the post-ventricular atrial blanking period constant.   57. The medical device of any of clauses 46-56, wherein the control circuit is further configured to: set a post-ventricular atrial blanking ending time interval that begins prior to an expiration time of the at least one post-ventricular atrial blanking period; and determine the amplitude by comparing the motion signal sensed during the post-ventricular atrial blanking ending time interval to an amplitude threshold.   58. The medical device of clause 57, wherein the control circuit is further configured to: based on the comparing of the motion signal to the amplitude threshold sensed during the post-ventricular atrial blanking ending time interval: decrease the post-ventricular atrial blanking period in response to the motion signal not crossing the amplitude threshold during the post-ventricular atrial blanking ending time interval or hold the post-ventricular atrial blanking period constant in response to the motion signal crossing the amplitude threshold during the post-ventricular atrial blanking ending time interval.   59. The medical device of any of clauses 46-58, wherein the control circuit is further configured to: disable motion signal sensing by the motion sensor during each of a first plurality of the post-ventricular atrial blanking periods; detect a change in a heart rate; in response to detecting the change in heart rate, enable motion signal sensing by the motion sensor during at least a portion of the at least one post-ventricular atrial blanking period.   60. The medical device of any of clauses 46-59, wherein the control circuit is further configured to: sense atrial event signals from the motion signal outside of the post-ventricular atrial blanking periods; detect a change in at least one of a frequency and a timing of the atrial event signals sensed from the motion signal; and determine the amplitude of the motion signal sensed during the at least one post-ventricular atrial blanking period in response to detecting the change in at least one of the frequency and the timing of the atrial event signals sensed from the motion signal.   61. The medical device of any of clauses 46-60, wherein the control circuit is further configured to: determine that the amplitude of the motion signal is greater than an amplitude threshold during a threshold time interval from an expiration of the at least one post-ventricular atrial blanking period; and adjust the post-ventricular atrial blanking period by increasing the post-ventricular atrial blanking period in response to the amplitude of the motion signal being greater than the amplitude threshold during the threshold time interval from the expiration of the at least one post-ventricular atrial blanking period.   62. The medical device of any of clauses 46-61, wherein the control circuit is further configured to sense an atrial event signal from the motion signal after an expiration of the post-ventricular atrial blanking period; and the pulse generator is configured to generate a ventricular pacing pulse in response to the control circuit sensing the atrial event signal.   63. A method comprising: sensing a motion signal; identifying a plurality of ventricular events; setting a post-ventricular atrial blanking period following each of a plurality of ventricular events; determining an amplitude of the motion signal sensed during at least one post-ventricular atrial blanking period of the post-ventricular atrial blanking periods; and adjusting the post-ventricular atrial blanking period based on the determined amplitude of the motion signal.   64. The method of clause 63, further comprising generating ventricular pacing pulses, wherein identifying the plurality of ventricular events comprises identifying at least one ventricular pacing pulse.   65. The method of any of clauses 63-64, further comprising: receiving a cardiac electrical signal; sensing ventricular events from the cardiac electrical signal; and identifying the plurality of ventricular events by identifying at least ventricular event sensed from the cardiac electrical signal.   66. The method of any of clauses 63-65, further comprising: determining the amplitude by determining a peak amplitude from the motion signal sensed during at least a portion of the at least one post-ventricular atrial blanking period; determining whether the peak amplitude is less than an amplitude threshold; and adjusting the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the peak amplitude being less than the amplitude threshold.   67. The method of any of clauses 63-66, further comprising: determining the amplitude by determining a peak amplitude of the motion signal sensed during at least a portion of the at least one post-ventricular atrial blanking period; determining whether the peak amplitude is greater than an amplitude threshold; and adjusting the post-ventricular atrial blanking period by increasing the post-ventricular atrial blanking period in response to the peak amplitude being greater than the amplitude threshold.   68. The method of any of clauses 63-67, further comprising adjusting the post-ventricular atrial blanking period by adjusting the duration of a post-ventricular atrial blanking period applicable to a future cardiac cycle.   69. The method of any of clauses 63-68, further comprising: determining the amplitude by detecting a latest amplitude threshold crossing by the motion signal during the post-ventricular atrial blanking period; determining that the amplitude threshold crossing is earlier than a threshold time interval from an expiration of the post-ventricular atrial blanking period; and adjusting the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the amplitude threshold crossing being earlier than the threshold time interval from the expiration of the post-ventricular atrial blanking period.   70. The method of clause 69, further comprising setting the threshold time interval based on a decrement interval used to adjust the post-ventricular atrial blanking period.   71. The method of any of clauses 63-70, further comprising: detecting a change in a heart rate; and determining the amplitude of the motion signal in response to detecting the change in the heart rate.   72. The method of any of clauses 63-71, further comprising: setting an amplitude analysis window during the at least one post-ventricular atrial blanking period; determining the amplitude by determining a peak amplitude during the amplitude analysis window; determining that the peak amplitude is less than an amplitude threshold; and adjusting the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the peak amplitude being less than the amplitude threshold.   73. The method of any of clauses 63-72, further comprising: detecting a decrease in a heart rate; increasing the post-ventricular atrial blanking period in response to detecting the decrease in the heart rate; detecting an increase in the heart rate; and based on the determined amplitude and the increase in the heart rate: adjusting the post-ventricular atrial blanking period by decreasing the post-ventricular atrial blanking period, or holding the post-ventricular atrial blanking period constant.   74. The method of any of clauses 63-73, further comprising: setting a post-ventricular atrial blanking ending time interval that begins before an expiration time of the at least one post-ventricular atrial blanking period; and determining the amplitude by comparing the motion signal sensed during the post-ventricular atrial blanking ending time interval to an amplitude threshold.   75. The method of clause 74, further comprising: based on the comparing of the motion signal to the amplitude threshold during the post-ventricular atrial blanking ending time interval: decreasing the post-ventricular atrial blanking period in response to the motion signal not crossing the amplitude threshold during the post-ventricular atrial blanking ending time interval; or holding the post-ventricular atrial blanking period constant in response to the motion signal crossing the amplitude threshold during the post-ventricular atrial blanking ending time interval.   76. The method of any of clauses 63-75, further comprising: disabling motion signal sensing by the motion sensor during each of a first plurality of the post-ventricular atrial blanking periods; detecting a change in a heart rate; and in response to detecting the change in heart rate, enabling motion signal sensing by the motion sensor during at least a portion of the at least one post-ventricular atrial blanking period.   77. The method of any of clauses 63-76, further comprising sensing atrial event signals from the motion signal outside of the post-ventricular atrial blanking periods; detecting a change in at least one of a frequency and a timing of the atrial event signals sensed from the motion signal; and determining the amplitude of the motion signal sensed during the at least one post-ventricular atrial blanking period in response to detecting the change in at least one of the frequency and the timing of the atrial event signals sensed from the motion signal.   78. The method of any of clauses 63-77, further comprising: determining that the amplitude of the motion signal is greater than an amplitude threshold during a threshold time interval from an expiration of the at least one post-ventricular atrial blanking period; and adjusting the post-ventricular atrial blanking period by increasing the post-ventricular atrial blanking period in response to the amplitude of the motion signal being greater than the amplitude threshold during the threshold time interval from the expiration of the at least one post-ventricular atrial blanking period.   79. The method of any of clauses 63-78, further comprising: sensing an atrial event signal from the motion signal after an expiration of the post-ventricular atrial blanking period; and generating a ventricular pacing pulse in response to sensing the atrial event signal.   80. A non-transitory, computer-readable medium storing instructions which, when executed by a processor of a medical device, cause the medical device to: sense a motion signal; identify a plurality of ventricular events; set a post-ventricular atrial blanking period following each of the plurality of ventricular events; determine an amplitude of the motion signal sensed during at least one post-ventricular atrial blanking period of the post-ventricular atrial blanking periods; and adjust the post-ventricular atrial blanking period based on the determined amplitude of the motion signal.   81. The non-transitory, computer-readable medium of clause 80, wherein the instructions further cause the medical device to: generate ventricular pacing pulses; and identify the plurality of ventricular events by identifying at least one ventricular pacing pulse.   82. The non-transitory, computer-readable medium of any of clauses 80-81, wherein the instructions further cause the medical device to: receive a cardiac electrical signal; sense ventricular events from the cardiac electrical signal; and identify the plurality of ventricular events by identifying at least ventricular event sensed from the cardiac electrical signal.   83. The non-transitory, computer-readable medium of any of clauses 80-82, wherein the instructions further cause the medical device to: determine the amplitude by determining a peak amplitude from the motion signal sensed during at least a portion of the at least one post-ventricular atrial blanking period; determine whether the peak amplitude is less than an amplitude threshold; and adjust the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the peak amplitude being less than the amplitude threshold.   84. The non-transitory, computer-readable medium of any of clauses 80-83, wherein the instructions further cause the medical device to: determine the amplitude by determining a peak amplitude of the motion signal sensed during at least a portion of the at least one post-ventricular atrial blanking period; determine whether the peak amplitude is greater than an amplitude threshold; and adjust the post-ventricular atrial blanking period by increasing the post-ventricular atrial blanking period in response to the peak amplitude being greater than the amplitude threshold.   85. The non-transitory, computer-readable medium of any of clauses 80-84, wherein the instructions further cause the medical device to adjust the post-ventricular atrial blanking period by adjusting the duration of a post-ventricular atrial blanking period applicable to a future cardiac cycle.   86. The non-transitory, computer-readable medium of any of clauses 80-85, wherein the instructions further cause the medical device to: determine the amplitude by detecting a latest amplitude threshold crossing by the motion signal during the post-ventricular atrial blanking period; determine that the amplitude threshold crossing is earlier than a threshold time interval from an expiration of the post-ventricular atrial blanking period; and adjust the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the amplitude threshold crossing being earlier than the threshold time interval from the expiration of the post-ventricular atrial blanking period.   87. The non-transitory, computer-readable medium of clause 86, wherein the instructions further cause the medical device to set the threshold time interval based on a decrement interval used to adjust the post-ventricular atrial blanking period.   88. The non-transitory, computer-readable medium of any of clauses 80-87, wherein the instructions further cause the medical device to: detect a change in a heart rate; and determine the amplitude of the motion signal in response to detecting the change in the heart rate.   89. The non-transitory, computer-readable medium of any of clauses 80-88, wherein the instructions further cause the medical device to: set an amplitude analysis window during the at least one post-ventricular atrial blanking period; determine the amplitude by determining a peak amplitude during the amplitude analysis window; determine that the peak amplitude is less than an amplitude threshold; and adjust the post-ventricular atrial blanking period by shortening the post-ventricular atrial blanking period in response to the peak amplitude being less than the amplitude threshold.   90. The non-transitory, computer-readable medium of any of clauses 80-89, wherein the instructions further cause the medical device to: detect a decrease in a heart rate; increase the post-ventricular atrial blanking period in response to detecting the decrease in the heart rate; detect an increase in the heart rate; and based on the determined amplitude and the increase in the heart rate: adjust the post-ventricular atrial blanking period by decreasing the post-ventricular atrial blanking period, or hold the post-ventricular atrial blanking period constant.   91. The non-transitory, computer-readable medium of any of clauses 80-90, wherein the instructions further cause the medical device to: set a post-ventricular atrial blanking ending time interval that begins before an expiration time of the at least one post-ventricular atrial blanking period; and determine the amplitude by comparing the motion signal sensed during the post-ventricular atrial blanking ending time interval to an amplitude threshold.   92. The non-transitory, computer-readable medium of clause 91, wherein the instructions further cause the medical device to: based on the comparing of the motion signal to the amplitude threshold during the post-ventricular atrial blanking ending time interval: decrease the post-ventricular atrial blanking period in response to the motion signal not crossing the amplitude threshold during the post-ventricular atrial blanking ending time interval; or hold the post-ventricular atrial blanking period constant in response to the motion signal crossing the amplitude threshold during the post-ventricular atrial blanking ending time interval.   93. The non-transitory, computer-readable medium of any of clauses 80-92, wherein the instructions further cause the medical device to: disable motion signal sensing by the motion sensor during each of a first plurality of the post-ventricular atrial blanking periods; detect a change in a heart rate; and in response to detecting the change in heart rate, enable motion signal sensing by the motion sensor during at least a portion of the at least one post-ventricular atrial blanking period.   94. The non-transitory, computer-readable medium of any of clauses 80-93,wherein the instructions further cause the medical device to: sense atrial event signals from the motion signal outside of the post-ventricular atrial blanking periods; detect a change in at least one of a frequency and a timing of the atrial event signals sensed from the motion signal; and determine the amplitude of the motion signal sensed during the at least one post-ventricular atrial blanking period in response to detecting the change in at least one of the frequency and the timing of the atrial event signals sensed from the motion signal.   95. The non-transitory, computer-readable medium of any of clauses 80-94, wherein the instructions further cause the medical device to: determine that the amplitude of the motion signal is greater than an amplitude threshold during a threshold time interval from an expiration of the at least one post-ventricular atrial blanking period; and adjust the post-ventricular atrial blanking period by increasing the post-ventricular atrial blanking period in response to the amplitude of the motion signal being greater than the amplitude threshold during the threshold time interval from the expiration of the at least one post-ventricular atrial blanking period.   96. The non-transitory, computer-readable medium of any of clauses 80-95, wherein the instructions further cause the medical device to: sense an atrial event signal from the motion signal after an expiration of the post-ventricular atrial blanking period; and generate a ventricular pacing pulse in response to sensing the atrial event signal.   

     The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a conceptual diagram illustrating a medical device system that may be used to sense cardiac electrical signals and motion signals induced by cardiac motion and flowing blood and provide pacing therapy to a patient&#39;s heart. 
         FIG.  2    is a conceptual diagram of the pacemaker shown in  FIG.  1   . 
         FIG.  3    is a conceptual diagram of an example configuration of the pacemaker shown in  FIG.  1   . 
         FIG.  4    is an example of a motion sensor signal that may be acquired by a motion sensor included in the pacemaker of  FIG.  1    over a cardiac cycle. 
         FIG.  5    is an example of motion sensor signals acquired over two different cardiac cycles. 
         FIG.  6    is a flow chart of a method for adjusting a PVAB period according to one example. 
         FIG.  7    is a conceptual diagram of a motion signal illustrating amplitude data that may be determined by the pacemaker of  FIG.  1    during the PVAB period according to some examples. 
         FIG.  8    is a flow chart of a method for controlling PVAB period adjustments according to another example. 
         FIG.  9    is a flow chart of a method for adjusting the PVAB period according to another example. 
         FIG.  10    is a flow chart of a method for setting the PVAB period and atrial synchronized ventricular pacing according to another example. 
         FIG.  11    is a timing diagram of one example of ventricular events and corresponding PVAB periods that are adjusted according to the example techniques of  FIG.  10   . 
         FIG.  12    is a flow chart of a method for adjusting the PVAB period and controlling atrial synchronized ventricular pacing according to one example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure describes techniques for adjusting a PVAB period by a medical device. As described below, atrial event signals that correspond to atrial systole, e.g., atrial myocardial depolarization or atrial contraction, may be sensed from a cardiac signal by a ventricular pacemaker configured to deliver atrial synchronized ventricular pacing. The atrial event signal may be sensed from a variety of cardiac signals, e.g., by sensing an atrial P-wave attendant to atrial myocardial depolarization from a cardiac electrical signal or sensing an atrial systolic event signal attendant to atrial contraction from a cardiac motion signal. In some examples, the cardiac motion signal is an acceleration signal sensed by an accelerometer. 
     A ventricular pacemaker, which may be wholly implantable within a ventricular heart chamber may include a motion sensor such as an accelerometer for sensing a cardiac motion signal, e.g., an intraventricular motion signal. Atrial event signals corresponding to atrial mechanical contraction and the active filling phase of the ventricle, sometimes referred to as the “atrial kick,” can be detected from the motion signal sensed from within the ventricle for use in controlling the timing of ventricular pacing pulses synchronized to atrial events. The techniques disclosed herein provide techniques for promoting reliable sensing of atrial event signals without falsely sensing ventricular event signals associated with ventricular systole by setting and adjusting a PVAB period. 
