WO2007047681B1

WO2007047681B1 - Leadless cardiac pacemaker and system

Info

Publication number: WO2007047681B1
Application number: PCT/US2006/040564
Authority: WO
Inventors: Peter M Jacobson
Original assignee: Nanostim Inc; Peter M Jacobson
Priority date: 2005-10-14
Filing date: 2006-10-13
Publication date: 2008-11-06
Also published as: EP2471450A1; JP5514259B2; US9872999B2; US20130103109A1; EP2471576B1; JP2012179425A; US9072913B2; JP2012157763A; CN101578067B; US10238883B2; US20070088405A1; US9227077B2; JP2012179424A; EP2471452B1; EP2471452A1; US20070088398A1; US20180126180A1; US9409033B2; JP2012179426A; US20140309706A1

Abstract

In a cardiac pacing system, a leadless cardiac pacemaker is configured for implantation in electrical contact with a cardiac chamber and configured for leadless pacing.

Claims

AMENDED CLAIMS received by the International Bureau on 07 June 2008 (07.06.2008)

1. A leadless biostimulator comprising: a housing; a plurality of electrodes formed integrally to the housing or coupled to the housing at a maximum distance of 2 centimeters; a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator generating and delivering electrical pulses via the electrode plurality; a processor hermetically contained within the housing and communicatively coupled to the pulse generator and the electrode plurality, the processor controlling electrical pulse delivery according to programmed instructions; and a power supply hermetically contained within the housing and coupled to the pulse generator, the power supply supplying energy for operations and electrical pulse generation as a source internal to the housing, the power supply further comprising a primary battery with an energy density of at least 3 watt-hours/cubic centimeter.

2. The biostimulator according to Claim 1 further comprising: the processor that controls electrical pulse delivery according to at least one programmable parameter, the processor being programmable by conducted communication signals transmitted via the electrode plurality.

3. The biostimulator according to Claim 1 further comprising: the processor that communicates with a device external to the biostimulator by conducted communication signals transmitted via the electrode plurality.

4. The biostimulator according to Claim 1 further comprising: the pulse generator selectively generating and delivering electrical energy in a stimulation pulse to at least two of the electrode plurality for causing a contraction of a patient's heart in response to control signals from the processor.

5. The biostimulator according to Claim 4 further comprising: the pulse generator generating and delivering electrical energy with the stimulation pulse interrupted by at least one notch that conveys information to a device external to the biostimulator.

6. The biostimulator according to Claim 4 further comprising: the pulse generator generating and delivering electrical energy with the stimulation pulse interrupted by at least one notch that conveys information to a device external to the biostimulator; and the processor that communicates control signals to the pulse generator specifying characteristics of the at least one notch and defining the conveyed information.

7. The biostimulator according to Claim 4 further comprising: the pulse generator generating and delivering electrical energy with the stimulation pulse interrupted by at least one notch that conveys information to a device external to the biostimulator, the conveyed information comprising data selected from a group consisting of programmable parameter settings, event counts, power- supply voltage, and power-supply current.

8. The biostimulator according to Claim 4 further comprising: the pulse generator generating and delivering electrical energy with the stimulation pulse interrupted by at least one notch that conveys information to a device external to the biostimulator wherein width of the at least one notch is approximately 15 microseconds.

9. The biostimulator according to Claim 4 further comprising: the pulse generator generating and delivering electrical energy with the stimulation pulse interrupted by at least one notch that conveys information to a device external to the biostimulator wherein the at least one notch occurs in at least one timing window.

10. The biostimulator according to Claim 4 further comprising: the pulse generator generating and delivering electrical energy with the stimulation pulse interrupted by at least one notch that conveys information to a device external to the biostimulator wherein the at least one notch occurs in at least one timing window and spacing between timing windows is approximately 100 microseconds.

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11. The biostimulator according to Claim 4 further comprising: the pulse generator generating and delivering electrical energy in a series of stimulation pulses with time between the stimulation pulses selectively varied to convey information to a device external to the biostimulator.

12. The biostimulator according to Claim 11 wherein: the variation of time between pulses is less than a total of 10 milliseconds.

13. The system according to Claim 1 further comprising: the pulse generator configured for generating and delivering electrical energy in a series of stimulation pulses with pacing pulse width selectively varied to convey information to a device external to the biostimulator.

14. The biostimulator according to Claim 1 further comprising: the power supply comprising a primary battery formed of a beta-voltaic converter that obtains electrical energy from radioactivity.

