Leadless cardiac pacemaker system for usage in combination with an implantable cardioverter-defibrillator

• US 20070088394A1
• Filed: 10/13/2006
• Published: 04/19/2007
• Est. Priority Date: 10/14/2005
• Status: Active Grant

• Current Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated), Nostix, LLC (Teleflex Incorporated)
• Original Assignee: Nanostim, Inc (Abbott Laboratories Incorporated)
• Inventors: Jacobson, Peter
• Granted Patent: US 8,457,742 B2
• Time in Patent Office: Days
• US Class Current: 607/4
• CPC Class Codes: A61M 25/0662 Guide tubes
• CPC Class Codes: A61N 1/056 Transvascular endocardial electrode systems
• CPC Class Codes: A61N 1/0573 chacterised by means penetrating the heart tissue, e.g. helix needle or hook
• CPC Class Codes: A61N 1/0587 Epicardial electrode systems; Endocardial electrodes piercing the pericardium
• CPC Class Codes: A61N 1/059 Anchoring means
• CPC Class Codes: A61N 1/3621 for treating or preventing abnormally high heart rate
• CPC Class Codes: A61N 1/3627 for treating a mechanical deficiency of the heart, e.g. congestive heart failure or cardiomyopathy
• CPC Class Codes: A61N 1/36514 controlled by a physiological quantity other than heart potential, e.g. blood pressure controlled by two or more physical parameters A61N1/36585
• CPC Class Codes: A61N 1/36542 controlled by body motion, e.g. acceleration
• CPC Class Codes: A61N 1/368 comprising more than one electrode co-operating with different heart regions A61N1/3622, A61N1/3627 take precedence
• CPC Class Codes: A61N 1/3684 for stimulating the heart at multiple sites of the ventricle or the atrium
• CPC Class Codes: A61N 1/36842 Multi-site stimulation in the same chamber
• CPC Class Codes: A61N 1/3704 Circuits specially adapted therefor, e.g. for sensitivity control
• CPC Class Codes: A61N 1/3706 Pacemaker parameters stimulation threshold A61N1/371
• CPC Class Codes: A61N 1/3708 for power depletion
• CPC Class Codes: A61N 1/372 Arrangements in connection with the implantation of stimulators
• CPC Class Codes: A61N 1/37205 Microstimulators, e.g. implantable through a cannula
• CPC Class Codes: A61N 1/37217 characterised by the communication link, e.g. acoustic or tactile
• CPC Class Codes: A61N 1/37252 Details of algorithms or data aspects of communication system, e.g. handshaking, transmitting specific data or segmenting data
• CPC Class Codes: A61N 1/3727 characterised by the modulation technique
• CPC Class Codes: A61N 1/37288 : Communication to several implantable medical devices within one patient
• CPC Class Codes: A61N 1/37512 : Pacemakers
• CPC Class Codes: A61N 1/37518 : Anchoring of the implants, e.g. fixation
• CPC Class Codes: A61N 1/3756 : Casings with electrodes thereon, e.g. leadless stimulators
• CPC Class Codes: A61N 1/3925 : Monitoring; Protecting
• CPC Class Codes: A61N 1/3956 : Implantable devices for applying electric shocks to the heart, e.g. for cardioversion
• CPC Class Codes: A61N 1/39622 : Pacing therapy
• CPC Class Codes: A61N 2001/058 : Fixing tools
• CPC Class Codes: H04B 13/005 : Transmission systems in which the medium consists of the human body

Abstract

A cardiac pacing system comprising one or more leadless cardiac pacemakers configured for implantation in electrical contact with a cardiac chamber and configured to perform cardiac pacing functions in combination with a co-implanted implantable cardioverter-defibrillator (ICD). The leadless cardiac pacemaker comprises at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with the co-implanted ICD.