       FIG.  1    is a conceptual diagram illustrating an implantable medical device (IMD) system  10  that may be used to sense cardiac electrical signals and motion signals induced by cardiac motion and flowing blood and provide pacing therapy to a patient&#39;s heart  8 . IMD system  10  includes a ventricular intracardiac pacemaker  14 . Pacemaker  14  may be a transcatheter intracardiac pacemaker which is adapted for implantation wholly within a heart chamber, e.g., wholly within the right ventricle (RV) or wholly within the left ventricle (LV) of heart  8  for sensing cardiac signals and delivering ventricular pacing pulses. Pacemaker  14  may be reduced in size compared to subcutaneously implanted pacemakers and may be generally cylindrical in shape to enable transvenous implantation via a delivery catheter. 
     Pacemaker  14  is shown positioned in the RV, along an endocardial wall, e.g., near the RV apex though other locations are possible. The techniques disclosed herein are not limited to the pacemaker location shown in the example of  FIG.  1   . For example, pacemaker  14  may be positioned along the interventricular septal wall. Pacemaker  14  may be positioned within or on the RV or LV to provide respective right ventricular or left ventricular pacing and for sensing cardiac motion signals by a motion sensor from a ventricular location for providing atrial synchronized ventricular pacing. In some examples, pacemaker  14  may be adapted for implantation in the right atrium for delivering ventricular pacing pulses via a tip electrode that is advanced into the area of the bundle of His for delivering pacing pulses that capture the native ventricular conduction system and/or ventricular myocardial tissue. Various examples of a pacemaker configured for delivering atrial synchronous ventricular pacing from an atrial chamber implant site are generally described in U.S. Patent Publication No. 2019/0083779 (Yang, et al.), incorporated herein by reference in its entirety. 
     Pacemaker  14  is capable of producing electrical stimulation pulses, e.g., pacing pulses, delivered to heart  8  via one or more electrodes on the outer housing of the pacemaker. Pacemaker  14  may be a leadless pacemaker configured to deliver ventricular pacing pulses and sense a cardiac electrical signal using housing based electrodes for producing an intracardiac electrogram (EGM) signal. The cardiac electrical signals may be sensed using the housing based electrodes that are also used to deliver ventricular pacing pulses. 
     According to the techniques described herein, atrial systolic events e.g., contractions, that can be associated with the active ventricular filling phase are detected by pacemaker  14  from a motion sensor signal such as an acceleration signal sensed by an accelerometer enclosed by the housing of pacemaker  14 . The motion signal produced by an accelerometer implanted within a ventricular chamber, which may be referred to as an “intraventricular motion signal,” includes motion signals caused by ventricular and atrial events. For example, acceleration of blood flowing into the RV through the tricuspid valve  16  between the RA and RV caused by atrial systole, and referred to as the “atrial kick,” may be detected by pacemaker  14  from the acceleration signal produced by an accelerometer included in pacemaker  14 . Other cardiac motion signals that may be detected by pacemaker  14 , such as motion signals caused by ventricular contraction, ventricular relaxation, and passive ventricular filling, are described below in conjunction with  FIG.  4   . 
     Pacemaker  14  is configured to control the delivery of ventricular pacing pulses in a manner that promotes synchrony between atrial activation and ventricular activation, e.g., by maintaining a target atrioventricular (AV) interval between atrial events and ventricular pacing pulses. That is, pacemaker  14  controls pacing pulse delivery to maintain a desired AV interval between a time of sensing an atrial event signal corresponding to atrial systole and the time of generating a ventricular pacing pulse delivered to cause ventricular depolarization and ventricular systole. 
     A target AV interval may be a default value or a programmed value selected by a clinician and can be the time interval from the detection of the atrial event until delivery of the ventricular pacing pulse. In some instances, the target AV interval may be started from the time the atrial systolic event is detected based on a motion sensor signal or starting from an identified fiducial point of the atrial event signal. The target AV interval may be identified as being hemodynamically optimal for a given patient based on clinical testing or assessments of the patient or based on clinical data from a population of patients. The target AV interval may be determined to be optimal based on relative timing of electrical and mechanical events as identified from the cardiac electrical signal received by pacemaker  14  and the motion sensor signal received by pacemaker  14 . 
     Pacemaker  14  may be capable of bidirectional wireless communication with an external device  20  for programming the AV pacing interval and other pacing control parameters as well as cardiac event sensing parameters, which may be utilized for detecting ventricular mechanical events and/or the atrial systolic event from the motion sensor signal. External device  20  is often referred to as a “programmer” because it is typically used by a physician, technician, nurse, clinician or other qualified user for programming operating parameters in pacemaker  14 . External device  20  may be located in a clinic, hospital or other medical facility. External device  20  may alternatively be embodied as a home monitor or a handheld device that may be used in a medical facility, in the patient&#39;s home, or another location. Operating parameters, including sensing and therapy delivery control parameters, may be programmed into pacemaker  14  using external device  20 . 
     External device  20  may include a processor  52 , memory  53 , display  54 , user interface  56  and telemetry unit  58 . Processor  52  controls external device operations and processes data and signals received from pacemaker  14 . Display unit  54  may generate a display, which may include a graphical user interface, of data and information relating to pacemaker functions to a user for reviewing pacemaker operation and programmed parameters as well as cardiac electrical signals, cardiac motion signals or other physiological data that may be acquired by pacemaker  14  and transmitted to external device  20  during an interrogation session. User interface  56  may include a mouse, touch screen, keypad or the like to enable a user to interact with external device  20  to initiate a telemetry session with pacemaker  14  for retrieving data from and/or transmitting data to pacemaker  14 , including programmable parameters for controlling cardiac event sensing and therapy delivery. 
     External device telemetry unit  58  is configured for bidirectional communication with implantable telemetry circuitry included in pacemaker  14 . Telemetry unit  58  includes a transceiver and antenna for establishing a wireless communication link  24  with pacemaker  14  and is configured to operate in conjunction with processor  52  for sending and receiving data relating to pacemaker functions via the communication link  24 . Communication link  24  may be established using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, Medical Implant Communication Service (MICS) or other communication bandwidth. In some examples, external device  20  may include a programming head that is placed proximate pacemaker  14  to establish and maintain a communication link  24 , and in other examples external device  20  and pacemaker  14  may be configured to communicate using a distance telemetry algorithm and circuitry that does not require the use of a programming head and does not require user intervention to maintain a communication link. 
     It is contemplated that external device  20  may be in wired or wireless connection to a communications network via a telemetry circuit that includes a transceiver and antenna or via a hardwired communication line for transferring data to a centralized database or computer to allow remote management of the patient. Remote patient management systems including a centralized patient database may be configured to utilize the presently disclosed techniques to enable a clinician to review EGM, motion sensor signal, and marker channel data and authorize programming of sensing and therapy control parameters in pacemaker  14 , e.g., after viewing a visual representation of EGM, motion sensor signal and marker channel data. 
       FIG.  2    is a conceptual diagram of the pacemaker  14  shown in  FIG.  1   . Pacemaker  14  includes electrodes  162  and  164  spaced apart along the housing  150  of pacemaker  14  for sensing cardiac electrical signals and delivering pacing pulses. Electrode  164  is shown as a tip electrode extending from a distal end  102  of pacemaker  14 , and electrode  162  is shown as a ring electrode along a mid-portion of housing  150 , for example adjacent housing proximal end  104 . Distal end  102  is referred to as “distal” in that it is expected to be the leading end as pacemaker  14  is advanced through a delivery tool, such as a catheter, and placed against a targeted implant and pacing site. 
     Electrodes  162  and  164  form an anode and cathode pair for bipolar cardiac pacing and sensing. In alternative embodiments, pacemaker  14  may include two or more ring electrodes, two tip electrodes, and/or other types of electrodes exposed along pacemaker housing  150  for delivering electrical stimulation to heart  8  and sensing cardiac electrical signals. Electrodes  162  and  164  may be, without limitation, titanium, platinum, iridium or alloys thereof and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black, among others. Electrodes  162  and  164  may be positioned at locations along pacemaker  14  other than the locations shown. 
     Housing  150  is formed from a biocompatible material, such as a stainless steel or titanium alloy. In some examples, the housing  150  may include an insulating coating. Examples of insulating coatings include parylene, urethane, PEEK, or polyimide, among others. The entirety of the housing  150  may be insulated, but only electrodes  162  and  164  uninsulated. Electrode  164  may serve as a cathode electrode and be coupled to internal circuitry, e.g., a pacing pulse generator and cardiac electrical signal sensing circuitry, enclosed by housing  150  via an electrical feedthrough crossing housing  150 . Electrode  164  may be a button electrode, hemispherical electrode, ring electrode, segmented electrode, helical electrode, fishhook electrode or other tissue-piercing electrode or other shape or configuration in various examples. 
     Electrode  162  may be formed as a conductive portion of housing  150  defining a ring electrode circumscribing a lateral sidewall of housing  150  that is electrically isolated from the other portions of the housing  150  as generally shown in  FIG.  2   . The lateral sidewall extends from proximal end  104  to distal end  102  of housing  150 . In other examples, the entire periphery of the housing  150  may function as an electrode that is electrically isolated from tip electrode  164 , instead of providing a localized ring electrode such as anode electrode  162 . Electrode  162  formed along an electrically conductive portion of housing  150  may serve as a return anode during pacing and sensing with electrode  164  serving as the cathode electrode. Electrode  162  may alternatively circumscribe a portion of the lateral sidewall or be configured as a button, segmented or other type of electrode. 
     The housing  150  encloses a control electronics subassembly  152  and a battery subassembly  160 , which provides power to the control electronics subassembly  152 . Battery subassembly  160  may include one or more chargeable or non-rechargeable batteries for powering one or more processor(s), sensor(s), pulse generator, sensing circuit, and other circuitry of control electronics subassembly  152 . Control electronics subassembly  152  houses the electronics for sensing cardiac signals, generating pacing pulses and controlling therapy delivery and other functions of pacemaker  14  as described below in conjunction with  FIG.  3   . A motion sensor may be implemented as an accelerometer enclosed within housing  150  in some examples. The accelerometer may provide the sensed acceleration signal to a processor included in control electronics subassembly  152  for signal processing and analysis for detecting atrial systolic events, e.g., for use in controlling the timing of ventricular pacing pulses, as described below. 
     The accelerometer may be a three-dimensional accelerometer. In some examples, the accelerometer may have one “longitudinal” axis that is parallel to or aligned with the longitudinal axis  108  of pacemaker  14  and two orthogonal axes that extend in radial directions relative to the longitudinal axis  108 . Practice of the techniques disclosed herein, however, are not limited to a particular orientation of the accelerometer within or along housing  150 . In other examples, a one-dimensional accelerometer may be used to sense a motion signal from which atrial systolic events are detected. In still other examples, a two dimensional accelerometer or other multi-dimensional accelerometer may be used. Each axis of a single or multi-dimensional accelerometer may be defined by a piezoelectric element, micro-electrical mechanical system (MEMS) device or other sensor element capable of producing an electrical signal in response to changes in acceleration imparted on the sensor element, e.g., by converting the acceleration to a force or displacement that is converted to the electrical signal. In a multi-dimensional accelerometer, the sensor elements may be arranged orthogonally with each sensor element axis orthogonal relative to the other sensor element axes. Orthogonal arrangement of the elements of a multi-axis accelerometer, however, is not necessarily required. 
     Each sensor element may produce an acceleration signal corresponding to a vector aligned with the axis of the sensor element. Pacemaker  14  may be configured to select a vector signal of a multi-dimensional accelerometer (also referred to as a “multi-axis” accelerometer) for use in sensing atrial systolic events. In some cases one, two or all three axis signals produced by a three dimensional accelerometer may be selected as a vector signal for use in detecting atrial systolic events, e.g., for controlling atrial synchronized ventricular pacing delivered by pacemaker  14 . 
     Pacemaker  14  may include a set of fixation tines  166  to secure pacemaker  14  to patient tissue, e.g., by actively engaging with the ventricular endocardium and/or interacting with the ventricular trabeculae. Fixation tines  166  are configured to anchor pacemaker  14  to position electrode  164  in operative proximity to a targeted tissue for delivering therapeutic electrical stimulation pulses. Numerous types of active and/or passive fixation members may be employed for anchoring or stabilizing pacemaker  14  in an implant position. It is to be understood that the size, shape, and locations of electrodes  162  and  164  and fixation times  166 , if present, may vary depending on the implant location of pacemaker  14  as needed for sensing cardiac signals and delivering ventricular pacing pulses to a target tissue. Pacemaker  14  may optionally include a delivery tool interface  158 . Delivery tool interface  158  may be located at the proximal end  104  of pacemaker  14  and is configured to connect to a delivery device, such as a catheter, used to position pacemaker  14  at an implant location during an implantation procedure, for example within a heart chamber. 
       FIG.  3    is a conceptual diagram of an example configuration of pacemaker  14  shown in  FIG.  1   . Pacemaker  14  includes a pulse generator  202 , a cardiac electrical signal sensing circuit  204 , a control circuit  206 , memory  210 , telemetry circuit  208 , motion sensor  212  and a power source  214 . The various circuits represented in  FIG.  3    may be combined on one or more integrated circuit boards which include a specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine or other suitable components that provide the described functionality. 
     Motion sensor  212  includes an accelerometer in the examples described herein. Motion sensor  212  is not limited to being an accelerometer, however, and other motion sensors may be utilized successfully in pacemaker  14  for detecting cardiac motion signals according to the techniques described herein. Examples of motion sensors that may be implemented in motion sensor  212  include piezoelectric sensors and MEMS devices. In other examples, a sensor capable of sensing a signal responsive to mechanical function, e.g., heart chamber contraction and relaxation, heart valve opening and closure, etc., may be included in motion sensor  212 . Other examples of sensors that may be included in a pacemaker configured to perform the techniques disclosed herein for controlling a PVAB period include an impedance sensor and a pressure sensor, for instance. Pacemaker  14  may include an impedance measurement circuit for sensing an impedance signal from which atrial events signals may be detected in some examples. In other examples, pacemaker  14  may include a pressure sensor for sensing a pressure signal and detecting an atrial event signal from the pressure signal. In these examples, a PVAB period may be set following a ventricular event and adjusted based on heart rate and/or an amplitude of the signal determined during the PVAB period according to the techniques disclosed herein. 
     Motion sensor  212  may include a multi-axis accelerometer, e.g., a two-dimensional or three-dimensional accelerometer, with each axis providing an axis signal that may be analyzed individually or in combination for detecting cardiac mechanical events. Motion sensor  212  produces an electrical signal correlated to motion or vibration of sensor  212  (and pacemaker  14 ), e.g., when subjected to flowing blood and/or cardiac motion. The motion sensor  212  may include one or more filter, amplifier, rectifier, analog-to-digital converter (ADC) and/or other components for producing a motion signal that is passed to control circuit  206 . For example, each vector signal produced by each individual axis of a multi-axis accelerometer, or one or more selected axis signals, may be filtered by a high pass filter, e.g., a 10 Hz high pass filter. The filtered signal may be digitized by an ADC and rectified for use by atrial event detector circuit  240  for detecting atrial systolic events. The high pass filter may be lowered (e.g., to 5 Hz) if needed to detect atrial signals that have lower frequency content. In some examples, high pass filtering is performed with no low pass filtering. In other examples, each accelerometer axis signal is filtered by a low pass filter, e.g., a 30 Hz low pass filter, with or without high pass filtering. 
     One example of an accelerometer for use in implantable medical devices that may be implemented in conjunction with the techniques disclosed herein is generally disclosed in U.S. Pat. No. 5,885,471 (Ruben, et al.), incorporated herein by reference in its entirety. An implantable medical device arrangement including a piezoelectric accelerometer for detecting patient motion is disclosed, for example, in U.S. Pat. No. 4,485,813 (Anderson, et al.) and U.S. Pat. No. 5,052,388 (Sivula, et al.), both of which patents are hereby incorporated by reference herein in their entirety. Examples of three-dimensional accelerometers that may be implemented in pacemaker  14  and used for detecting cardiac mechanical events using the presently disclosed techniques are generally described in U.S. Pat. No. 5,593,431 (Sheldon) and U.S. Pat. No. 6,044,297 (Sheldon), both of which are incorporated herein by reference in their entirety. Other accelerometer designs may be used for producing an electrical signal that is correlated to motion, e.g., acceleration, imparted on pacemaker  14  due to ventricular and atrial events. 