15. The biostimulator according to Claim 1 further comprising: the power supply comprising a primary battery that has a volume less than approximately

1 cubic centimeter and sources sufficient energy to support circuitry that consumes a maximum of 64 microwatts.

16. The biostimulator according to Claim 1 further comprising: a regulator circuit electrically connected between the power supply and biostimulator circuitry, the regulator circuit that regulates voltage supply for powering biostimulator circuitry.

17. The biostimulator according to Claim 1 further comprising: the power supply comprising a primary battery; and a battery ammeter in the power supply indicating battery current drain and indirect device health for usage by the processor.

18. The biostimulator according to Claim 1 further comprising: the power supply comprising a primary battery; and a battery voltmeter in the power supply indicating battery voltage for usage by the processor.

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19. The biostimulator according to Claim 1 further comprising: a tissue connector that affixes the housing to cardiac muscle, the tissue connector selected from a group consisting of a helix configured to rotatingly advance into the cardiac muscle, at least one member pierced with a hole for passing a suture, and at least one tine.

20. The biostimulator according to Claim 1 wherein: the housing is cylindrical; and the electrode plurality comprises are annular electrodes and located at extremities of the housing.

21. The biostimulator according to Claim 1 wherein: the housing is constructed from a ceramic material; and the electrode plurality is deposited on the ceramic material.

22. The biostimulator according to Claim 1 wherein: the housing is operative as an electrode and constructed from titanium or stainless steel and is coated over part of an exterior surface with a silicone rubber or polyurethane insulating material.

23. The biostimulator according to Claim 1 wherein: the biostimulator is a leadless cardiac pacemaker.

24. The system according to Claim 1 further comprising: a receiving amplifier/filter with multiple controllable gain settings; and a processor that controls gain setting for the receiving amplifier/filter, invokes a low- gain setting for normal operation and detecting presence of an electrical pulse, and invokes a high-gain setting for detecting and decoding information encoded in the detected electrical pulse.

25. The system according to Claim 1 further comprising: a tank capacitor coupled across a pair of the electrode plurality that charges and discharges for generating an electrical pulse; a charge pump circuit coupled to the tank capacitor that controls charging of the tank capacitor; and

- 106 - a processor that controls 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 biostimulator.

26. A leadless biostimulator comprising: a housing; a plurality of electrodes formed integrally to the housing or coupled to the housing at a maximum distance of 2 centimeters; a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator that generates and delivers electrical pulses to the electrode plurality; an amplifier hermetically contained within the housing and electrically coupled to the electrode plurality, the amplifier that amplifies signals received from the electrode plurality; a processor hermetically contained within the housing and communicatively coupled to the pulse generator, the amplifier, and the electrode plurality, the processor operating in combination to receive amplifier output signals from the amplifier and control electrical pulse delivery according to programmed instructions; and a power supply hermetically contained within the housing and coupled to the pulse generator, the power supply supplying energy for operations and electrical pulse generation as a source internal to the housing wherein the power supply has a volume less than approximately 1 cubic centimeter and sources sufficient energy to support circuitry contained within the housing.

27. The biostimulator according to Claim 26 wherein: the biostimulator is a leadless cardiac pacemaker; and the amplifier comprises a cardiac sensing amplifier that detects signals associated with a cardiac contraction from at least two of the electrode plurality and sends a contraction output signal to the processor in response to the cardiac contraction.

28. The biostimulator according to Claim 26 wherein: the amplifier comprises a communication amplifier that detects an incoming communication signal from at least one device external to the biostimulator from at least two of the electrode plurality and sends a communication output signal to the processor in response to the communication signal conveying information from the at least one external device.

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29. The biostimulator according to Claim 28 wherein: the conveyed information comprises programmable parameter settings.

30. The biostimulator according to Claim 28 wherein: the communication amplifier amplifies signals in a range of approximately 10 kHz to 100 kHz.