Claims

1. A cardiac pacing system comprising:

at least one leadless cardiac pacemaker configured for implantation in electrical contact with a cardiac chamber and configured to perform cardiac pacing functions in combination with a co-implanted implantable cardioverter-defibrillator (ICD), the at least one leadless cardiac pacemaker comprising at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with the co-implanted ICD.

2. The system according to claim 1 further comprising:

the at least one leadless cardiac pacemaker configured to intercommunicate and/or to communicate with a non-implanted programmer and/or the implanted ICD via the at least two electrodes that are also used for delivering pacing pulses.

3. The system according to claim 1 further comprising:

the at least one leadless cardiac pacemaker configured for antenna-less and telemetry coil-less communication.

4. The system according to claim 1 further comprising:

the at least one leadless cardiac pacemaker configured to intercommunicate and to communicate with a non-implanted programmer via communication that has transmitted communication power requirements essentially met by power consumed in cardiac pacing.

5. A system according to claim 1 further comprising:

the at least one leadless cardiac pacemaker comprising;

a hermetic housing configured for placement on or attachment to the inside or outside of a cardiac chamber; and

the at least two leadless electrodes proximal to the housing configured for bidirectional communication with at least one other device within or outside the body.

6. The system according to claim 1 further comprising:

the at least one leadless cardiac pacemaker configured to communicate bidirectionally among the leadless cardiac pacemaker plurality and/or the implanted ICD to coordinate pacing pulse delivery using messages that identify an event at an individual pacemaker originating the message and pacemakers receiving the message react as directed by the message depending on the message origin.

7. The system according to claim 1 further comprising:

the at least one leadless cardiac pacemaker configured to communicate bidirectionally among the leadless cardiac pacemaker plurality and/or the implanted ICD to facilitate sensitivity and specificity of the ICD.

8. The system according to claim 1 further comprising:

the at least one leadless cardiac pacemaker configured to communicate with a subcutaneous-only implantable defibrillator to facilitate sensitivity and specificity of the subcutaneous-only implantable defibrillator whereby atrial depolarization and ventricular depolarization are uniquely and specifically identified.

9. The system according to claim 1 further comprising:

the at least two leadless electrodes configured to communicate bidirectionally among the leadless cardiac pacemaker plurality and/or the ICD and transmit data including designated codes for events detected or created by an individual pacemaker, the individual pacemakers configured to issue a unique code corresponding to an event type and a location of the individual pacemaker.

10. The system according to claim 1 further comprising:

individual pacemakers of the leadless cardiac pacemaker plurality configured to deliver a coded pacing pulse with a code assigned according to pacemaker location and configured to transmit a message to at least one pacemaker of the leadless cardiac pacemaker plurality via the coded pacing pulse wherein the code identifies the individual pacemaker originating an event, the at least one pacemaker receiving the message being adapted to respond to the message in a predetermined manner depending on type and location of the event.

11. The system according to claim 1 further comprising:

individual pacemakers of the leadless cardiac pacemaker plurality configured to deliver a coded pacing pulse with a code assigned according to pacemaker location and configured to transmit a message to at least one pacemaker of the leadless cardiac pacemaker plurality via the coded pacing pulse wherein the code identifies the individual pacemaker originating an event, the individual pacemakers further configured to deliver a pacing pulse in absence of encoding whereby, for dual-chamber cardiac pacing, a pacing pulse that is not generated in a first cardiac pacemaker that senses the pacing pulse is necessarily generated in a second cardiac pacemaker.

12. The system according to claim 1 further comprising:

individual pacemakers of the leadless cardiac pacemaker plurality configured to communicate to at least one pacemaker of the leadless cardiac pacemaker plurality occurrence of a sensed heartbeat at the individual pacemaker location via generation of a coded pacing pulse triggered by the sensed heartbeat in a natural refractory period following the sensed heartbeat.

13. The system according to claim 1 further comprising:

individual pacemakers of the leadless cardiac pacemaker plurality configured to receive conducted communication from a co-implanted cardioverter-defibrillator (ICD) that configures the individual pacemakers to deliver overdrive anti-tachycardia pacing in response to a detected tachyarrhythmia.