     Cardiac electrical signal sensing circuit  204 , also referred to herein as “sensing circuit”  204 , is configured to receive a cardiac electrical signal via electrodes  162  and  164  by a pre-filter and amplifier circuit  220 . Pre-filter and amplifier circuit may include a high pass filter to remove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or a wideband filter having a passband of 2.5 Hz to 100 Hz to remove DC offset and high frequency noise. Pre-filter and amplifier circuit  220  may further include an amplifier to amplify the “raw” cardiac electrical signal passed to analog-to-digital converter (ADC)  226 . ADC  226  may pass a multi-bit, digital EGM signal to control circuit  206  for performing morphology analysis of the EGM signal, storing EGM signal segments in memory  210  for transmission to an external device or other processing and analysis. For example, the EGM signal may be received by atrial event detector circuit  240  for use in identifying ventricular electrical events (e.g., R-waves or T-waves) and/or atrial electrical events, e.g., P-waves. Identification of cardiac electrical events may be used in algorithms for establishing atrial sensing control parameters and for detecting atrial systolic events from the motion sensor signal. The digital signal from ADC  226  may be passed to rectifier and amplifier circuit  222 , which may include a rectifier, bandpass filter, and amplifier for passing a cardiac signal to cardiac event detector  224 . 
     Cardiac event detector  224  may include a sense amplifier or other detection circuitry that compares the incoming rectified, cardiac electrical signal to an R-wave sensing threshold, which may be an auto-adjusting threshold, for sensing intrinsic R-waves attendant to intrinsic ventricular myocardial depolarizations. When the incoming signal crosses the R-wave sensing threshold, the cardiac event detector  224  senses a ventricular event and produces a ventricular sensed event signal that is passed to control circuit  206 . In other examples, cardiac event detector  224  may receive the digital output of ADC  226  for detecting R-waves by a comparator, morphological signal analysis of the digital EGM signal or other R-wave detection techniques. Processor  244  of control circuit  206  may provide sensing control signals to sensing circuit  204 , e.g., for controlling the R-wave sensing threshold including the R-wave sensing sensitivity, and various blanking and refractory intervals that may be applied to the cardiac electrical signal for controlling R-wave sensing. 
     Ventricular sensed event signals passed from cardiac event detector  224  to control circuit  206  may be used for scheduling ventricular pacing pulses by pace timing circuit  242  and for use in identifying the timing of ventricular electrical events. For example, ventricular event intervals (RRIs), sometimes referred to as VV intervals (or “VVIs” but not to be confused with a VVI pacing mode), may be determined by control circuit  206  as time intervals between consecutively received ventricular sensed event signals. The RRIs (or corresponding heart rate) may be determined and used in setting or adjusting a PVAB period according to techniques disclosed herein. 
     In some examples, sensing circuit  204  may include multiple sensing channels including a ventricular sensing channel for sensing R-waves by cardiac event detector  224  and an atrial sensing channel for sensing P-waves attendant to atrial myocardial depolarization, for example, by cardiac event detector  224 . In these examples, cardiac event detector  224  may generate both ventricular sensed event signals and atrial sensed event signals that may be passed to control circuit  206  for use in controlling atrial synchronized ventricular pacing pulse timing. The atrial event signals may be sensed from a cardiac electrical signal sensed by the same electrodes  162  and  164  but may undergo different filtering, amplification, and blanking than the cardiac electrical signal that the R-waves are sensed from. Cardiac event detector  224  may include a sense amplifier, comparator or other detection circuitry configured for sensing an atrial P-wave in response to a P-wave sensing threshold crossing by the cardiac electrical signal. The illustrative examples presented herein for controlling a PVAB period for inhibiting sensing of an atrial event signal from a motion signal following a ventricular event may be adapted for use in controlling a PVAB period applied to the cardiac electrical signal in some examples. The PVAB period may be started in response to an identified ventricular event, e.g., a ventricular pacing pulse or ventricular sensed event signal from sensing circuit  204 . Atrial P-wave sensing by sensing circuit  204  may be inhibited during the PVAB period. The PVAB period may be adjusted based on an analysis of the cardiac electrical signal amplitude during the PVAB period according to the techniques disclosed herein. 
     Control circuit  206  may include an atrial event detector circuit  240 , pace timing circuit  242 , and processor  244 . Control circuit  206  may receive ventricular sensed event signals and/or digital cardiac electrical signals from sensing circuit  204  for use in detecting and confirming cardiac events and controlling ventricular pacing. For example, ventricular sensed event signals may be passed to pace timing circuit  242  for inhibiting scheduled ventricular pacing pulses or scheduling ventricular pacing pulses by starting a pacing escape interval when pacemaker  14  is operating in a non-atrial tracking ventricular pacing mode. Ventricular sensed event signals may be passed to atrial event detector circuit  240  for use in setting the PVAB period and, in some examples, a refractory period and/or one or more time windows used by control circuit  206  in sensing atrial event signals from the motion sensor signal. 
     Atrial event detector circuit  240  is configured to detect atrial event signals from the motion signal received from motion sensor  212 . Techniques for detecting atrial event signals are described below, e.g., in conjunction with  FIG.  5   . In some examples, one or more ventricular mechanical event signals may be detected from the motion sensor signal in a given cardiac cycle to facilitate positive detection of the atrial event signal from the motion sensor signal during the ventricular cycle. As disclosed herein, control circuit  206  may be configured to determine an amplitude of the motion sensor signal, which may correspond to the amplitude or relative timing of a ventricular event signal in the motion signal, for use in setting or adjusting the duration of the PVAB period. 
     Atrial event detector circuit  240  receives a motion signal from motion sensor  212  and may start the PVAB period in response to a ventricular electrical event. The ventricular electrical event may be a ventricular event sensed by sensing circuit  204 , which may be identified by control circuit  206  based on a ventricular sensed event signal received from sensing circuit  204  corresponding to an intrinsic R-wave sensed by sensing circuit  204 . The ventricular electrical event may be the delivery of a ventricular pacing pulse by pulse generator  202 . The PVAB period may extend for a time period after the ventricular electrical event during which ventricular mechanical events, e.g., ventricular contraction followed by closure of the aortic and pulmonic valves, marking the approximate offset or end of ventricular mechanical systole, are expected to occur. When ventricular pacing is properly synchronized to atrial events, an atrial event signal is not expected to occur during the PVAB period, generally corresponding to ventricular systole. Motion signal peaks that may occur during the PVAB period, therefore, are not sensed as atrial event signals by atrial event detector circuit  240 . 
     The motion sensor signal, however, may still be sensed during all or a portion of the PVAB period in some examples. Control circuit  206  may receive the motion sensor signal during the PVAB period during at least some ventricular cycles for processing and analysis for use in setting or adjusting the PVAB period. As described below, an amplitude of the motion sensor signal may be determined during the PVAB period, which may be a maximum peak amplitude or a predetermined amplitude threshold crossing. In some examples, an associated time of the determined amplitude, e.g., a maximum peak amplitude time and/or a latest time of a predetermined amplitude threshold crossing during the PVAB period, may be determined for use setting or adjusting the PVAB period. 
     Atrial event detector circuit  240  determines if the motion sensor signal satisfies atrial systolic event detection criteria outside of the PVAB period. Atrial event detector circuit  240  may set time windows corresponding to the passive ventricular filling phase and the active ventricular filling phase of the ventricular cycle based on the timing of a preceding ventricular electrical event, either a ventricular sensed event signal received from sensing circuit  204  or a ventricular pacing pulse delivered by pulse generator  202 . A motion sensor signal crossing of an atrial event sensing threshold during either of these windows may be detected as the atrial event. As described below, two different atrial event sensing threshold values may be established for applying a first, higher threshold value during the passive filling phase window (also referred to herein as an “A3 window”) and a second, lower threshold value after the passive filling phase window (e.g., during an active filling phase window also referred to below as an “A4 window”). The earliest crossing of the atrial event sensing threshold by the motion signal may be detected as the atrial event signal by atrial event detector circuit  240 . 
     Atrial event detector circuit  240  may pass an atrial event detection signal to processor  244  and/or pace timing circuit  242  in response to sensing an atrial event signal. Pace timing circuit  242  (or processor  244 ) may additionally receive ventricular sensed event signals from cardiac event detector  224  for use in controlling the timing of pacing pulses delivered by pulse generator  202 . Processor  244  may include one or more clocks for generating clock signals that are used by pace timing circuit  242  to time out an AV pacing interval that is started upon receipt of an atrial event detection signal from atrial event detector circuit  240 . Pace timing circuit  242  may include one or more pacing escape interval timers or counters that are used to time out the AV pacing interval, which may be a programmable interval stored in memory  210  and retrieved by processor  244  for use in setting the AV pacing interval used by pace timing circuit  242 . 
     Pace timing circuit  242  may additionally include a lower pacing rate interval timer for controlling a minimum ventricular pacing rate. For example, if an atrial event signal is not sensed from the motion sensor signal that triggers a ventricular pacing pulse at the programmed AV pacing interval, a ventricular pacing pulse may be delivered by pulse generator  202  upon expiration of the lower pacing rate interval to prevent ventricular asystole and maintain a minimum ventricular rate. The lower pacing rate interval may be adjusted to a rate smoothing interval based on recent RRIs to avoid a sudden change in the ventricular rate. At times, control circuit  206  may control pulse generator  202  in a non-atrial tracking ventricular pacing mode (also referred to as “asynchronous ventricular pacing”), e.g., when the atrial rate is greater than an upper tracking rate limit, during rate response pacing, or during various processes that control circuit  206  may perform for establishing sensing control parameters used for sensing atrial event signals from the motion signal. 
     In some instances, pacemaker  14  may operate in a non-atrial tracking ventricular pacing mode with dual chamber sensing, which may be denoted as a VDI pacing mode, in which ventricular pacing pulses are delivered in the absence of a sensed R-wave and inhibited in response to a ventricular sensed event signal from sensing circuit  204 . Dual chamber sensing may be performed during the non-atrial tracking ventricular pacing mode by sensing ventricular electrical events by sensing circuit  204  and sensing atrial event signals from the motion signal received by atrial event detector circuit  240  from motion sensor  212 . Some atrial event sensing parameters may be established during the VDI pacing mode, which may include an atrial event sensing vector of the motion sensor for producing the motion signal from which the atrial event signals are sensed, the end of the passive ventricular filling window, and the atrial event sensing threshold amplitude values applied during and after the passive ventricular filling window. Techniques for establishing and adjusting atrial event sensing control parameters are generally disclosed in U.S. Pat. No. 10,449,366 (Splett, et al.), U.S. Publication No. 2021/0236825 (Sheldon, et al.), and U.S. Publication No. 2021/0236826 (Sheldon, et al.), all of which are incorporated herein by reference in their entirety. 
     Pulse generator  202  generates electrical pacing pulses that are delivered to the patient&#39;s heart via cathode electrode  164  and return anode electrode  162 . In addition to providing control signals to pace timing circuit  242  and pulse generator  202  for controlling the timing of ventricular pacing pulses, processor  244  may retrieve programmable pacing control parameters from memory  210 , such as pacing pulse amplitude and pacing pulse width, which are passed to pulse generator  202  for controlling pacing pulse delivery. 
     Pulse generator  202  may include charging circuit  230 , switching circuit  232  and an output circuit  234 . Charging circuit  230  may include a holding capacitor that may be charged to a pacing pulse amplitude by a multiple of the battery voltage signal of power source  214  under the control of a voltage regulator. The pacing pulse amplitude may be set based on a control signal from control circuit  206 . Switching circuit  232  may control when the holding capacitor of charging circuit  230  is coupled to the output circuit  234  for delivering the pacing pulse. For example, switching circuit  232  may include a switch that is activated by a timing signal received from pace timing circuit  242  upon expiration of an AV pacing interval (or lower rate pacing interval) and kept closed for a programmed pacing pulse width to enable discharging of the holding capacitor of charging circuit  230 . The holding capacitor, previously charged to the pacing pulse voltage amplitude, is discharged across electrodes  162  and  164  through the output capacitor of output circuit  234  for the programmed pacing pulse duration. Examples of pacing circuitry generally disclosed in U.S. Pat. No. 5,507,782 (Kieval, et al.) and in U.S. Pat. No. 8,532,785 (Crutchfield, et al.), both of which patents are incorporated herein by reference in their entirety, may be implemented in pacemaker  14  for charging a pacing capacitor to a predetermined pacing pulse amplitude under the control of control circuit  206  and delivering a pacing pulse. 
     Memory  210  may include computer-readable instructions that, when executed by control circuit  206 , cause control circuit  206  to perform various functions attributed throughout this disclosure to pacemaker  14 . The computer-readable instructions may be encoded within memory  210 . Memory  210  may include any non-transitory, computer-readable storage media including any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or other digital media with the sole exception being a transitory propagating signal. Memory  210  may store timing intervals and other data used by control circuit  206  to control the delivery of pacing pulses by pulse generator  202 , e.g., by setting PVAB periods according to the techniques disclosed herein, sensing atrial event signals by atrial event detector circuit  240  from the motion sensor signal outside of the PVAB periods, and setting a pacing escape interval timer included in pace timing circuit  242  to an AV pacing interval in response to sensed atrial event signals. 
     Power source  214  provides power to each of the other circuits and components of pacemaker  14  as required. Power source  214  may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source  214  and other pacemaker circuits and components are not shown in  FIG.  3    for the sake of clarity but are to be understood from the general block diagram of  FIG.  3   . For example, power source  214  may provide power as needed to charging and switching circuitry included in pulse generator  202 , amplifiers, ADC  226  and other components of sensing circuit  204 , telemetry circuit  208 , memory  210 , and motion sensor  212 . Power source  214  may correspond to battery subassembly  160  shown in  FIG.  2   . 
     Telemetry circuit  208  includes a transceiver  209  and antenna  211  for transferring and receiving data via a radio frequency (RF) communication link. Telemetry circuit  208  may be capable of bi-directional communication with external device  20  ( FIG.  1   ) as described above. Motion sensor signals and cardiac electrical signals, and/or data derived therefrom may be transmitted by telemetry circuit  208  to external device  20 . Programmable control parameters and algorithms for performing atrial event sensing and ventricular pacing control may be received by telemetry circuit  208  and stored in memory  210  for access by control circuit  206 . 
     The functions attributed to pacemaker  14  herein may be embodied as one or more processors, controllers, hardware, firmware, software, or any combination thereof. Depiction of different features as specific circuitry is intended to highlight different functional aspects and does not necessarily imply that such functions must be realized by separate hardware, firmware or software components or by any particular circuit architecture. Rather, functionality associated with one or more circuits described herein may be performed by separate hardware, firmware or software components, or integrated within common hardware, firmware or software components. For example, sensing atrial event signals from the motion sensor signal and ventricular pacing control operations performed by pacemaker  14  may be implemented in control circuit  206  in hardware, firmware and/or software executing instructions stored in memory  210  and relying on input from sensing circuit  204  and motion sensor  212 . Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modern pacemaker, given the disclosure herein, is within the abilities of one of skill in the art. 
       FIG.  4    is an example of a motion sensor signal  250  that may be acquired by motion sensor  212  over a cardiac cycle. Vertical dashed lines  252  and  262  denote the timing of two consecutive ventricular events (an intrinsic ventricular depolarization or a ventricular pacing pulse), marking the respective beginning and end of the ventricular cycle  251 . The motion signal includes an A1 event  254 , an A2 event  256 , an A3 event  258  and an A4 event  260 . The A1 event  254  is an acceleration signal (in this example when motion sensor  212  is implemented as an accelerometer) that occurs during ventricular contraction and marks the approximate onset of ventricular mechanical systole. The A1 event is also referred to herein as a “ventricular contraction event.” The A2 event  256  is an acceleration signal that may occur with closure of the aortic and pulmonic valves, marking the approximate offset or end of ventricular mechanical systole. The A3 event  258  is an acceleration signal that occurs during passive ventricular filling and marks ventricular mechanical diastole. The A3 event is also referred to herein as the “ventricular passive filling event.” The A1 through A3 events are ventricular events associated with ventricular myocardial contraction and relaxation. 