31. A leadless cardiac pacemaker comprising: a housing; a plurality of electrodes formed integrally to the housing or coupled to the housing at a maximum distance of 2 centimeters; a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator generating and delivering electrical pulses to the electrode plurality and causing cardiac contractions, the pulse generator further conveying information to at least one device external to the pacemaker by conductive communication encoded on pacing pulses via the electrode plurality; at least one amplifier hermetically contained within the housing and electrically coupled to the electrode plurality, the at least one amplifier amplifying signals received from the electrode plurality and detecting cardiac contractions, the at least one amplifier further receiving information from the at least one external device; a processor hermetically contained within the housing and communicatively coupled to the pulse generator, the at least one amplifier, and the electrode plurality, the processor receiving amplifier output signals from the amplifier, control communications, and controlling electrical pulse delivery according to programmed instructions; and a power supply hermetically contained within the housing and coupled to the pulse generator, the power supply supplying energy for operations, communication, and electrical pulse generation as a source internal to the housing.

32. The pacemaker according to Claim 31 further comprising: the pulse generator and rate limiter consuming a maximum electrical power of 27 microwatts averaged over one cardiac cycle.

33. The pacemaker according to Claim 31 further comprising: the amplifier consuming a maximum electrical power of 30 microwatts.

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34. The pacemaker according to Claim 31 further comprising: the power supply consuming a maximum electrical power of 2 microwatts and supplying a minimum electrical power of approximately 64 microwatts.

35. The pacemaker according to Claim 31 further comprising: the processor consuming a maximum electrical power of 5 microwatts averaged over one cardiac cycle.

36. A leadless cardiac pacemaker comprising: a housing; a plurality of electrodes formed integrally to the housing or coupled to the housing at a maximum distance of 2 centimeters; a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator generating and delivering electrical pulses to the electrode plurality powered from a source contained entirely within the housing; a logic hermetically contained within the housing and communicatively coupled to the pulse generator and the electrode plurality, the logic controlling electrical pulse delivery and communication with an external device according to logic execution of program instructions; and a battery configured to power the leadless biostimulator and having a volume of less than one cubic centimeter and a minimum lifetime of five years.

37. A leadless biostimulator comprising: a housing; a plurality of electrodes formed integrally to the housing or coupled to the housing at a maximum distance of 2 centimeters; a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator for generating and delivering electrical pulses to the electrode plurality; and a processor hermetically contained within the housing and communicatively coupled to the pulse generator and the electrode plurality, the processor controlling electrical pulse delivery and communicating with at least one device external to the biostimulator by conductive communication signals encoded on biostimulation pulses via the electrode plurality.

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38. The biostimulator according to Claim 37 further comprising: the processor that controls electrical pulse delivery according to at least one programmable parameter, the processor being programmable by conducted communication signals transmitted via the electrode plurality.

39. The biostimulator according to Claim 37 further comprising: the processor that communicates to the at least one device external to the biostimulator by communication signals transmitted via the electrode plurality.

40. The biostimulator according to Claim 37 wherein: the biostimulator is a leadless cardiac pacemaker.

41. A cardiac pacing system comprising: a leadless cardiac pacemaker that implants in electrical contact with a cardiac chamber for leadless pacing and powered by a battery contained within a volume of less than one cubic centimeter that sources sufficient energy to support circuitry that consumes a maximum of 64 microwatts.

42. A method of operating a biostimulator comprising: configuring operational circuits and components in a leadless biostimulator to consume a maximum of 64 total microwatts; and powering the leadless biostimulator from a battery contained within a volume of less than one cubic centimeter that sources sufficient energy to support the operational circuits and components.

43. The method according to Claim 42 further comprising: recharging a tank capacitor in preparation for delivering a biostimulation pulse; and throttling a charge pump circuit for recharging the tank capacitor at constant power from the battery.

44. The method according to Claim 42 further comprising: encoding transmitted telemetry communications in a pacing pulse.

45. A cardiac pacing system comprising: a leadless cardiac pacemaker that implants in electrical contact with a left ventricular cardiac chamber for leadless triggered left-ventricular pacing for cardiac

- 110 - resynchronization therapy (CRT) in response to conducted signals from a pacing pulse generator.

46. The system according to Claim 45 further comprising: the leadless cardiac pacemaker for leadless triggered left-ventricular pacing in response to conducted signals from at least one implanted leadless or electrode-lead pacing pulse generator.

47. The system according to Claim 45 further comprising: at least one implanted leadless or electrode-lead pacing pulse generator that conducts signals to the leadless cardiac pacemaker that trigger left-ventricular pacing.

48. The system according to Claim 45 further comprising: the leadless cardiac pacemaker that operates as a left-ventricular pacemaker for cardiac resynchronization therapy using a cardioverter-defibrillator (CRT-D) or cardiac resynchronization therapy using an otherwise conventional pacemaker (CRT-P), in response to wireless conducted signals from at least one implanted leadless or electrode-lead pacing pulse generator, the wireless conducted signals being conducted pacing and/or cardiac signals.