14. The system according to claim 1 further comprising:

individual pacemakers of the leadless cardiac pacemaker plurality configured for operation in a particular location and a particular functionality at manufacture and/or at programming by an external programmer.

15. The system according to claim 1 further comprising:

the leadless cardiac pacemaker plurality comprising a right ventricular leadless cardiac pacemaker and a left ventricular leadless cardiac pacemaker configured to operate with atrio-ventricular (AV) delays whereby a left ventricular pacing pulse can be delivered before, after, or substantially simultaneously with a right ventricular pacing pulse.

16. The system according to claim 15 further comprising:

the right-ventricular leadless cardiac pacemaker configured to;

set the ventricular-to-ventricular (VV) escape interval longer than a predetermined atrial-to-atrial (AA) escape interval to enable backup ventricular pacing at a low rate corresponding to the VV escape interval in case of failure of a triggered signal from a co-implanted atrial leadless cardiac pacemaker.

17. The system according to claim 1 further comprising:

the at least two leadless electrodes configured to communicate bidirectionally among the leadless cardiac pacemaker plurality and transmit data including designated codes for events detected or created by an individual pacemaker, the codes encoding information using pacing pulse width.

18. The system according to claim 1 further comprising:

an atrial leadless cardiac pacemaker further configured to;

time a prolonged post-ventricular atrial refractory period (PVARP) after recycling in presence of the premature signal whereby pacemaker-mediated tachycardia is prevented.

19. The system according to claim 1 further comprising:

an implanted cardioverter-defibrillator (ICD) comprising a case and pair of electrodes mounted on or near the case, and configured to receive and transmit conducted communication using a pulse modulated or frequency modulated carrier signal whereby the ICD is configured to detect communication pulses from co-implanted leadless cardiac pacemakers and transmit programming information to the co-implanted leadless cardiac pacemakers.

20. The system according to claim 1 further comprising:

an implanted cardioverter-defibrillator (ICD) configured to receive conducted communication using two implantable electrodes.

21. The system according to claim 1 further comprising:

individual pacemakers of the leadless cardiac pacemaker plurality comprising;

a receiving amplifier/filter adapted for multiple controllable gain settings; and

a processor configured to control gain setting for the receiving amplifier/filter, invoking a low-gain setting for normal operation and detecting presence of a cardiac pacing pulse, and invoking a high-gain setting for detecting and decoding information encoded in the detected cardiac pacing pulse.

22. The system according to claim 1 further comprising:

individual pacemakers of the leadless cardiac pacemaker plurality comprising;

a tank capacitor coupled across a pair of the at least two leadless electrodes and adapted for charging and discharging wherein a pacing pulse is generated;

a charge pump circuit coupled to the tank capacitor and adapted for controlling charging of the tank capacitor; and

a processor configured to control recharging of the tank capacitor wherein recharging is discontinued when a battery terminal voltage falls below a predetermined value to ensure sufficient voltage for powering the leadless cardiac pacemaker.

23. A cardiac pacing system comprising:

an implantable cardioverter-defibrillator (ICD); and

at least one leadless cardiac pacemaker configured for implantation in electrical contact with a cardiac chamber and for performing cardiac rhythm management functions in combination with the implantable ICD, the implantable ICD and the at least one leadless cardiac pacemaker configured for leadless intercommunication by information conduction through body tissue.

24. The system according to claim 23 further comprising:

the at least one leadless cardiac pacemaker comprising at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with the co-implanted ICD.

25. The system according to claim 23 further comprising:

the at least one leadless cardiac pacemaker configured to intercommunicate and/or to communicate with a non-implanted programmer via communication that has transmitted communication power requirements essentially met by power consumed in cardiac pacing.