     The A4 event  260  is an acceleration signal that occurs during atrial contraction and active ventricular filling and marks atrial mechanical systole. The A4 event  260  is also referred to herein as the “atrial event signal” that is sensed or detected from motion sensor signal  250 . With reference to  FIG.  3   , atrial event detector circuit  240  may be configured to detect the A4 event  260  from the motion signal  250  received from motion sensor  212 . Processor  244  may control pace timing circuit  242  to trigger a ventricular pacing pulse by starting an AV pacing interval in response to detecting the A4 event  260 . 
       FIG.  5    is an example of motion sensor signals  400  and  410  acquired over two different cardiac cycles. A ventricular pacing pulse is delivered at time 0.0 seconds for both cardiac cycles. The top sensor signal  400  is received over one cardiac cycle, and the bottom sensor signal  410  is received over a different cardiac cycle. The two signals  400  and  410  are aligned in time at 0.0 seconds, the time of the ventricular pacing pulse delivery. While motion signals  400  and  410  and motion signal  250  of  FIG.  4    are shown as raw accelerometer signals, it is recognized that control circuit  206  may receive a digitized filtered, amplified and rectified signal from motion sensor  212  for processing and analysis. 
     The A1 events  402  and  412  of the respective motion sensor signals  400  and  410 , which occur during ventricular contraction, are observed to be well-aligned in time following the ventricular pacing pulse at time 0.0 seconds. Similarly, the A2 events  404  and  414  (which may mark the end of ventricular systole) and the A3 events  406  and  416  (occurring during passive ventricular filling) are well-aligned in time. Because the A1, A2 and A3 events are ventricular events, occurring during ventricular contraction, at the end of ventricular systole and during passive ventricular filling, respectively, these events are expected to occur at relatively consistent intervals relative to each other following a ventricular electrical event. The time relationship of the A1, A2 and A3 events may be different following a ventricular pacing pulse compared to following a sensed intrinsic R-wave; however, during a stable paced or intrinsic ventricular rhythm, the relative timing of ventricular A1, A2 and A3 events to each other and the immediately preceding ventricular electrical event is expected to be consistent from beat-to-beat. 
     The A4 events  408  and  418  of the first and second motion sensor signals  400  and  410  respectively are not aligned in time. The A4 event occurs during atrial systole and as such the time interval of the A4 event following the immediately preceding ventricular electrical event (sensed R-wave or ventricular pacing pulse) and the preceding A1 through A3 events may vary between cardiac cycles as changes in the atrial rate occur or when the ventricles are paced asynchronously with the atrial events. 
     A PVAB period  436  may be set to inhibit sensing of the A4 event following the ventricular electrical event (at time 0.0) to avoid sensing the A1 and A2 signals and promote reliably sensing of A4 events  408  and  418 . The PVAB period  436  may be set to extend through an estimated onset of ventricular diastole, e.g., at least past an expected time of the A2 event  404  and  414 , so that the PVAB period  436  includes both the A1 and A2 events. The A2 events  404  and  414  are shown as negative-going peaks in this non-rectified signal, but in a rectified signal that is compared to an A4 sensing threshold the A2 events  404  and  414  may have a large enough amplitude to be falsely sensed as the A4 event if the PVAB period  436  is too short. 
     During the PVAB period  436 , the motion sensor  212  may be powered down and/or processing of the motion signal by motion sensor  212  and/or control circuit  206  may be disabled. Control circuit  206  may disable sensing and/or processing of the motion signal during the PVAB period  436  to conserve power source  214  (shown in  FIG.  4   ). For example, control circuit  206  may disable motion sensor  212  or at least one axis of motion sensor  212 , e.g., by disabling or reducing power supplied to the motion sensor  212  (or at least one motion sensor axis) from power source  214 , at the onset of the PVAB period  436 . Control circuit  206  may enable motion sensor  212 , e.g., by providing power from power source  214  to power up motion sensor  212 , at or just prior to (e.g., 10 to 20 milliseconds prior to) the expiration of the PVAB period  436  so that the motion signal is available for sensing atrial event signals after the expiration of the PVAB period  436 . 
     In the example shown in  FIG.  5   , motion signal sensing is disabled for a power conservation time period  435  for conserving power source  214 . Power conservation time period  435  may begin upon the ventricular event that caused control circuit  206  to start the PVAB period  436  and may be terminated prior to the expiration of PVAB period  436 . At time point  437 , at the expiration of time period  435 , control circuit  206  may power up the motion sensor  212  so that by the expiration of the PVAB period  436 , the motion signal  400  or  410  is available for sensing the A4 events  408  and  418  any time after the PVAB period  436 . Because an inherent delay may exist between applying power to the motion sensor  212  and receiving of the motion signal by control circuit  206 , the power conservation time period  435  may be terminated earlier, e.g., 10 to 100 milliseconds (ms) earlier, or 80 to 90 ms earlier as an example, than the expiration of the PVAB period  436 . In other examples, the motion signal may be received by control circuit  206  during the power conservation time period  435 , but control circuit  206  may disable processing and analysis of the motion signal until the expiration of the power conservation time period  435 . In this case, the power conservation time period  435  may expire simultaneously with the PVAB period  436 . During a telemetry session with external device  20 , the power conservation time period  435  may be disabled to enable transmission of the motion signal to external device  20  for display to a user without interruption or discontinuities. 
     An A3 window  424  may be set as a time interval having a starting time  420  corresponding to the end of the PVAB period  436 . A3 window  424  expires at an ending time  422 . The ending time  422  may be considered a starting time of an A4 sensing window  450 , though A4 events may be sensed during the A3 window in some instances. Because the timing of atrial systole may vary between ventricular cycles, the A4 event may sometimes occur earlier in the ventricular cycle, such that the atrial kick may occur during the passive ventricular filling phase, corresponding to the A3 window. A4 events may be sensed during the A3 window using a higher A4 sensing threshold amplitude  444  applied during the A3 window than after the A3 window ending time. 
     A4 events  408  and  418  may be detected based on a multi-level A4 sensing threshold  444 . As seen by the lower motion sensor signal  410 , the A4 event  418  may occur earlier after the A3 window  424  due to changes in atrial rate. In some instances, as the atrial rate increases, the A4 event  418  may occur within the A3 window  424 . When this occurs, the A3 event  416  and the A4 event  418  may fuse as passive and active ventricular filling occur together. The fused A3 /A4 event may have a high amplitude, even greater than the amplitude of either the A3 event  416  or the A4 event  418  when they occur separately. As such, in some examples a first, higher A4 sensing threshold amplitude  446  may be established for detecting an early A4 signal that is fused with the A3 signal during the A3 window  424 . A second, lower A4 sensing threshold amplitude  448  may be established for detecting relatively later A4 signals, after the ending time  422  of the A3 window  424 , during an A4 window  450 . The A4 window  450  may extend from the ending time  422  of the A3 window  424  until an atrial event is sensed or until the next ventricular electrical event, sensed or paced. The earliest crossing of the A4 sensing threshold  444  by the motion sensor signal after the starting time  420  of the A3 window (or after the expiration of the PVAB period  436 ) may be sensed as the atrial event signal. Example techniques for establishing the A3 window ending time  422 , an early A4 sensing threshold amplitude  446  used during the A3 window  424  and a late A4 sensing threshold amplitude  448  used during the A4 window  450  are generally described in the above-incorporated U.S. Pat. No. 10,449,366 (Splett, et al.), U.S. Publication No. 2021/0236825 (Sheldon, et al.), and U.S. Publication No. 2021/0236826 (Sheldon, et al.). 
     As the heart rate increases, the time from the ventricular electrical event to the end of ventricular systole may decrease such that the time from the ventricular electrical event, e.g., a pacing pulse or a sensed R-wave, to the A2 signal  404  or  414  and to the A3 signal  406  or  416  may shorten. The A4 events may also occur earlier after a ventricular electrical event as the ventricular rate increases when the ventricular rate is properly tracking the atrial rate. The PVAB period  436  may be appropriately shortened when the heart rate increases so that the A3 window  420  may start earlier following a ventricular electrical event. When the heart rate decreases, the PVAB period  436  may need to be increased, to extend through the A2 event  404  or  414 , so that the A3 window  420  is not started before or during the A2 event  404  or  414 . Using the techniques disclosed herein, the PVAB period  436  may be adjusted between two or more predetermined time periods as the ventricular rate increases and decreases. The PVAB period  436  may be adjusted between a minimum and a maximum PVAB period in a step change when the heart rate crosses a threshold rate in some examples. In other examples, the PVAB period  436  may be adjusted in a more linear manner between a minimum and maximum PVAB period as heart rate increases and decreases. Control circuit  206  may control the adjustment of the PVAB period  436  based on an analysis of the motion signal amplitude during the blanking period  436  in some examples. 
       FIG.  6    is a flow chart  300  of a method for adjusting the PVAB period according to one example. At block  301 , control circuit  206  sets a PVAB period that is started in response to each ventricular electrical event. Control circuit  206  may identify ventricular electrical events by identifying ventricular sensed event signals (e.g., R-waves) sensed by sensing circuit  204  and/or by identifying ventricular pacing pulses generated by pulse generator  202 . Control circuit  206  may set the PVAB period in response to each identified ventricular event. The PVAB period may extend 400 ms to 600 ms or 500 to 550 ms after the ventricular electrical event, as examples. It is recognized that the PVAB period that is set post-sense and the PVAB period that is set post-pace may be different due to different relative timings of the A2 and A3 event signals following the time of a ventricular sensed event signal compared to the relative timings of the A2 and A3 event signals following the time of a delivered ventricular pacing pulse. 
     At block  302 , control circuit  206  may enable sensing of the motion signal during at least a portion of the PVAB period. As described above in conjunction with  FIG.  5   , during a ventricular pacing mode that includes atrial event sensing, the motion sensor or processing of the motion signal may be disabled during a power conservation time period that may extend for all or a portion of the PVAB period. Before adjusting the PVAB period, however, control circuit  206  may enable motion signal sensing and analysis at block  302  during the PVAB period for at least one or more PVAB periods. Motion signal sensing and analysis may be enabled for the entirety of the PVAB period or for at least a latest portion of the PVAB period. For example, motion signal sensing and analysis may be enabled for the latest 100 ms, the latest 200 ms, the latest 300 ms or other selected portion of the PVAB period. 
     While motion signal sensing is enabled during the PVAB period, atrial event signal sensing may not be enabled during the PVAB period at block  302 . Control circuit  206  may set the A4 sensing threshold amplitude to a maximum amplitude to inhibit A4 event sensing during the PVAB period when motion signal sensing and analysis is enabled during the PVAB for use in adjusting the PVAB period. Alternatively, if the motion signal crosses an A4 sensing threshold amplitude during the PVAB period, control circuit  206  may detect the threshold crossing but be configured to withhold starting an AV pacing interval by ignoring any atrial event detected by atrial event detector circuit  240  during the PVAB period. 
     At block  304 , control circuit  206  may determine motion signal amplitude data during the PVAB period during one or more ventricular cycles. When separate post-sense and post-pace PVAB periods are set by control circuit  206 , the amplitude data determined at block  304  for multiple ventricular cycles may be separated into post-sense amplitude data and post-pace amplitude data, which may be stored in memory buffers allocated for storing amplitude data. 
     The amplitude data may be determined during a latest portion of the PVAB period, e.g., the latest 50 to 300 ms of the PVAB period in some examples. The amplitude data may include an absolute maximum peak amplitude of the motion signal, a time of the absolute maximum peak amplitude, whether an amplitude threshold crossing occurs during the PVAB period, the time of an amplitude threshold crossing, and/or whether the motion signal amplitude is greater than or equal to an amplitude threshold during a latest portion of a PVAB period. When control circuit  206  determines the motion signal amplitude data during each PVAB period of multiple ventricular cycles, control circuit  206  may determine an average, median, maximum, minimum or other representative value(s) of the determined amplitudes and/or associated times of a peak amplitude or amplitude threshold crossing relative to the time of the preceding ventricular electrical event and/or relative to the expiration of the PVAB period. The representative value(s) may be determined for use in adjusting the PVAB period. 
       FIG.  7    is a conceptual diagram  500  of a motion signal  502  illustrating techniques for determining amplitude data by control circuit  206  during the PVAB period of one ventricular cycle according to some examples. Motion signal  502  includes an A1 event  504 , an A2 event  506 , an A3 event  508  and an A4 event  510 . Following a ventricular pacing pulse  560 , the PVAB period  536  is started. A passive ventricular filling window (A3 window)  524  begins upon expiration of the PVAB period  536 . A first, higher A4 sensing threshold amplitude  546  is applied to the motion signal  502  during the A3 window  524 . The A4 sensing threshold  544  is decreased to the second, lower A4 sensing threshold amplitude  548  after expiration of the A3 window  524 , during an A4 sensing window  550  that may extend until the A4 event  510  is sensed or until a ventricular pacing pulse is delivered or a ventricular event signal is sensed by the sensing circuit  204 . 
     In the example shown, the A4 event  510  is sensed upon a crossing of the second, lower A4 sensing threshold amplitude  548  by motion signal  502 . Control circuit  206  sets an AV pacing interval  566  in response to an atrial event detection signal  562  generated by atrial event detector circuit  240 . Pulse generator  202  generates a ventricular pacing pulse  564  at the expiration of the AV pacing interval  566 . 
     As described above, the sensing and analysis of the motion signal  502  may be disabled during a power conservation time period  535 , which may extend until or just prior to the expiration of the PVAB period  536  to enable powering up of the motion sensor so that processing and analysis of the motion signal  502  can begin upon expiration of the PVAB period  536 . When control circuit  206  is determining whether to adjust the PVAB period  536 , however, the power conservation time period  535  may shortened or cancelled altogether so that the motion signal  502  can be received at least during a latest portion  538  of the PVAB period  536  by control circuit  206  for processing and analysis. In the example shown, the motion sensor may be powered up during the PVAB period  536  so that processing and analysis of the motion signal  502  for determining amplitude data may be begin at time point  537 . The later portion  538  of the PVAB period  536  may be the latest 50 ms, 100 ms, 200 ms, 300 ms or other predetermined portion or percentage of PVAB period  536 . The later portion  538  of the PVAB period  536  is also referred to herein as an “amplitude analysis window” because control circuit  206  may determine the motion signal amplitude data at block  304  of  FIG.  6    from the motion signal  502  received during this later portion  538  of the PVAB period  536 . In an example, the power conservation time period  535  may normally expire 85 ms before the expiration of the PVAB period  536 . Control circuit  206  may terminate the power conservation time period  535  85 ms before the amplitude analysis window  538  when amplitude data is determined at block  304 . It is to be understood, however, that the amplitude analysis window  538  may extend for all or any portion(s) of the PVAB period  536  in various examples and is not necessarily limited to only the later portion of the PVAB period  536 . In some examples, the amplitude analysis window  538  may be enabled by control circuit  206  intermittently or during multiple intervals within the PVAB period  536 . 
     Control circuit  206  may determine the maximum peak amplitude  570  of the rectified motion sensor signal  502  during the amplitude analysis window  538 . Control circuit  206  may determine the time of the maximum peak amplitude  570  relative to the expiration of the PVAB period  536 . For example, the peak amplitude to PVAB period expiration time interval  572  may be determined as amplitude data at block  304  of  FIG.  6   . 