49. The system according to Claim 45 further comprising: the leadless cardiac pacemaker comprising: a hermetic housing for placement on or attachment to the inside or outside of a cardiac chamber; and at least two electrodes abutting or adjacent to the housing for delivering pacing pulses and receiving triggering signals from the pacing pulse generator.

50. The system according to Claim 49 further comprising: the at least two electrodes that sense electrical activity from cardiac chamber muscle.

51. The system according to Claim 49 further comprising: the at least two electrodes that bidirectionally communicate with at least one other device within or outside the body.

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52. The system according to Claim 49 further comprising: the leadless pacemaker communicating with a non-implanted programmer or at least one implanted pulse generator via the at least two electrodes that are also used for delivering pacing pulses,the electrodes driving communication signals in absence of an antenna or telemetry coil dedicated to communication.

53. The system according to Claim 49 further comprising: the leadless pacemaker communicating to a non-implanted programmer or at least one implanted pulse generator via communication that adds nothing to transmitted power requirements above power requirements for cardiac pacing.

54. The system according to Claim 45 further comprising: the leadless cardiac pacemaker for leadless triggered left-ventricular pacing in response to conducted signals selected from a group consisting of a signal from a separate implanted pulse generator, a signal from at least one electrode-lead of a separate implanted pulse generator, a right-ventricular pacing pulse delivered by an implanted pulse generator, an atrial pacing pulse delivered by an implanted pulse generator, a signal delivered in combination with a cardioversion function, and a signal delivered in combination with a defibrillation function.

55. The system according to Claim 45 further comprising: the leadless cardiac pacemaker operative as a "slave" left- ventricular leadless cardiac pacemaker triggered by an atrial pacing pulse or right-ventricular pacing pulse of the pulse generator operative for right-ventricular and/or atrial pacing.

56. The system according to Claim 45 further comprising: the leadless pacemaker for implantation adjacent to the inside or outside wall of a cardiac chamber and leadless conductive communication with the pacing pulse generator.

57. The system according to Claim 45 further comprising: the leadless pacemaker for left-ventricular pacing triggered by conducted communication from the pulse generator implanted within the body, the left-ventricular pacing triggered by a right-ventricular pacing pulse or atrial pacing pulse delivered by the pulse generator.

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58. The system according to Claim 45 further comprising: at least two electrodes abutting or adjacent to the housing that deliver pacing pulses and are operative as an incoming communication channel for receiving triggering signals from the pacing pulse generator, the triggering information comprising an electrical potential difference resulting from a right-ventricular pacing pulse or an atrial pacing pulse of an implanted pulse generator and electrode-lead system.

59. The system according to Claim 58 further comprising: a controller coupled to the at least two electrodes that examines triggering information validity and, for a valid condition, activating delivery of a pacing pulse following a predetermined delay of zero or more milliseconds.

60. The system according to Claim 58 further comprising: the incoming communication channel that communicates information selected from a group consisting of pacing rate, pulse duration, sensing threshold, delay intervals, refractory time intervals, stimulation pulse amplitudes, and parameters commonly programmed from an external programmer in a pacemaker; and the at least two electrodes further operative as an outgoing communication channel adapted to communicate information selected from a group consisting of programmable parameter settings, pacing and sensing event counts, battery voltage, battery current, information commonly displayed by external programmers used with pacemakers, and echoed information from the incoming channel to confirm correct programming.

61. The system according to Claim 58 further comprising: a controller coupled to the at least two electrodes that examines the potential difference resulting from a pacing pulse, decodes information encoded in the pacing pulse, and evaluates the decoded information for pacing pulse signature validation.

62. The system according to Claim 45 further comprising: a controller coupled to the at least two electrodes that examines output pulse duration from an implanted pulse generator for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activates delivery of a pacing pulse following a predetermined delay of zero or more milliseconds, the predetermined delay being determined from a method in a group consisting of preset at manufacture, programmed via an external programmer, and adaptively monitoring and conforming to duration of a triggering signal.

63. The system according to Claim 45 further comprising: a controller coupled to the at least two electrodes that examines output pulse amplitude, duration, and rate from an implanted pulse generator for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activates delivery of a pacing pulse following a predetermined delay of zero or more milliseconds.