26. The system according to claim 23 further comprising:

the at least one leadless cardiac pacemaker configured to intercommunicate and/or to communicate with a non-implanted programmer via communication that has negligible transmission power requirements in addition to power consumed in cardiac pacing.

27. A cardiac pacing system comprising:

at least one leadless cardiac pacemaker configured for implantation in electrical contact with a cardiac chamber and configured to perform cardiac pacing functions in combination with a co-implanted implantable cardioverter-defibrillator (ICD), the at least one leadless cardiac pacemaker configured to intercommunicate and/or to communicate with a non-implanted programmer and/or the implanted ICD via at least two electrodes that are used for delivering pacing pulses, the individual pacemakers configured for antenna-less and telemetry coil-less communication.

28. The system according to claim 27 further comprising:

the at least one leadless cardiac pacemaker comprising at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with the co-implanted ICD.

29. The system according to claim 27 further comprising:

the at least one leadless cardiac pacemaker configured to intercommunicate and/or to communicate with a non-implanted programmer via communication that has transmitted communication power requirements essentially met by power consumed in cardiac pacing.

30. The system according to claim 27 further comprising:

the at least one leadless cardiac pacemaker configured to intercommunicate and/or to communicate with a non-implanted programmer via communication that has negligible transmission power requirements in addition to power consumed in cardiac pacing.

31. A cardiac pacing system comprising:

an implantable cardioverter-defibrillator (ICD); and

at least one leadless cardiac pacemaker configured for implantation in electrical contact with a cardiac chamber and configured for performing cardiac pacing functions in combination with the ICD comprising delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with a co-implanted ICD and/or at least one pacemaker of a plurality of leadless cardiac pacemakers, the at least one leadless cardiac pacemaker further configured to communicate a code that signifies occurrence of sensed cardiac electrical signals and/or delivered pacing pulses and identifies an event type and/or location.

32. The system according to claim 31 further comprising:

at least two electrodes coupled to the ICD and configured to transmit and/or receive conducted communication using a pulse-modulated or frequency-modulated carrier signal, the ICD configured to detect communication pulses from at least one co-implanted leadless cardiac pacemaker and transmit programming information to the at least one co-implanted leadless cardiac pacemaker.

33. The system according to claim 31 further comprising:

the at least one leadless cardiac pacemaker configured to broadcast information to the co-implanted ICD and/or at least one pacemaker of a plurality of leadless cardiac pacemakers.

34. The system according to claim 31 further comprising:

the at least one leadless cardiac pacemaker configured to receive the code and react based on the code, location of the receiving leadless cardiac pacemaker, and predetermined system functionality.

35. The system according to claim 31 further comprising:

the at least one leadless cardiac pacemaker configured to perform at least one cardiac pacing function selected from a group consisting of single-chamber pacing, dual-chamber pacing, cardiac resynchronization therapy with cardioversion/defibrillation (CRT-D), single-chamber overdrive pacing for prevention of tachyarrhythmias, single-chamber overdrive pacing for conversion of tachyarrhythmias, multiple-chamber pacing for prevention of tachyarrhythmias, multiple-chamber pacing for conversion of tachyarrhythmias.

36. The system according to claim 31 further comprising:

the plurality of leadless cardiac pacemakers configured for co-implantation in a single patient and multiple-chamber pacing, the bidirectional communication among the leadless cardiac pacemaker plurality adapted to communicate notification of a sensed heartbeat or delivered pacing pulse event and encoding type and location of the event to at least one pacemaker of the leadless cardiac pacemaker plurality, the at least one pacemaker that receives the communication adapted to decode the information and react depending on location of the receiving pacemaker and predetermined system functionality.