     Additionally or alternatively, control circuit  206  may determine amplitude data at block  304  of  FIG.  6    by determining whether the motion signal  502  crosses an amplitude threshold  578  during amplitude analysis window  538 . Control circuit  206  may set the amplitude threshold  578  based on the first, higher A4 sensing threshold amplitude  546  or based on the second, lower sensing threshold amplitude  548  or a combination of both in some examples. For instance, the amplitude threshold  578  may be a percentage, e.g., 50%, 60%, 70%, 80%, 90% or 100% of the first, higher A4 sensing threshold amplitude  546 . 
     Control circuit  206  may alternatively set the amplitude threshold  578  to the first, higher A4 sensing threshold amplitude  546  less an offset. 
     When the motion signal  502  does cross the amplitude threshold  578  during the amplitude analysis window  538 , control circuit  206  may determine the time interval  576  from an amplitude threshold crossing  574  to the expiration time of PVAB period  536 . The threshold crossing  574  is a latest negative-going threshold crossing during amplitude analysis window  538  in the example shown in  FIG.  7   . Control circuit  206  may additionally or alternatively determine a time interval from a latest positive-going threshold crossing to the expiration of the PVAB period  536  in other examples. 
     Referring again to  FIG.  6    with continued reference to  FIG.  7   , control circuit  206  determines whether the amplitude data determined at block  304  meets PVAB adjustment criteria at block  306 . The PVAB adjustment criteria may include one or more thresholds, ranges or other amplitude-based or time-based requirements that are applied to the amplitude data to determine if the amplitude of the motion signal is likely to be greater than an A4 sensing threshold amplitude outside of a PVAB period. Control circuit  206  may apply the PVAB adjustment criteria by determining if amplitude-based requirements are met, e.g., by comparing a maximum peak amplitude to an amplitude threshold and/or determining whether and/or when an amplitude threshold crossing is detected. Control circuit  206  may apply the PVAB adjustment criteria by determining if time-based requirements are met, e.g., by comparing a time of the maximum peak amplitude and/or an amplitude threshold crossing time (e.g., relative to the expiration of the current PVAB period) to a threshold time interval as described in the examples below. 
     In some examples, control circuit  206  may compare the maximum peak amplitude  570  during the amplitude analysis window  538  to an amplitude threshold, e.g., amplitude threshold  578 . When the maximum peak amplitude during the amplitude analysis window  538  is less than the amplitude threshold, PVAB adjustment criteria may be determined to be met at block  306 . Control circuit  206  may adjust the PVAB period at block  308  by shortening the PVAB period, e.g., by decreasing the duration of the PVAB period from the ventricular electrical event to the expiration of the PVAB period. When the maximum peak amplitude during the amplitude analysis window of the PVAB period is greater than the amplitude threshold, however, control circuit  206  may determine that the PVAB adjustment criteria are not met at block  306 . Control circuit  206  may not adjust the PVAB period at block  310  when the motion signal amplitude is relatively high at a time that is relatively late during the PVAB period as determined based on the PVAB adjustment criteria not being met at block  306 . In other examples, e.g., as described below in conjunction with  FIG.  8   , control circuit  206  may increase the duration of the PVAB period in response to determining that the motion signal amplitude is greater than or equal to an amplitude threshold during the amplitude analysis window (or during at least a portion of the current PVAB period). The adjusted PVAB period, e.g., the decreased or increased PVAB period, may be applicable to one or more future cardiac cycles. 
     The amplitude threshold, e.g., amplitude threshold  578 , that the maximum peak amplitude  570  is compared to may be based on the first, higher A4 sensing threshold amplitude  546 . The amplitude threshold may be set equal to the first, higher A4 sensing threshold amplitude  546  applied during the A3 window  524  or to a percentage, e.g., 40% to 80%, of the first higher A4 sensing threshold amplitude. When the maximum peak amplitude  570  during the amplitude analysis window  538  of the current PVAB period is less than the amplitude threshold, control circuit  206  may determine that the PVAB adjustment criteria are met. The PVAB period  536  may be safely shortened, e.g., by a predetermined decrement that is equal to or shorter than the amplitude analysis window  538 . When the motion signal amplitude is less than the amplitude threshold during the amplitude analysis window  538 , the duration of the PVAB period  536  may be decreased with a low likelihood of oversensing the A2 event as a fused A3 /A4 event. 
     Additionally or alternatively, control circuit  206  may compare the time interval  572  from maximum peak amplitude  570  to the expiration time of the current PVAB period  536  to a threshold time interval  580 . When the maximum peak amplitude  570  is within a threshold time interval  580  before the expiration time of the current PVAB period  536 , control circuit  206  may determine that PVAB adjustment criteria are not met at block  306 . Control circuit  206  may withhold an adjustment to the PVAB period at block  310 . If control circuit  206  determines that the maximum peak amplitude  570  of the motion signal  502  during the amplitude analysis window  538  occurs at a time earlier than the threshold time interval  580  from the expiration time of the current PVAB period  536 , PVAB adjustment criteria may be determined to be met. The PVAB period  536  may be safely shortened at block  308 . The PVAB period  536  may be decreased by 10 ms to 100 ms as examples and is decreased by 20 ms to 60 ms in some examples. 
     In some examples, the threshold time interval  580  may be set according to the amount of time that the PVAB period  536  is to be shortened. For example, control circuit  206  may be configured to adjust the PVAB period  536  by a predetermined decrement interval, which may be 50 ms as an example, when PVAB adjustment criteria are met. Control circuit  206  may set the threshold time interval  580  equal to the decrement interval plus a safety interval, e.g., a safety interval of 0 to 30 ms. For instance if the PVAB period is to be shortened by 50 ms, the threshold time interval  580  may be set to extend 50 ms plus 20 ms, or a total of 70 ms, earlier than the expiration time of the PVAB period  536 . In this way, the PVAB period  536  may be shortened by a predetermined decrement interval at block  308  that is equal to or less than threshold time interval  580  with a low likelihood of the A2 event  506  being oversensed as a fused A3 /A4 event after the adjusted expiration time of the PVAB period . The threshold time interval  580  represents a time interval beginning prior to and extending to the expiration of the PVAB period  536  in some examples and is therefore also referred to herein as a “PVAB ending time interval.” 
     Additionally or alternatively, control circuit  206  may determine if the motion signal  502  crosses an amplitude threshold  578  during the amplitude analysis window  538  at block  306 . When the motion signal  502  does not cross the amplitude threshold  578  during the amplitude analysis window  538 , control circuit  206  may determine that the PVAB adjustment criteria are met at block  306 . Control circuit  206  may shorten the PVAB period  536  at block  308 . The PVAB period  536  may be shortened by an interval that is equal to or less than the amplitude analysis window  538  with a low likelihood of oversensing the A2 event  506  when the motion signal  502  does not cross the amplitude threshold  578  during the amplitude analysis window. 
     In some examples, the time of the latest threshold crossing  574  relative to the expiration of the current PVAB period  536  may be determined by control circuit  206  for selecting a decrement time interval used for shortening the duration of the PVAB period  536 . For instance, control circuit  206  may determine the time interval  576  from the latest threshold crossing  574  to the expiration of the PVAB period  536 . Control circuit  206  may decrease the PVAB period by a portion of the time interval  576 , e.g., 40%, 50%, 60%, 70%, or 80% of the time interval  576 . In this way, the PVAB period  536  may be shortened by control circuit  206  to expire earlier than the current PVAB period  536  but after the latest threshold crossing  574  to safely minimize the likelihood of oversensing the A2 event  506  after the expiration of an adjusted PVAB blanking period applied following a future ventricular event. 
     In still other examples, control circuit  206  may compare the threshold time interval  580  to the time interval  576  from a latest threshold crossing  574  by the motion signal during the amplitude analysis window  538  to the expiration of the PVAB blanking period  536 . The amplitude threshold  578  used for determining the threshold crossing  574  may be based on the first, higher A4 sensing threshold amplitude  546  applied during the A3 window  524  as described above. Control circuit  206  may determine the latest threshold crossing  574  as the latest positive-going or the latest negative-going crossing of the amplitude threshold  578 . When the latest threshold crossing  574  is earlier than the threshold time interval  580  from the expiration of the current PVAB period  536 , control circuit  206  may determine that the PVAB adjustment criteria are met at block  306 . The PVAB period  536  may be shortened at block  308  by control circuit  206 . When the time of the latest threshold crossing  574  is within the PVAB ending time interval  580 , e.g., within the threshold time interval from the expiration of the current PVAB period  536 , control circuit  206  may determine that the PVAB adjustment criteria are not met at block  306 . In response, control circuit  206  may hold the PVAB period constant at the current setting at block  310 . In other examples, control circuit  206  may extend or lengthen the duration of the PVAB period  536  when the latest threshold crossing  574  is within the PVAB ending time interval  580 . As described above, the PVAB ending time interval  580  may be set based on the amount of time that PVAB period  536  is to be adjusted by at block  308 . For instance, when the PVAB period  536  is to be adjusted by a 50 ms time interval, the PVAB ending time interval  580  may be set to 50 ms or 50 ms plus a predetermined offset or percentage greater than 50 ms. The PVAB ending time interval  580  may be between  20  and 100 ms, as examples. 
     Referring again to  FIG.  6   , control circuit  206  may adjust the PVAB period  536  at block  308  by shortening the PVAB period by a predetermined decrement interval when the PVAB adjustment criteria are determined to be met. The decrement interval may be 10 ms, 20 ms, 30 ms, 40 ms, 50 ms, 60 ms, 70 ms, 80 ms, 90 ms or 100 ms as examples. In other examples, the PVAB period  536  may be decreased by a predetermined percentage or decreased based on a current ventricular rate, which may be a ventricular paced rate or sensed ventricular rate. For example, the PVAB period  536  may be adjusted in a linear manner between two heart rates. To illustrate, the PVAB period  536  may be adjusted between a maximum PVAB period of 550 ms at a lower heart rate limit of 80 beats per minute and decrease to a minimum of 500 ms at an upper heart rate limit of 110 beats per minute. At heart rates below the lower heart rate limit, the PVAB period  536  may be held at the maximum of 550 ms. At heart rates above the upper heart rate limit, the PVAB period  536  may be held at the minimum of 500 ms. The maximum PVAB period may be set to 500 to 800 ms as examples. The lower heart rate limit may be between 60 and 90 beats per minute as examples. The minimum PVAB period may be set to 400 ms to 700 ms as examples. The upper heart rate limit may be between 90 and 120 beats per minute as examples. 
     The amplitude analysis window  538  and the threshold time interval  580  are shown as two different time intervals in  FIG.  7   . In some examples, the PVAB adjustment criteria may include multiple requirements that utilize both the amplitude analysis window  538  and the threshold time interval  580 . For instance, the PVAB adjustment criteria for shortening the PVAB period  536  may require that the maximum peak amplitude during the amplitude analysis window  538  is less than a first amplitude threshold and that the motion signal does not cross a second amplitude threshold, which may be equal to or less than the first amplitude threshold, during the threshold time interval  580 . In this case, control circuit  206  may set both the amplitude analysis window  538  and the threshold time interval  580  to two different time intervals and may set one or two different amplitude thresholds applied to the motion signal during the PVAB period  536  for determining amplitude data. 
     It is to be understood, however, that the amplitude analysis window  538  may be equal to the threshold time interval  580  in some examples. Control circuit  206  may set a single window or time interval during the later portion of the PVAB period  536  for determining when the PVAB adjustment criteria are met based on amplitude data. For example, control circuit  206  may set the PVAB ending time interval  580  as both as the threshold time interval extending to the expiration of the PVAB period  536  and as the amplitude analysis window. In some examples, control circuit  206  may determine that the PVAB adjustment criteria are met when the motion signal amplitude is not greater than, or the motion signal  502  does not cross, an amplitude threshold  578  during the PVAB ending time interval  580 . 
     The flow chart  300  of  FIG.  6    has been described with regard to a PVAB period adjustment that is a decrease in the PVAB period  536  at block  308 . Control circuit  206  may perform the process of flow chart  300  in response to detecting an increase in the ventricular rate. When the ventricular rate is increased, such that the time intervals between atrial synchronized ventricular pacing pulses are shortened for example, a shortening of the PVAB period may be warranted. In order to avoid oversensing the A2 event, control circuit  206  may perform the method of flow chart  300  to verify that an amplitude of the motion signal near the expiration of the current PVAB blanking period, or during a threshold time interval such as PVAB ending time interval  580  that would be after the expiration of a potentially shortened PVAB blanking period, is not near or greater than the first, higher A4 sensing threshold amplitude  546  or another selected amplitude threshold. When the ventricular rate decreases again, the PVAB period may be increased without an analysis of the amplitude of the motion signal during the PVAB period in some examples. By increasing the PVAB period, the likelihood of oversensing the A2 event is decreased since the expiration of the PVAB period is later in the ventricular cycle and most likely after the A2 event. Accordingly, in some examples, control circuit  206  may perform the method of flow chart  300  only when a potential adjustment to the PVAB period, e.g., due to an increased ventricular rate, is a decrease or shortening of the PVAB period. In other examples, however, control circuit  206  may determine and analyze motion signal amplitude data to verify that the current PVAB period is appropriate or should be shortened (decreased) or lengthened (increased) for avoiding A2 event oversensing, which may be in response to both increases and decreases in ventricular rate and/or in response to changes in the frequency and/or timing of A4 event sensing, for example. 
       FIG.  8    is a flow chart  600  of a method for controlling PVAB period adjustments according to another example. At block  601 , control circuit  206  may determine that it is time to adjust the PVAB period. Control circuit  206  may perform the method of flow chart  600  at the time of pacemaker implant, on a periodic basis, e.g., once per minute, once per hour, every four hours, every eight hours, every twelve hours, once per day or other scheduled frequency. Control circuit  206  may determine that it is time to adjust the PVAB period at block  601  in response to a scheduled time of day or the expiration of a PVAB adjustment timer in some examples. 
     Additionally or alternatively, control circuit  206  may determine that it is time to adjust the PVAB period at block  601  in response to detecting a change in the heart rate, which may be an atrial rate and/or ventricular rate. Control circuit  206  may determine RRIs as the time intervals between consecutive pacing pulses and/or sensed ventricular event signals during an atrial synchronized ventricular pacing mode (or during an asynchronous ventricular pacing mode that includes atrial sensing). A mean or median of the most recent X RRIs may be determined as a representative value corresponding to the current ventricular rate and compared to a previously determined RRI. For the sake of illustration, the median RRI may be determined after every 8 RRIs as the median of the 8 RRIs. The most recent median RRI may be compared to a previously determined median RRI, which may be the most recent preceding median RRI and/or one or more earlier median RRIs that may represent the ventricular rate at an earlier time, e.g., approximately 30 seconds earlier, one minute earlier, two minutes earlier, five minutes earlier, etc. to account for relatively faster and slower changes that could occur in the ventricular rate. When the difference between the current median RRI and a previously determined median RRI is greater than a rate change threshold, e.g., corresponding to an increase or decrease in the ventricular rate of 10 beats per minute (bpm), 20 bpm, 30 bpm or other rate change threshold, control circuit  206  may determine that it is time to adjust the PVAB period at block  601 . 
     Additionally or alternatively, control circuit  206  may determine it is time to adjust the PVAB period when the heart rate crosses one or more rate thresholds. For example, control circuit  206  may determine that the ventricular rate has changed between being greater than or equal to 100 bpm to being less than 100 bpm. Control circuit  206  may determine that it is time to adjust the PVAB period at block  601  based on the ventricular rate crossing the rate threshold of 100 bpm. For instance, control circuit  206  may determine a median RRI (which may be based on paced and/or sensed RRIs determined between consecutive ventricular events) and compare the median RRI to a threshold interval. The threshold interval may be 500 ms, 600 ms, 700 ms or other selected threshold interval that corresponds to a ventricular rate threshold. When the median RRI has changed from being less than or equal to the threshold interval to being greater than the threshold interval or vice versa, control circuit  206  may determine that it is time to adjust the PVAB period at block  601 . 