64. The system according to Claim 45 further comprising: a controller coupled to the at least two electrodes that triggers delivery of a left- ventricular pacing pulse from an atrial pacing pulse of an implanted pulse generator for cardiac resynchronization therapy (CRT) after a selected atrioventricular delay of 50 to 300 milliseconds, the controller further varying the atrio-ventricular delay according to time since a last delivered left-ventricular pacing pulse whereby a shorter atrio-ventricular delay is selected for a higher atrial rate.

65. A cardiac pacing system comprising: a leadless cardiac pacemaker that implants in electrical contact with a cardiac chamber and receives and evaluates triggering information from an implanted pulse generator via an electrical signal conducted from an atrial or ventricular pacing pulse delivered by the implanted pulse generator.

66. The system according to Claim 65 further comprising: at least one implanted leadless or electrode-lead pulse generator conducting signals that trigger left-ventricular pacing to the leadless cardiac pacemaker by direct conduction using modulated signals at a frequency in a range from approximately

10 kHz to 100 kHz.

67. The system according to Claim 65 further comprising: the leadless cardiac pacemaker comprising: a hermetic housing for placement on or attachment to the inside or outside of a cardiac chamber; and

- 114 - at least two electrodes abutting or adjacent to the housing for delivering pacing pulses and receiving triggering signals from the pulse generator.

68. The system according to Claim 65 further comprising: at least two electrodes abutting or adjacent to the housing that deliver pacing pulses and operate as an incoming communication channel for receiving triggering signals from the pulse generator, the triggering information comprising an electrical potential difference resulting from a pacing pulse.

69. The system according to Claim 65 further comprising: a controller coupled to the at least two electrodes that examines triggering information validity and, for a valid condition, activates delivery of a pacing pulse following a predetermined delay of zero or more milliseconds.

70. The system according to Claim 65 further comprising: a controller coupled to the at least two electrodes that examines output pulse duration from an implanted pulse generator for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activates delivery of a pacing pulse following a predetermined delay of zero or more milliseconds, the predetermined delay being determined from a method in a group consisting of preset at manufacture, programmed via an external programmer, and adaptively monitoring and conforming to duration of a triggering signal.

71. The system according to Claim 65 further comprising: a controller coupled to the at least two electrodes that examines output pulse amplitude from an implanted pulse generator for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activates delivery of a pacing pulse following a predetermined delay of zero or more milliseconds.

72. The system according to Claim 65 further comprising: a controller coupled to the at least two electrodes that triggers delivery of an atrial pacing pulse in response to sensing of an atrial heartbeat in sinus rhythm and in response to detection of sinus rhythm below a selected rate for atrial demand pacing.

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73. The system according to Claim 65 further comprising: a controller coupled to the at least two electrodes that limits synchronous pacing pulse delivery rate to a selected maximum rate.

74. The system according to Claim 65 further comprising: a plurality of leadless cardiac pacemakers for implantation in electrical contact with at least one cardiac chamber and distributed epicardially, the leadless cardiac pacemaker plurality timed from an initial triggering pulse for generating simultaneous pulses for defibrillation or cardioversion therapy.

75. The system according to Claim 65 further comprising: the leadless cardiac pacemaker that further is retriggered by the implanted pulse generator whereby the leadless cardiac pacemaker generates a pacing pulse after a predetermined time with no received triggering signal and the predetermined time is preset slightly longer than a pacing interval of the implanted pulse generator, the leadless cardiac pacemaker functioning as a synchronized redundant pacemaker.

76. A cardiac pacing system comprising: a leadless cardiac pacemaker for implantation in electrical contact with a cardiac chamber that delivers a pacing pulse and encodes outgoing communication in the pacing pulse whereby a power requirement for the outgoing communication adds nothing to a power requirement for current in delivering the pacing pulse.

77. The system according to Claim 76 wherein: the outgoing communication power requirement does not add to a power requirement for pacing whereby power is used for a dual purpose.

78. The system according to Claim 76 further comprising: the leadless cardiac pacemaker comprising: a hermetic housing for placement on or attachment to the inside or outside of a cardiac chamber; and at least two electrodes abutting or adjacent to the housing that deliver pacing pulses and receive triggering signals from the pulse generator, and bi- directional communication with the at least one implanted and/or non- implanted device.