37. The system according to claim 31 further comprising:

the leadless cardiac pacemaker plurality comprising an atrial leadless cardiac pacemaker implanted in electrical contact to an atrial cardiac chamber and configured for dual-chamber pacing in combination with the co-implanted ICD, the atrial leadless cardiac pacemaker configured to;

wait for an earliest occurring event of a plurality of events comprising a sensed atrial heartbeat, a communication of an event sensed on the at least two leadless electrodes encoding a pacing pulse marking a heartbeat at a ventricular leadless cardiac pacemaker, or timeout of an escape interval;

respond to the sensed atrial heartbeat by generating an atrial pacing pulse that signals to at least one pacemaker of the leadless cardiac pacemaker plurality and optionally to the co-implanted ICD that an atrial heartbeat has occurred and that encodes the atrial pacing pulse with a code signifying an atrial location and a sensed event type;

time a predetermined atrial-to-atrial escape interval beginning at delivery of an atrial pacing pulse; and

respond to timeout of an escape interval by delivering an atrial pacing pulse, causing an atrial heartbeat, the atrial pacing pulse encoding paced type and atrial location of an atrial heartbeat event.

38. The system according to claim 37 further comprising:

the atrial leadless cardiac pacemaker configured to;

detect a signal originating from a co-implanted ventricular leadless cardiac pacemaker;

examine an elapsed amount of the atrial-to-atrial escape interval since a most recent atrial heartbeat;

determine whether the signal originating from the co-implanted ventricular leadless cardiac pacemaker is premature;

in absence of a premature signal, wait for an event with no effect on atrial pacing;

for a premature signal, restart a ventricle-to-atrium escape interval that is shorter than the atrial-to-atrial escape interval and representative of a typical time from a ventricular beat to a next atrial beat in sinus rhythm; and

respond to timeout of an escape interval by delivering an atrial pacing pulse, causing an atrial heartbeat, the atrial pacing pulse encoding paced type and atrial location of an atrial heartbeat event.

39. The system according to claim 31 further comprising:

the leadless cardiac pacemaker plurality comprising a right-ventricular leadless cardiac pacemaker implanted in electrical contact to a right-ventricular cardiac chamber and configured for dual-chamber pacing in combination with the co-implanted ICD, the right-ventricular leadless cardiac pacemaker configured to;

wait for an earliest occurring event of a plurality of events comprising a sensed right-ventricular heartbeat, a sensed communication of a pacing pulse marking a heartbeat at an atrial leadless cardiac pacemaker, and timeout of an escape interval;

respond to the sensed right-ventricular heartbeat by generating a right-ventricular pacing pulse that signals to at least one pacemaker of the leadless cardiac pacemaker plurality and optionally to the co-implanted ICD that a right-ventricular heartbeat has occurred and that encodes the right-ventricular pacing pulse with a code signifying a right-ventricular location;

time a predetermined right ventricular-to-right ventricular escape interval; and

respond to timeout of an escape interval by delivering a right ventricular pacing pulse, causing a right ventricular heartbeat, the right ventricular pacing pulse encoding paced type and right-ventricular location of a right ventricular heartbeat event.

40. The system according to claim 39 further comprising:

the right-ventricular leadless cardiac pacemaker configured to;

detect a signal originating from a co-implanted atrial leadless cardiac pacemaker;

examine an elapsed amount of the ventricular-to-ventricular escape interval since a most recent right-ventricular heartbeat;

determine whether the signal originating from the co-implanted atrial leadless cardiac pacemaker is premature;

for a premature signal, wait for an event with no effect on ventricular pacing; and

in absence of a premature signal, start a right atrium to right ventricular (AV) escape interval that is representative of a typical time from an atrial beat to a right-ventricular beat in sinus rhythm; and

respond to timeout of an escape interval by delivering a right ventricular pacing pulse, causing a right ventricular heartbeat, the right ventricular pacing pulse encoding paced type and right-ventricular location of a right ventricular heartbeat event.