     Control circuit  206  may compare the median RRI to two more threshold intervals in some examples, such as a 1000 ms threshold, a 700 ms threshold and a 500 ms threshold corresponding to changes in heart rate from less than or equal to 60 bpm, between 60 bpm and about 85 bpm, between about 86 bpm and 120 bpm and greater than 120 bpm. Other threshold intervals corresponding to other heart rate thresholds may be selected. When a threshold interval is crossed by the median RRI compared to a previously determined median RRI, control circuit  206  may determine that it is time to adjust the PVAB period at block  601 . While the foregoing examples relating to heart rate changes refer to changes in the ventricular rate determined based on RRIs, the heart rate or changes in the heart rate may be determined based on a rate of sensed atrial events in some examples. 
     In still other examples, control circuit  206  may determine that it is time to adjust the PVAB period at block  601  based on an analysis of the frequency and/or timing of sensed atrial event signals. In some examples, control circuit  206  may determine that it is time to adjust the PVAB in response to regular, atrial event sensing occurring after the A3 window (during the A4 window) in a 1:1 ratio with ventricular events. When atrial events are being reliably sensed during the A4 window consistently on a beat-to-beat basis, control circuit  80  may analyze the motion signal amplitude during the PVAB period. If the motion signal amplitude is below a threshold amplitude, e.g., less than half of the first A4 sensing threshold amplitude applied during the A3 window, the PVAB period could be shortened because the likelihood of oversensing A2 events is relatively low when the motion signal amplitude is low during the PVAB period. 
     In another example, if an atrial event signal has not been sensed for one or more ventricular cycles, control circuit  206  may determine that it is time to adjust the PVAB period at block  601 . Control circuit  260  may determine if the atrial event signal has been sensed for less than X out of Y most recent ventricular cycles, e.g., less than four out of eight, less than two out of eight or other ratio or percentage of a predetermined number of most recent ventricular cycles. When fewer than a threshold number or percentage of atrial event signals have been sensed out of the most recent predetermined number of ventricular cycles, control circuit  206  may determine that it is time to adjust the PVAB period at block  601 . In some examples, control circuit  206  may adjust other atrial event sensing control parameters, e.g., the ending time of the A3 window and/or the A4 sensing threshold amplitude before adjusting the PVAB period when atrial event sensing is irregular or lost. Control circuit  206  may determine that it is time to adjust the PVAB period after adjustments to other atrial event sensing control parameters have been exhausted. 
     Additionally or alternatively, control circuit  206  may determine when atrial event signals are being sensed during the ventricular cycle. Referring again to  FIG.  7   , control circuit  206  may determine how often atrial event signals are sensed within an early portion  582  of the A3 window  524 . Atrial event signals sensed during the early portion of the A3 window  524  may correspond to oversensed A2 signals due to the PVAB period being too short. Accordingly, control circuit  206  may count the number of atrial event signals that are sensed during the early portion  582  (shown in  FIG.  7   ) of the A3 window. The early portion  582  of the A3 window  524  is also referred to herein as the “A3 window beginning time interval” because it extends from the start of the A3 window  524  for a predetermined time interval after the expiration of the PVAB period  536 . The duration of the A3 window beginning time interval  582  may be 50 ms, 100 ms, 150 ms or other selected time interval and may be set based on a predetermined increment that is used to increase the PVAB period, a maximum allowable PVAB period, or other adjustment that may be applied to the PVAB period  536  to increase the duration of the PVAB period following a ventricular event. When a threshold number or percentage of all sensed atrial events over a predetermined time interval or number of ventricular cycles are sensed during the A3 window beginning time interval  582 , control circuit  206  may determine that it is time to adjust the PVAB period  536 . To illustrate, when two, three, four, six or eight out of the eight most recently sensed atrial event signals are sensed during the A3 window beginning time interval  582 , control circuit  206  may determine that it is time to adjust the PVAB period. 
     In still other examples, control circuit  206  may be configured to determine that it is time to adjust the PVAB period at block  601  based on a threshold number of ventricular pacing pulses being delivered at a ventricular lower rate interval (e.g., due to the absence of sensed atrial event signals) during an atrial synchronized ventricular pacing mode. When a threshold number of asynchronous ventricular pacing pulses are delivered, the PVAB period may be too long causing missed sensing of the atrial event signals. 
     In various examples, control circuit  206  may be configured to determine that it is time to adjust the PVAB period at block  601  based on one or more requirements that may be met alone or in combination and may include any of the examples described above. Control circuit  206  may determine that it is time to begin the process of determining amplitude data from the motion signal based on the time of day, a scheduled PVAB period adjustment frequency, the ventricular rate, the rate or timing of the atrial sensed event signals, a maximum number of adjustments of other atrial event sensing control parameters being reached, the frequency or number of asynchronous ventricular pacing pulses, or any combination thereof. It is contemplated that other criteria may be used than the example criteria described above for causing control circuit  206  to determine that it may be time to adjust the PVAB period and begin determining amplitude data from the motion signal. 
     Upon determining that it is time to adjust the PVAB period at block  601 , control circuit  206  may set the PVAB period to the currently active setting at block  602  in response to an identified ventricular event, e.g., a ventricular pacing pulse delivered by pulse generator  202  or a ventricular event signal sensed from the cardiac electrical signal by sensing circuit  204 . Control circuit  206  may enable sensing and analysis of the motion signal during the PVAB period, or at least during an amplitude analysis window of the PVAB period, at block  604 . As described above, control circuit  206  may disable motion sensor  212  or at least processing of the motion signal to conserve power source  214  during the PVAB period during atrial synchronized ventricular pacing. Control circuit  206  may enable sensing and analysis of the motion signal starting from the expiration of the PVAB period (or a power conservation time period) until the start of the next PVAB period on a beat-by-beat basis until control circuit  206  determines that it is time to adjust the PVAB period at block  601 . At block  604 , in response to determining that it is time to adjust the PVAB period, control circuit  206  may maintain power to motion sensor  212  throughout one or more ventricular cycles to enable amplitude data to be determined during the PVAB period at block  606 . In some examples, control circuit  206  may disable motion sensor  212  during an early portion of the PVAB blanking period to still conserve power during a portion of the PVAB period and provide power to motion sensor  212  during at least an amplitude analysis window occurring during a later portion of the PVAB period for one or more ventricular cycles. 
     At block  606 , control circuit  206  determines amplitude data from the motion signal during the amplitude analysis window according to any of the examples described above, e.g., in conjunction with  FIG.  7   . At block  608 , control circuit  206  may determine whether the heart rate is increased, e.g., based on RRIs as generally described above in conjunction with block  601 . Regardless of what triggered control circuit  206  to determine that it is time to adjust the PVAB period, e.g., a scheduled time or reduced and/or early atrial event sensing, control circuit  206  may determine the heart rate, e.g., by determining a median RRI and determining if the median RRI represents an increase in heart rate based on a comparison to a previously determined median RRI. In this way, control circuit  206  may determine that a decrease in the PVAB period may be needed. In some cases, it is a change in the heart rate that is determined at block  601 , that triggers control circuit  206  to determine that it is time to adjust the PVAB period and start the process of flow chart  600 . In that case, control circuit  206  does not necessarily redetermine whether the ventricular rate has increased or decreased at block  608  but may rely on the earlier determination. In other examples, control circuit  206  may determine if the heart rate has crossed a rate threshold, e.g., from a lower to a higher heart rate at block  608 . 
     Additionally or alternatively to determining if the ventricular rate is increased at block  608 , control circuit  206  may determine if atrial events are being sensed infrequently at block  608 . Control circuit  206  may determine if a reduced frequency of atrial events, e.g., less than a 1:1 ratio with ventricular events, is occurring. In this case, the PVAB period may be too long and interfering with atrial event sensing, particularly when the reduced number of sensed atrial events are only or mostly being sensed during the early portion of the A3 window, e.g., during an A3 window beginning time interval  582  (shown in  FIG.  7   ). In other instances, if intermittent sensing of atrial events (e.g., less than a 1:1 ratio with ventricular events) occurs after a period of regular sensing of atrial events during the A3 window and in a 1:1 ratio with ventricular events, the PVAB period may need to be shortened. Any of the example methods described above for determining a reduced frequency and/or early atrial event signal sensing during the A3 window may be used by control circuit  206  at block  608 . In some cases, it is a determination of reduced frequency of atrial event sensing and/or early atrial event sensing during the A3 window that is determined by control circuit  206  at block  601  as being the trigger for starting the process of determining amplitude data for adjusting the PVAB period. In this case, control circuit  206  does not necessarily need to redetermine the frequency and/or timing of atrial event sensing at block  608  and may rely on the previous determination made at block  601 . 
     When control circuit  206  determines that the heart rate is increased and/or the frequency of atrial event sensing is reduced (“yes” branch of block  608 ), control circuit  206  may determine that a possible shortening of the PVAB period is needed to restore or maintain reliable atrial event sensing. Before shortening the PVAB period at block  612 , control circuit  206  analyzes the amplitude data determined at block  606 . For example, control circuit  206  may verify that the amplitude of the motion signal is less than an amplitude threshold during the PVAB ending time interval, e.g., time interval  580  shown in  FIG.  7   . Any of the techniques described above in conjunction with  FIGS.  6  and  7    for determining if PVAB adjustment criteria are met for shortening the PVAB period may be used at block  610  for determining that the PVAB period can be shortened at block  612 . 
     When the amplitude data meet criteria applied at block  610  for enabling shortening of the PVAB period, control circuit  206  may adjust the PVAB period by a predetermined decrement interval or percentage, to a predetermining shortened PVAB period, to a percentage of the current PVAB period, or based on the current ventricular cycle interval in various examples. For instance, when the ventricular rate is increased above a threshold rate, and the motion signal amplitude is less than an amplitude threshold during the PVAB ending time interval  580  ( FIG.  7   ), control circuit  206  may decrease the PVAB period to a predetermined shortened PVAB period, e.g., to 500 ms. The PVAB period may be decreased according to any of the examples given above. It is to be understood that a minimum PVAB period limit may be set by control circuit  206 , e.g., based on the ventricular rate, or stored in memory  210  as a fixed minimum. When the PVAB period is at the minimum PVAB period limit, control circuit  206  does not decrease the PVAB period further. After adjusting the PVAB period at block  612 , control circuit  206  returns to block  601  to wait until the next determination is made that it is time to adjust the PVAB period. 
     If the amplitude of the motion signal does not meet criteria applied at block  610 , e.g., if the motion signal amplitude is equal to or greater than a threshold amplitude during the PVAB ending time interval, control circuit  206  may determine that the PVAB period should not be shortened. Control circuit  206  may hold the PVAB period constant at the current setting at block  616  and withhold making an adjustment to the PVAB period. Control circuit  206  may then return to block  601  to wait for the next determination that it is time to adjust the PVAB period. 
     Returning to block  608 , when the ventricular rate is decreased or relatively unchanged (“no” branch) and/or regular atrial event signal sensing is occurring (e.g., 1:1 with the ventricular events), particularly when regular atrial event signal sensing is occurring only or mostly very early, e.g., in the A3 window beginning time interval  582  (shown in  FIG.  7   ), an increase in the PVAB period may be appropriate to reduce the likelihood of oversensing A2 events. Control circuit  206  may advance to block  614  to determine if the amplitude data meet criteria for increasing the PVAB period before increasing the PVAB period at block  628 . 
     At block  614 , control circuit  206  may compare the amplitude data to criteria for justifying a PVAB period increase. For instance, control circuit  206  may determine if the motion signal amplitude is greater than a threshold during the PVAB ending time interval  580  (shown in  FIG.  7   ). When the motion signal amplitude is greater than an amplitude threshold near the end of the PVAB period, an increase in the PVAB period may be warranted to reduce the likelihood of the A2 event signal extending into the A3 window and causing false sensing of atrial event signals. In response to determining that motion signal amplitude is greater than an amplitude threshold during the PVAB ending time interval  580 , control circuit  206  may increase the PVAB period at block  628 . 
     In other examples, when the ventricular rate is decreased, control circuit  206  may advance directly to block  628  to increase the PVAB period without an analysis of the determined amplitude data. The PVAB period may be increased at block  628  by a predetermined increment, by a percentage of the current PVAB period, based on the current median RRI, or to a predetermined extended PVAB period, e.g., to 550 ms. If the PVAB period is already at a maximum limit, control circuit  206  may not increase the PVAB period to be longer than the maximum limit. The maximum limit may be stored in memory  210  or determined by control circuit  206  based on the ventricular rate. 
     When the amplitude of the motion signal during the PVAB ending time interval (or during the amplitude analysis window as shown in  FIG.  7   ) is less than an amplitude threshold (“no” branch of block  614 ), control circuit  206  may maintain the current PVAB period at block  616  without making an adjustment. After either holding the PVAB period at the current setting (block  616 ) or increasing the PVAB period (block  628 ), control circuit  206  may return to block  601  to wait for the next determination that it is time to adjust the PVAB period. 
       FIG.  9    is a flow chart  700  of a method for adjusting the PVAB period according to another example. At block  701 , control circuit  206  determines a median RRI. As described above, after every X RRIs where X may be four, eight, sixteen, or thirty-two as examples with no limitation intended, control circuit  206  may determine a median of the X RRIs. The X RRIs may include paced and/or sensed ventricular events. It is recognized that other methods may be used to determine a representative RRI or associated ventricular rate (or heart rate). Other methods may include determining a running average, mean, trimmed median, minimum, maximum or other metric of a predetermined number of RRIs. Other methods may include determining a running average, mean, trimmed median, minimum, maximum or other metric of all RRIs that occur over a predetermined time interval, e.g.,  30  seconds, one minute, two minutes, five minutes or other time interval, which may result in a variable number of RRIs. 
     At block  702 , control circuit  206  determines if a ventricular rate change is detected based on the median (or other representative) RRI. In some examples, control circuit  206  may compare the median RRI to a rate interval threshold at block  702 . If the median RRI has crossed a rate interval threshold at block  702 , a ventricular rate change is detected. For example, if the current median RRI is greater than a rate interval threshold and the preceding median RRI is less than or equal to the rate interval threshold or vice versa, control circuit  206  may detect a ventricular rate change at block  702 . In an illustrative example, control circuit  206  determines if the ventricular rate corresponding to the current median RRI has increased to a rate greater than a threshold rate such as 100 bpm or has decreased to a rate that is less than or equal to the threshold rate of 100 bpm from a previously determined ventricular rate. If a ventricular rate change is not detected at block  702 , control circuit  206  returns to block  701 . When a ventricular rate change is not detected, control circuit  206  may hold the PVAB period at the current setting without any adjustment. It is recognized that in some examples a heart rate change may be detected at block  702  based on determining a rate of sensed atrial events, e.g., by determining time intervals between consecutively sensed atrial event signals. 
     When control circuit  206  detects a heart rate change at block  702 , control circuit  206  may enable motion signal sensing and analysis during the PVAB period at block  704  as described above. It is to be understood that while sensing and analysis of the motion signal is enabled during the PVAB period, sensing of atrial event signals remains disabled or any atrial event signals sensed during the blanking period are ignored by control circuit  206  for the purposes of starting an AV pacing interval. 
     At block  706 , control circuit  206  determines amplitude data from the motion signal received during at least a portion of the PVAB period according to any of the examples described above. At block  708 , control circuit  206  determines if the current heart rate is greater than the rate threshold. If a rate increase was detected at block  702 , the heart rate is greater than the rate threshold at block  708 . If a rate decrease was detected at block  702 , the heart rate is less than or equal to the rate threshold at block  708 . 
     When the heart rate, e.g., the ventricular rate, is greater than the rate threshold (“yes” branch of block  708 ), control circuit  206  may determine if the amplitude of the motion signal is less than an amplitude threshold during the PVAB ending time interval at block  710 . Any of the techniques described above in conjunction with  FIGS.  6  and  7    may be used at block  710  to determine that adjustment criteria are met based on the determined amplitude data for allowing the duration of the PVAB period to be shortened at block  712 . 
     When the amplitude of the motion signal is less than the amplitude threshold during the PVAB ending time interval (e.g., time interval  580  shown in  FIG.  7   ), control circuit  206  adjusts the PVAB period to a short blanking period at block  712 . Whenever control circuit  206  determines that the ventricular rate has increased to a rate greater than a rate threshold, e.g., a rate threshold of 90 to 120 bpm, and the amplitude of the motion signal is not greater than an amplitude threshold during the PVAB ending time interval of the current PVAB period, control circuit  206  may adjust the PVAB period to a predetermined short blanking period at block  712 . The short blanking period may be 500 ms to 550 ms as examples. 
     When control circuit  206  determines that the amplitude of the motion signal is greater than or equal to the amplitude threshold during the PVAB ending time interval (“no” branch of block  710 ), control circuit  206  withholds an adjustment to the PVAB period in response to the detected increase in ventricular rate. When the motion signal amplitude is greater than or equal to the amplitude threshold during a PVAB period, which may be currently set to the long blanking period duration, control circuit  206  may withhold selecting a short blanking period duration in response to the heart rate being faster than a threshold rate, e.g., based on a representative RRI being less than a threshold interval. The PVAB period may be maintained at the current PVAB period at block  716 , which may be a predetermined long blanking period that can be set whenever the ventricular rate is less than or equal to the rate threshold (e.g., when a RRI is greater than or equal to a corresponding threshold interval) and/or the motion signal amplitude is high during the PVAB period. The long blanking period may be 550 to 600 ms as examples. Control circuit  206  may select the long blanking period duration at block  716  in response to determining that the amplitude is greater than or equal to the threshold amplitude (“no” branch of block  710 ) during one or more PVAB periods (that may be set to the long blanking period duration) and the ventricular rate is faster than a rate threshold (“yes” branch of block  708 ), e.g., based on a ventricular event interval being less than a threshold interval. 
     Returning to block  708 , when control circuit  206  determines that the heart rate, e.g., the ventricular rate, has decreased to a rate that is less than or equal to the rate threshold (“no” branch of block  708 ), control circuit  206  may increase the PVAB period from the short blanking period to the long blanking period at block  714 . In the example shown, control circuit  206  does not necessarily perform an analysis of the amplitude data when a ventricular rate decrease to less than or equal to the rate threshold is detected. Extending the PVAB period to the predetermined long blanking period is expected to safely maintain reliable atrial event sensing because the long blanking period is likely to encompass the A2 event, and the long blanking period is selected to expire before an expected time of the A3 event. An analysis of the amplitude data may be performed by control circuit  206 , for example, only when a ventricular rate increase to greater than the threshold rate is detected because shortening the PVAB period from the long blanking period to the short blanking period in response to a ventricular rate increase could lead to oversensing of the A2 event if the amplitude of the motion signal is relatively high near the end of the long blanking period. 
     Control circuit  206  is described in conjunction with  FIG.  9    as detecting a ventricular rate that is greater than or less than a single rate threshold for toggling the PVAB period between a long blanking period and a short blanking period. Adjusting to the short blanking period is withheld, however, when the motion signal amplitude is greater than a threshold during a late portion of the long blanking period. It is to be understood, however, that control circuit  206  may detect ventricular rate changes between three or more rate zones, e.g., a low rate zone less than or equal to 60 bpm, a moderate rate zone greater than 60 bpm and less than or equal to 100 bpm and a high rate zone greater than 100 bpm. In this case, control circuit  206  may adjust the PVAB period between a long blanking period (e.g., 600 ms), a moderate blanking period (e.g., 550 ms), and short blanking period (e.g., 500 ms) in response to detecting a heart rate change from one zone to another. The PVAB periods, e.g., a short or minimum PVAB period and a long or maximum PVAB period and any intermediate PVAB periods, may be programmable by a user, e.g., using external device  20 . Additionally or alternatively, the heart rate(s) at which switching between one PVAB period and another PVAB period may be programmable by a user. 
     When the PVAB period is shortened from the long blanking period to the moderate or short blanking period, control circuit  206  may first verify that the motion signal amplitude is less than an amplitude threshold during a PVAB period ending time of the long blanking period. As described above, the PVAB period ending time may be set to the amount of time that the PVAB period is to be decreased. Continuing the illustrative example of a long 600 ms blanking period applied during a low ventricular rate zone, a moderate 550 ms blanking period applied during a moderate rate zone, and a short 500 ms blanking period during a high rate zone, the PVAB period ending time used during the motion signal amplitude analysis performed at block  710  may be set to 50 ms plus an optional safety offset when the ventricular rate has increased from the low rate zone to the moderate rate zone or from the moderate rate zone to the high rate zone. The PVAB period may be safely decreased from the long blanking period to the moderate blanking period or from the moderate blanking period to the short blanking period when the motion signal amplitude is less than an amplitude threshold during the respective PVAB period ending time. The PVAB period ending time may be set to 100 ms plus an optional safety offset when the ventricular rate has increased from the low rate zone to the high rate zone to allow the PVAB blanking period to be safely decreased from the long blanking period to the short blanking period when the motion signal amplitude is less than an amplitude threshold during the relatively longer PVAB period ending time. 
       FIG.  10    is a flow chart  800  of a method for setting the PVAB period according to another example. The process of flow chart  800  may be performed by control circuit  206  during a variety of operating or pacing modes. In this way, a selected duration of the PVAB period may be set to a currently relevant value for use in atrial event signal sensing during a current operating mode and/or during a subsequent operating mode, e.g., after a pacing mode switch. For example, control circuit  206  may be configured to operate in an atrial synchronous ventricular pacing mode, which may be denoted as a VDD pacing mode. Control circuit  206  may additionally be configured to operate in one or more asynchronous ventricular pacing modes, which may include dual chamber or single chamber sensing and may include a rate response ventricular pacing mode. Examples of asynchronous pacing modes may be denoted as a VDI pacing mode, VVI pacing mode, and a VDIR pacing mode. Control circuit  206  may additionally be configured to operate in a sensing without pacing mode which may include single chamber or dual chamber sensing, which may be denoted as an OVO mode or ODO mode, respectively. It is to be understood that during operating modes that include dual chamber sensing, the atrial event sensing may be performed by atrial event detector circuit  240  ( FIG.  3   ) using the motion signal received from motion sensor  212 , and ventricular event sensing may be performed by sensing circuit  204  using a cardiac electrical signal, e.g., sensed from electrodes  162  and  164 . 
     The process of flow chart  800  may be performed during any pacing mode, including sensing without pacing modes, for selecting and updating the PVAB period on an ongoing basis. As described below, the duration of the PVAB period may be selected based on a recently determined representative RRI. When the pacing mode is switched to the atrial synchronous ventricular pacing mode, e.g., the VDD pacing mode, from any other pacing mode, e.g., an asynchronous ventricular pacing mode, a single chamber sensing mode, or any other pacing mode which may include any of the VDI, VVI, VVIR, OVO or ODO pacing modes listed above, the PVAB period is set to a currently relevant duration based on the most recently determined RRI(s) so that reliable atrial event sensing is promoted during the atrial synchronous ventricular pacing mode. Furthermore, during any asynchronous ventricular pacing mode or sensing only mode that includes dual chamber sensing, e.g., VDI or ODO, the PVAB period may be set to a currently relevant duration based on the most recently determined RRI(s) for promoting reliable atrial event signal sensing. Atrial event signal sensing may be performed during an asynchronous ventricular pacing mode for use in diagnostic functions or other device functions such as setting atrial event sensing control parameters, e.g., as described in the above-incorporated U.S. Pat. No. 10,449,366 (Splett, et al.), U.S. patent application Ser. No. 17/159,596 (Sheldon, et al.), and U.S. patent application Ser. No. 17/159,635 (Sheldon, et al.). 
     At block  802 , control circuit  206  identifies N ventricular events, e.g., 1 to 12 ventricular events or 6 to 8 ventricular events in some examples. As described above, ventricular events may be identified by control circuit  206  as sensed events, e.g., in response to a ventricular sensed event signal received from sensing circuit  204 , or ventricular pacing pulses delivered by pulse generator  202 . At block  804 , control circuit  206  may determine a representative RRI, e.g., a median RRI as shown in  FIG.  10   , from the N ventricular events. In an illustrative example, control circuit  206  may buffer the most recent 8 RRIs in memory  210 . Control circuit  206  may identify the 4 th  shortest interval out of the 8 buffered intervals as a “median” RRI at block  804 . In other examples, the representative RRI determined at block  804  may be an average, minimum, maximum, nth longest (or shortest), or trimmed mean or median RRI. 
     Control circuit  206  may compare the representative RRI to a threshold interval at block  806 . The threshold interval may be a predetermined or programmable value stored in memory  210  and may be between 800 ms and 500 ms, as examples, and can be between 750 ms and 600 ms. The threshold interval may correspond to a ventricular rate of 75 to 120 bpm or about 80 to 110 bpm, for instance. A default value of the threshold interval can be 665 ms corresponding to a ventricular rate of about 90 bpm, as an example. 
     When the RRI is less than the threshold interval, control circuit  206  may set the PVAB period to a minimum PVAB period at block  808 . The minimum PVAB period may be a duration of 400 to 650 ms, as examples, and may be programmable between 425 ms and 575 ms with a default value of 500 ms in some examples. In other examples, the minimum PVAB period may be established by control circuit  206  based on motion signal amplitude data determined according to any of the examples given above. Control circuit  206  may determine the amplitude of the motion signal during one or more PVAB periods, which may be set to the maximum PVAB period in some instances. Control circuit  206  may set the minimum duration of the PVAB period based on the determined amplitude of the motion signal. 
     For example, if the motion signal peak amplitude is less than a threshold amplitude during the maximum PVAB period, the minimum PVAB period may be set to the shortest PVAB period available or a default PVAB period. If the motion signal peak amplitude is greater than the threshold amplitude, however, the minimum PVAB period may be set relatively longer based on amplitude timing data in some examples. For instance, control circuit  206  may determine a time of a maximum peak amplitude or latest threshold crossing of the motion signal during one or more PVAB periods, which could be currently set to the maximum PVAB period. Control circuit  206  may set the minimum PVAB period based on the determined time of the maximum peak amplitude or latest threshold crossing. In one example, the minimum PVAB period is set to be at least a safety interval or offset longer than the time of the maximum peak amplitude or latest threshold crossing. 
     While not shown explicitly in  FIG.  10   , but as described above in conjunction with  FIG.  9   , in some examples, before selecting the minimum PVAB period at block  808  in response to the representative RRI being less than the threshold interval, control circuit  206  may enable sensing of the motion signal during one or more PVAB periods to check the amplitude of the motion signal. When the amplitude of the motion signal is greater than a threshold amplitude, e.g., during a PVAB period ending time as described in conjunction with  FIG.  7   , control circuit  206  may withhold selecting the minimum PVAB period in response to the representative RRI being less than the threshold interval. Control circuit  206  may select a maximum PVAB period (block  810 ) in response to determining that the motion signal amplitude is greater than or equal to a threshold amplitude during one or more PVAB periods when the RRI is less than the threshold interval. 
     When the representative RRI is greater than or equal to the threshold interval (“no” branch of block  806 ), control circuit  206  may set the PVAB period to a maximum PVAB period at block  810 . The maximum PVAB period may be a duration of 450 to 800 ms, as examples, and can be programmable between 450 ms to 600 ms with 550 ms as a default maximum PVAB period or other predetermined duration that is longer than the minimum PVAB period. The threshold interval and the minimum and maximum PVAB periods may be selected based on an individual patient&#39;s heart rate and systolic interval characteristics as well as the amplitude of the motion signal determined during the PVAB as described in any of the examples given above. In this example, control circuit  206  may select the maximum PVAB period when the representative RRI is equal to the threshold interval. It is to be understood that in other examples, control circuit  206  may be configured to select the minimum PVAB period when the representative RRI is equal to the threshold interval. 
     After setting the PVAB period to the minimum or maximum, control circuit  206  may return to block  802  to identify the next N ventricular events. The PVAB period set to either the minimum at block  808  or the maximum at block  810  may be started in response to each of the next N ventricular events. The current setting of the PVAB period may remain in effect for the next N ventricular events until the next representative RRI is determined. If PVAB period is currently set to the minimum, and the next representative RRI is still less than the threshold interval (block  806 ), the PVAB period may remain set to the minimum PVAB period at block  808  and be applied following each of the next N ventricular events. If PVAB period is currently set to the maximum, and the next representative RRI is still greater than or equal to the threshold interval (block  806 ), the PVAB period may remain set to the maximum PVAB period at block  808  and be applied following each of the next N ventricular events. It is to be understood that, depending on the value of the threshold interval, the criteria at block  806  can be less than or equal to the interval threshold in some examples. When the representative RRI changes from being less than the threshold interval to being greater than (or equal to) the threshold interval, or vice versa, control circuit  206  changes the PVAB period from the minimum PVAB period to the maximum PVAB period, or vice versa. 
       FIG.  11    is a timing diagram  900  of one example of ventricular events and corresponding PVAB periods that are adjusted according to the example techniques of  FIG.  10   . Ventricular events  901  are shown in  FIG.  11    as ventricular pacing pulses (VP) that are synchronized to sensed atrial event signals (AS) at an AV pacing interval (not shown in  FIG.  11   ) during an atrial synchronous ventricular pacing mode. It is to be understood however, that in other instances the ventricular events  901  may include all or any number of sensed ventricular events, e.g., R-waves sensed by sensing circuit  204 . Furthermore, depending on the pacing mode in effect, control circuit  206  may or may not be sensing atrial events and, if atrial events are being sensed, the ventricular events  901  may or may not be ventricular pacing pulses that are synchronized to sensed atrial events. 
     The PVAB periods  908  following each of the ventricular events  901  in a first group  902  of N (e.g., 8) RRIs are set to a maximum PVAB period duration. Control circuit  206  may disable atrial event sensing during a corresponding maximum power conservation time period  910  that may be set to end a fixed time interval earlier than the maximum PVAB period  908 , e.g., 20 to 100 ms earlier or 85 ms earlier in an example. During the power conservation time period  910 , atrial event signal sensing may be disabled, e.g., by disabling or powering off at least one axis of the multi-axis accelerometer included in motion sensor  212 . 
     As described above in conjunction with  FIG.  3   , atrial event detector circuit  240  may receive one, two or all three axis signals from a three-axis accelerometer included in motion sensor  212 . The axis signal(s) selected for producing a motion signal from which atrial event signals are sensed from may be selected by control circuit  206  or programmed by a user. Example techniques for selecting the axis signal(s) used for sensing atrial event signals are generally described in U.S. Publication No. 2020/0179708 (Splett, et al.), incorporated herein by reference in its entirety. When two or three axis signals are received, they may be summed or combined in another way for producing a motion signal from which atrial event signals are sensed. 
     At least one axis signal used for sensing atrial event signals may be powered down by control circuit  206  for the maximum power conservation time period  910  to conserve power source  214  during the maximum PVAB period  908 . The at least one axis signal that is powered down may be powered up again prior to the expiration of the maximum PVAB period  908 . In this way, the motion signal that is used for atrial event signal sensing, which may include a combination of one or more accelerometer axis signals, can be passed to control circuit  206  on or just before the expiration of the maximum PVAB period  908 . Referring to  FIG.  10   , at block  810 , in addition to setting the maximum PVAB period, control circuit  206  may set the power conservation time period to a maximum time period  910  that is a predetermined time interval shorter than the maximum PVAB period  908 . The maximum PVAB period  908  and the maximum power conservation time period  910  may be started by control circuit  206  in response to each ventricular event  901  during a first group  902  of N RRIs. 
     Control circuit  206  may identify N consecutive ventricular events, each ending an RRI, for determining the first group  902  of N consecutive RRIs. In the example shown, a first group  902  of 8 consecutive RRIs are determined and buffered in memory  210  by control circuit  206 . A second group  904  of 8 consecutive RRIs, consecutively following the first group  902 , are determined and buffered in memory  210 . The second group  904  of 8 RRIs may overwrite the first group  902  in the memory buffer. While the groups  902  and  904  of N RRIs are shown as being consecutive, non-overlapping groups of RRIs in FIG.  11 , the groups of N RRIs may be overlapping or running groups of RRIs in other examples. In still other examples, the groups of N RRIs may not be consecutive. For instance, control circuit  206  may identify N consecutive RRIs, e.g., 3 to 12 consecutive RRIs, for determining a representative RRI after every M consecutive ventricular events, e.g., after every 8 to 100 ventricular events or any selected number M that is greater than N. To illustrate, control circuit  206  may determine a representative RRI from the most recent 8 consecutive RRIs after every 30 consecutive RRIs or any other selected number of RRIs that is greater than 8. In this way the PVAB period may be updated based on the N most recent RRIs but can be updated less often than every N RRIs, e.g., once per M RRIs or after a predetermined time interval, e.g., after every 30 seconds, every 60 seconds, every 2 minutes or any other selected time interval. 