41. The system according to claim 31 further comprising:

the leadless cardiac pacemaker plurality comprising a left-ventricular leadless cardiac pacemaker implanted in electrical contact to a left-ventricular cardiac chamber and configured for dual-chamber pacing in combination with the co-implanted ICD, the left-ventricular leadless cardiac pacemaker configured to;

wait at the left-ventricular leadless cardiac pacemaker for an earliest occurring event of a plurality of events comprising a sensed communication of a pacing pulse marking a heartbeat at an atrial leadless cardiac pacemaker, and timeout of a left ventricular escape interval; and

respond to timeout of the left ventricular escape interval by delivering a left ventricular pacing pulse, causing a left ventricular heartbeat, the left ventricular pacing pulse encoding type and location of a left ventricular heartbeat event.

42. The system according to claim 31 further comprising:

the leadless cardiac pacemaker plurality comprising a left-ventricular leadless cardiac pacemaker implanted in electrical contact to a left-ventricular cardiac chamber and configured for dual-chamber pacing in combination with the co-implanted ICD, the left-ventricular leadless cardiac pacemaker configured to;

wait at the left-ventricular leadless cardiac pacemaker for an earliest occurring event of a plurality of events comprising a sensed communication of a pacing pulse marking a heartbeat at an atrial leadless cardiac pacemaker, and timeout of a left ventricular escape interval;

respond to the sensed communication of a pacing pulse marking a heartbeat at an atrial leadless cardiac pacemaker by examining an elapsed amount of the atrial-to-atrial escape interval since a most recent atrial heartbeat, determining whether the signal originating from the co-implanted ventricular leadless cardiac pacemaker is premature, and for a premature signal, restarting a ventricle-to-atrium escape interval that is shorter than the atrial-to-atrial escape interval and representative of a typical time from a ventricular beat to a next atrial beat in sinus rhythm; and

respond to timeout of an escape interval by delivering an atrial pacing pulse, causing an atrial heartbeat, the atrial pacing pulse encoding paced type and atrial location of an atrial heartbeat event.

43. A cardiac pacing system comprising:

at least one leadless cardiac pacemaker configured for implantation in electrical contact with a cardiac chamber and configured to perform cardiac pacing functions in combination with a co-implanted implantable cardioverter-defibrillator (ICD).

44. A cardiac pacing system comprising:

at least one leadless cardiac pacemaker configured for implantation in electrical contact with a cardiac chamber and configured to perform cardiac pacing functions in combination with a co-implanted implantable cardioverter-defibrillator (ICD), the at least one leadless cardiac pacemaker comprising at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and transmitting information to the co-implanted ICD.

45. A cardiac pacing system comprising:

at least one leadless cardiac pacemaker configured for implantation in electrical contact with a cardiac chamber and configured to perform cardiac pacing functions in combination with a co-implanted implantable cardioverter-defibrillator (ICD), the at least one leadless cardiac pacemaker comprising at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and receiving information from the co-implanted ICD.

46. A method of operating a cardiac pacing system comprising:

performing cardiac pacing functions in at least one pacemaker in combination with a co-implanted cardioverter-defibrillator (ICD) comprising;

delivering cardiac pacing pulses;

sensing evoked and/or natural cardiac electrical signals;

bidirectionally communicating among the co-implanted ICD and/or at least one pacemaker of a plurality of pacemakers; and

communicating a code that signifies occurrence of sensed cardiac electrical signals and/or delivered pacing pulses, identifying an event type and/or location.

47. The method according to claim 46 further comprising:

waiting in an atrial pacemaker for an earliest occurring event of a plurality of events comprising a sensed atrial heartbeat, a communication of a signal marking a heartbeat at a ventricular pacemaker, or timeout of an escape interval;

responding to the sensed atrial heartbeat by generating an atrial pacing pulse signaling to at least one pacemaker of the pacemaker plurality and optionally to the co-implanted ICD that an atrial heartbeat has occurred and that encodes the atrial pacing pulse with a code signifying an atrial location and a sensed event type;

timing a predetermined atrial-to-atrial escape interval beginning at delivery of an atrial pacing pulse;

responding to timeout of an escape interval by delivering an atrial pacing pulse, causing an atrial heartbeat; and

encoding the atrial pacing pulse with paced type and atrial location of an atrial heartbeat event.