     After the first group  902  of N RRIs, control circuit  206  determines a representative RRI of the group  902 . In the example shown, the representative RRI is determined as the fourth shortest RRI  912 . Control circuit  206  compares the RRI  912  to a threshold interval for selecting the PVAB period to be applied to the motion signal in response to the next group of ventricular events, e.g., following each ventricular event  901  in the second group  904  of 8 RRIs. In the example shown, control circuit  206  determines that the RRI  912  is less than a threshold interval, e.g., less than 665 ms or any other selected threshold value. In response to the representative RRI  912  being less than the threshold interval, control circuit  206  adjusts the PVAB period from the maximum PVAB period  908  to the minimum PVAB period  918 . During the second group  904  of RRIs, control circuit  206  starts the minimum PVAB period  918  in response to each ventricular event  901 . The last ventricular event  930  of the first group  902  of RRIs defines the end of the last RRI of the first group  902  and the beginning of the first RRI of the second group  904  of RRIs. As such, the PVAB period set in response to the ventricular event  930  may be set to the minimum PVAB period  918  based on the fourth shortest RRI  912  of the preceding group  902  being less than the threshold interval. 
     When the PVAB period is set to the minimum PVAB period at block  808  of  FIG.  10   , control circuit  206  may set a corresponding minimum power conservation time period  920 . The minimum power conservation time period  920  may be set to expire a predetermined interval earlier than the minimum PVAB period  918 . At least a portion of the motion sensor, e.g., at least one axis of the multi-axis accelerometer, may be disabled or powered down during the power conservation time period to reduce current drain from power source  214 . At least one axis of motion sensor  212  may be disabled during the minimum power conservation time period  920  and re-enabled at the expiration of time period  920  so that the motion sensor  212  is producing a motion signal used for sensing atrial event signals by the expiration of the minimum PVAB period  918 . It is to be understood that during the power conservation time periods  910  and  920  (and other power conservation time periods described herein), one or more axes of a multi-axis accelerometer included in motion sensor  212  may remain powered on and enabled for producing an acceleration signal that may be used by control circuit  206  for other purposes than sensing atrial event signals. 
     For example, control circuit  206  may be configured to determine a patient physical activity metric from an acceleration signal, which may be from a single axis of the accelerometer included in motion sensor  212 . Control circuit  206  may use the patient physical activity metric for setting a rate response pacing rate, e.g., during a VVIR or VDIR pacing mode. In this case, one axis used for determining the patient physical activity may remain powered on during the power conservation time periods  910  and  920 , but one or more other axes used for sensing atrial event signals may be powered off during the power conservation time periods  910  and  920 . Accordingly, when the motion sensor is disabled during the power conservation time period, it is to be understood that a portion of the motion sensor, e.g., at least one axis of a multi-axis accelerometer, may be powered down and disabled, and another portion, e.g., a different axis of the multi-axis accelerometer, may remain powered on or enabled during the power conservation time periods  910  and  920  to provide a motion signal to control circuit  206  that may not be used for sensing atrial event signals but may be used for other purposes. 
     In some instances, the axis signal used for determining patient physical activity may also be used, alone or in combination with one or more additional axis signals, for sensing atrial event signals. In some examples, when one (or more) axis signal(s) is/are used for sensing atrial event signals and the same one (or more) axis signal(s) is/are used for determining a patient physical activity metric, control circuit  206  may keep the accelerometer axis(es) used for monitoring patient physical activity powered on and enabled during the power conservation time periods  910  and  920  (or effectively cancel the power conservation time periods  910  and  920 ). When a combination of two or more axis signals are used for sensing atrial event signals and one of the two or more axis signals is also used for determining a patient physical activity metric, at least one axis used for atrial event signal sensing that is not used for monitoring patient physical activity may be powered off during the power conservation time periods  910  and  920 . In other instances, when the accelerometer axis used for monitoring patient physical activity is not used for atrial event signal sensing, all of the axis signals used for atrial event signal sensing may be powered off during the power conservation time periods  910  and  920 . For example, if the accelerometer of motion sensor  212  includes axis  1 , axis  2  and axis  3 , and axis  1  is used for patient physical activity monitoring and axes  2  and  3  are used for sensing atrial event signals, axes  2  and  3  may be disabled during the power conservation time periods  910  and  920 . In another illustrative example, when axis  1  is used for patient physical activity monitoring and axes  1  and  2  are used in combination for atrial event signal sensing, axis  2  may be powered off during power conservation time periods  910  and  920 . Techniques that may be used for reducing the current drain of power source  214  during the PVAB period in conjunction with the methods disclosed herein for adjusting the PVAB period are generally disclosed in U.S. Pat. No. 11,207,526 (Sheldon, et al.), incorporated herein by reference in its entirety. 
     In the example of  FIG.  11   , a minimum power conservation time period  920  and a maximum power conservation time period  910  are shown. In other examples, a single power conservation time period during which the motion sensor is at least partially disabled during the minimum and maximum PVAB periods to conserve power source  214  may be a fixed time interval shorter than the minimum PVAB period  918 . The power conservation time period  920 , for example, may not be increased when the maximum PVAB period  908  is applied. 
     After buffering the RRIs of the second group  904 , control circuit  206  determines the representative RRI of the second group  904  as the fourth shortest RRI  922 . Control circuit  206  may compare the representative RRI  922  to the threshold interval and, in this example, determine that RRI  922  is greater than the threshold interval. In response to RRI  922  being greater than the threshold interval, control circuit  206  adjusts the PVAB period back to the maximum PVAB period  908  that is started in response to the last ventricular event  932  of the second group  904  of RRIs. Control circuit  206  may additionally set the maximum power conservation time period  910 . 
     In the examples of  FIG.  10    and  FIG.  11   , a single threshold interval is used for selecting whether to set the PVAB period to a maximum or minimum PVAB period. In other examples, more than one threshold interval may be applied to a representative RRI. As described above, multiple ranges of RRIs (corresponding to multiple ventricular rates) may be separated by two or more threshold intervals to enable control circuit  206  to select between a minimum PVAB period, a maximum PVAB period and one or more intermediate PVAB periods. 
     In still other examples, more than one threshold interval may be used by control circuit  206  for controlling switching between a maximum PVAB period and a minimum PVAB period (or more generally between a relatively longer PVAB period and relatively shorter PVAB period). For instance, when the median RRI (or other representative RRI) is longer than a first threshold interval, the PVAB period may be increased to the maximum PVAB period. When the median RRI falls below a second threshold interval that is less than the first threshold interval, the PVAB period may be decreased to the minimum PVAB period. In this way, a different threshold interval may be used for causing the PVAB period to be increased than the threshold interval used for causing the PVAB period to be decreased. 
     For instance, with reference to  FIG.  10   , control circuit  206  may select the RRI threshold interval at block  806 , e.g., by selecting from a first, relatively longer threshold interval and a second, relatively shorter threshold interval based on the current duration of the PVAB period. When the PVAB period is at the minimum duration, the relatively longer RRI threshold interval may be selected for determining when to increase the PVAB period to the maximum duration. When the PVAB period is currently set at the maximum duration, the relatively shorter RRI threshold interval may be selected for determining when to decrease the PVAB period to the minimum duration. By using two different threshold intervals for switching between two different PVAB period durations, a hysteresis effect may be employed to reduce the likelihood of frequent adjustments to the PVAB period during a fluctuating heart rate, e.g., when the representative RRI is going back and forth from being greater than to being less than a single threshold interval. 
     In other examples, frequent adjustments between the maximum and minimum PVAB periods may be avoided by using a relatively higher number of RRIs, e.g., 12 to 30 RRIs, for determining the representative RRI that is compared to the threshold interval. In still other examples, frequent adjustments to the PVAB period may be avoided by allowing an adjustment to occur only after a predetermined number of RRIs, e.g., 20 to 100 RRIs or any other selected number of RRIs (which may be a greater number of RRIs than the number of RRIs used to determine the median or other representative RRI). In another example, the PVAB period may be adjusted at scheduled predetermined time intervals, e.g., no more than once every 30 seconds, once every 60 seconds or any other selected time interval. 
     All ventricular events are shown as ventricular pacing pulses in the example of  FIG.  11   . As mentioned previously herein, the PVAB period (and an associated power conservation period) may be set differently following a ventricular pacing pulse than following a ventricular event sensed by sensing circuit  204 . As such, one of a post-pace maximum PVAB period or a post-pace minimum PVAB period may be selected based on a representative RRI determined from a first group of RRIs (being greater than or less than a threshold interval), and the selected post-pace PVAB period (minimum or maximum) may be started in response to each ventricular pacing pulse that occurs during a second group of RRIs following the first group of RRIs. Additionally, one of a post-sense maximum PVAB period or a post-sense minimum PVAB period may be selected based on the representative RRI determined from the first group of RRIs, and the selected post-sense PVAB period (maximum or minimum) may be started in response to each ventricular event sensed by sensing circuit  204  that occurs during the second group of RRIs following the first group of RRIs. 
     The post-pace maximum and post-sense maximum PVAB periods may be different from each other. Control circuit  206  may select the maximum PVAB periods based on a representative RRI being greater than or equal to the threshold interval, but one or the other post-pace or post-sense maximum PVAB period is started in response to an individual ventricular event based on whether that ventricular event is a pacing pulse or a sensed event. The post-pace minimum and post-sense minimum PVAB periods may be different from each other. Control circuit  206  may select the minimum PVAB periods based on a representative RRI being less than the threshold interval, but one or the other post-pace or post-sense minimum PVAB period is started in response to a given ventricular event based on whether that ventricular event is a pacing pulse or a sensed event. 
     In some examples, one of the maximum PVAB periods or the minimum PVAB periods may be the same post-sense or post-pace. For example, a different post-pace maximum PVAB period may be set than the post-sense maximum PVAB period, but the post-pace minimum PVAB period and the post-sense minimum PVAB period may be equal. In another example, a different post-pace minimum PVAB period may be set than the post-sense minimum PVAB period, but the post-pace maximum and the post-sense maximum PVAB periods may be equal. 
       FIG.  12    is a flow chart  850  of a method for adjusting the PVAB period and controlling atrial synchronized ventricular pacing according to one example. Blocks  802  through  810  correspond to identically numbered blocks shown in  FIG.  10    and described above. After selecting the maximum or minimum PVAB period at block  808  or  810 , control circuit  206  starts the selected PVAB period in response to the next ventricular event and determines if the atrial event signal (A4 event) is sensed from the motion signal after the PVAB period at block  812 . As described above, e.g., in conjunction with  FIG.  5    or  FIG.  7   , control circuit  206  may sense the atrial event signal from the motion signal after the expiration of the PVAB period in response to the earliest A4 sensing threshold crossing of either the first, higher A4 sensing threshold amplitude during the A3 window or the second, lower A4 sensing threshold amplitude after the A3 window. 
     In response to sensing the atrial event signal outside the PVAB period, control circuit  206  may generate an output at block  816 , e.g., an atrial sensed event signal, that may be stored by memory  82 , e.g., with a time stamp. Atrial sensed event signals and associated data, such as an atrial sensed event interval, may be determined and stored in memory  210  for use in various functions such as automatically setting or adjusting atrial sensing control parameters, determining an atrial rate, controlling pacing mode switching or other functions. 
     It is to be understood that in any of the examples presented herein, control circuit  206  may start a post-ventricular atrial refractory period (PVARP) in response to each ventricular event in addition to starting the PVAB period. The PVARP may expire later than the PVAB period and may expire during the passive ventricular filling (A3) window  424  shown in  FIG.  5    in some examples. Control circuit  206  may sense an atrial event signal after the PVAB period expires but before the expiration of the PVARP during some ventricular cycles. Control circuit  206  may generate a refractory atrial sensed event signal at block  816 , e.g., for storing in memory  210  for determining an atrial rate or other diagnostic or sensing and/or therapy control purposes. When the atrial event signal is sensed after the expiration of the PVAB period but during a PVARP, pace timing circuit  242  may withhold starting the AV pacing interval in response to the refractory sensed atrial event signal. 
     When the atrial event signal is sensed outside the PVAB period and any PVARP, the pace timing circuit  242  ( FIG.  3   ) may receive the atrial sensed event signal from the atrial event detector circuit  240  at block  816  and, in response, start the AV pacing interval. Pulse generator  202  generates and delivers an atrial synchronous ventricular pacing pulse in response to the sensed atrial event signal, e.g., upon expiration of the AV pacing interval, at block  818 . In response to the delivered ventricular pacing pulse, control circuit  206  starts the PVAB period at block  820 , set to either the minimum or maximum PVAB period previously selected at block  808  or  810 . The ventricular pacing pulse delivered at block  818  is identified by control circuit  206  as a ventricular event of the next group of N ventricular events and may determine if N ventricular events have been identified at block  822 , corresponding to the next group of N RRIs buffered in memory  210 . 
     Referring again to block  812 , if the atrial event signal is not sensed at block  812  before another ventricular event is identified at block  814 , control circuit  206  may identify the ventricular event at block  814  and start the PVAB period at block  820  in response to the identified ventricular event (without delivering an atrial synchronized ventricular pacing pulse). If a ventricular event has not yet occurred, control circuit  206  may return to block  812  to continue waiting for a sensed atrial event signal or a ventricular event, whichever occurs first. In some instances, a ventricular event is sensed by sensing circuit  204  at block  814  before the atrial event signal is sensed. In other instances, a ventricular lower rate pacing interval may expire before the atrial event signal is sensed. An asynchronous ventricular pacing pulse may be delivered at the lower rate pacing interval (LRI) at block  814 . Control circuit  206  may identify the ventricular sensed event or ventricular pacing pulse as a ventricular event and start the PVAB period at block  820 . 
     At block  822 , control circuit  206  may determine if the next N ventricular events have been identified. If not, control circuit  206  may return to block  812  and continue to set the PVAB period according to the current maximum or minimum period most recently selected (at block  808  or  810 ). When the next N ventricular events are identified at block  822 , control circuit  206  may return to block  804  to determine the representative RRI from the N ventricular events. Control circuit  206  sets the PVAB period to the minimum period or the maximum period at one of blocks  808  or  810  based on the comparison of the threshold interval to the most recently determined representative RRI. 
     In the example of  FIG.  12   , control circuit  206  may adjust the PVAB period between a maximum PVAB period and a minimum PVAB period based on an RRI, which may be a representative RRI determined from multiple RRIs. The PVAB period may be adjusted from the maximum PVAB period to the minimum PVAB period without requiring an analysis of the amplitude of the motion signal during the maximum PVAB period in some examples. In other examples, techniques described herein for determining motion signal amplitude data during the PVAB period may be performed by control circuit  206  to analyze the amplitude of the motion signal during the maximum PVAB period, e.g., before shortening to the minimum PVAB period based on a representative RRI being less than a threshold interval. The techniques generally described in conjunction with  FIGS.  10  through  12    may be combined with any of the example techniques disclosed herein for analyzing the motion signal amplitude prior to making a decision by control circuit  206  to shorten or lengthen the PVAB period and/or by how much the PVAB period is shortened or lengthened (e.g., a maximum decrement or increment). 
     It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, a medical device. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer). 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPLAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     Thus, a medical device has been presented in the foregoing description with reference to specific examples. It is to be understood that various aspects disclosed herein may be combined in different combinations than the specific combinations presented in the accompanying drawings. It is appreciated that various modifications to the referenced examples may be made without departing from the scope of the disclosure and the following claims.