48. The method according to claim 47 further comprising:

detecting at the atrial pacemaker a signal originating from a co-implanted ventricular pacemaker;

examining an elapsed amount of the atrial-to-atrial escape interval since a most recent atrial heartbeat;

determining whether the signal originating from the co-implanted ventricular pacemaker is premature;

in absence of a premature signal waiting for an event with no effect on atrial pacing;

for a premature signal restarting a ventricle-to-atrium escape interval that is shorter than the atrial-to-atrial escape interval and representative of a typical time from a ventricular beat to a next atrial beat in sinus rhythm;

responding to timeout of an escape interval by delivering an atrial pacing pulse, causing an atrial heartbeat; and

encoding the atrial pacing pulse with paced type and atrial location of an atrial heartbeat event.

49. The method according to claim 48 further comprising:

waiting in a right-ventricular pacemaker for an earliest occurring event of a plurality of events comprising a sensed right-ventricular heartbeat, a communication of a signal marking a heartbeat at an atrial pacemaker, and timeout of an escape interval;

responding to the sensed right-ventricular heartbeat by generating a right-ventricular pacing pulse that signals to at least one pacemaker of the pacemaker plurality and optionally to the co-implanted ICD that a right-ventricular heartbeat has occurred and that encodes the right-ventricular pacing pulse with a code signifying a right-ventricular location;

timing a predetermined right ventricular-to-right ventricular escape interval;

responding to timeout of an escape interval by delivering a right ventricular pacing pulse, causing a right ventricular heartbeat; and

encoding the right ventricular pacing pulse with paced type and right-ventricular location of a right ventricular heartbeat event.

50. The method according to claim 49 further comprising:

detecting in a right-ventricular pacemaker a signal originating from a co-implanted atrial pacemaker;

examining an elapsed amount of the ventricular-to-ventricular escape interval since a most recent right-ventricular heartbeat;

determining whether the signal originating from the co-implanted atrial pacemaker is premature;

for a premature signal waiting for an event with no effect on ventricular pacing;

in absence of a premature signal starting a right atrium to right ventricular (AV) escape interval that is representative of a typical time from an atrial beat to a right-ventricular beat in sinus rhythm;

responding to timeout of an escape interval by delivering a right ventricular pacing pulse, causing a right ventricular heartbeat; and

encoding the right ventricular pacing pulse with paced type and right-ventricular location of a right ventricular heartbeat event.

51. The method according to claim 48 further comprising:

waiting at a left-ventricular pacemaker for an earliest occurring event of a plurality of events comprising a communication of a signal marking a heartbeat at an atrial pacemaker and timeout of a left ventricular escape interval;

responding to timeout of the left ventricular escape interval by delivering a left ventricular pacing pulse, causing a left ventricular heartbeat; and

encoding the left ventricular pacing pulse with paced type and left ventricular location of a left ventricular heartbeat event.

52. The method according to claim 48 further comprising:

waiting at a left-ventricular leadless cardiac pacemaker for an earliest occurring event of a plurality of events comprising a communication of a signal marking a heartbeat at an atrial pacemaker and timeout of a left ventricular escape interval;

responding to the sensed communication of a pacing pulse marking a heartbeat at an atrial leadless cardiac pacemaker by examining an elapsed amount of the atrial-to-atrial escape interval since a most recent atrial heartbeat;

determining whether the signal originating from the co-implanted ventricular leadless cardiac pacemaker is premature;

for a premature signal restarting a ventricle-to-atrium escape interval that is shorter than the atrial-to-atrial escape interval and representative of a typical time from a ventricular beat to a next atrial beat in sinus rhythm;

responding to timeout of an escape interval by delivering an atrial pacing pulse, causing an atrial heartbeat; and

encoding the atrial pacing pulse with paced type and atrial location of an atrial heartbeat event.