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79. A method for communicating in an implantable device comprising: generating stimulation pulses at an implanted biostimulator; encoding information onto generated stimulation pulses at the implanted biostimulator by selective alteration of stimulation pulse morphology that is benign to therapeutic effect and energy cost of the stimulation pulse; and conducting the stimulation pulses into body tissue via electrodes that drive communication signals in absence of an antenna or telemetry coil dedicated to communication.

80. The method according to Claim 79 further comprising: generating cardiac pacing pulses at an implanted leadless cardiac pacemaker; encoding information onto generated cardiac pacing pulses at the implanted leadless cardiac pacemaker by selective alteration of pacing pulse morphology that is benign to therapeutic effect and energy cost of the pacing pulse; and conducting the cardiac pacing pulses into body tissue via electrodes that drive communication signals in absence of an antenna or telemetry coil dedicated to communication.

81. The method according to Claim 80 further comprising: encoding information onto generated cardiac pacing pulses at the implanted leadless cardiac pacemaker comprising pacemaker state information, battery voltage, lead impedance, sensed electrocardiogram amplitude, pacemaker current drain, and programmed parameters.

82. The method according to Claim 80 further comprising: encoding information onto generated cardiac pacing pulses at the implanted leadless cardiac pacemaker by selective alteration of pacing pulse morphology that is benign to therapeutic effect and energy cost of the pacing pulse; detecting at the implanted leadless cardiac pacemaker a natural cardiac depolarization; inhibiting cardiac pacing pulse delivery with delay for delivery during a refractory period following the natural cardiac depolarization; and conducting the cardiac pacing pulses into body tissue via electrodes that drive communication signals in absence of an antenna or telemetry coil dedicated to communication.

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83. The method according to Claim 80 further comprising: distinguishing a generated cardiac pacing pulse from a natural cardiac depolarization in an electrocardiogram comprising comparative pattern recognition of a pacing pulse and an R-wave produced during a cardiac cycle.

84. The method according to Claim 79 further comprising: generating stimulation pulses on stimulating electrodes of an implanted biostimulator; and encoding information onto generated stimulation pulses comprising gating the stimulation pulses for selected durations at selected timed sections in the stimulation pulses whereby gating removes current flow through the stimulating electrodes and timing of the gated sections encodes the information.

85. The method according to Claim 79 further comprising: generating stimulation pulses on stimulating electrodes of an implanted biostimulator; and encoding information onto generated stimulation pulses comprising selectively varying timing between consecutive stimulation pulses.

86. The method according to Claim 79 further comprising: generating stimulation pulses on stimulating electrodes of an implanted biostimulator; and encoding information onto a series of generated stimulation pulses comprising selectively varying pacing pulse width.

87. The method according to Claim 79 further comprising: charging a tank capacitor in preparation for stimulation pulse generation; generating stimulation pulses on stimulating electrodes of an implanted biostimulator; and encoding information onto generated stimulation pulses.

88. The method according to Claim 87 further comprising: timing one or more windows between pulse generation; disabling charging of the tank capacitor during the one or more timed windows; and enabling a receive amplifier in the implanted biostimulator while the tank capacitor is disabled whereby operation of a communications amplifier and charging of the tank capacitor are made mutually exclusive.

89. The method according to Claim 79 further comprising: controlling gain setting for a receiving amplifier/filter;

- 118 - invoking a low-gain setting for normal operation and detecting presence of a biostimulation pulse; and invoking a high-gain setting for detecting and decoding information encoded in the detected biostimulation pulse.

90. A biostimulation system comprising: a biostimulator that implants in electrical contact with a biological tissue and receives and evaluates triggering information from an implanted pulse generator via an electrical signal conducted from a stimulation pulse delivered by the implanted pulse generator.

91. A cardiac pacing system comprising: a leadless cardiac pacemaker that implants in electrical contact with a left ventricular cardiac chamber for leadless triggered left- ventricular pacing in response to conducted signals from a pulse generator and powered by a battery contained within a volume of less than one cubic centimeter.

92. A cardiac pacing system comprising: a plurality of leadless cardiac pacemakers that individually implant in electrical contact with a cardiac chamber in combination for multi-chamber cardiac pacing, the leadless cardiac pacemaker plurality individually comprising at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating among the leadless cardiac pacemaker plurality.

93. The system according to Claim 92 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality configured to intercommunicate and to communicate with a non-implanted programmer via the at least two electrodes that are also used for delivering pacing pulses, the electrodes driving communication signals in absence of an antenna or telemetry coil dedicated to communication.

94. The system according to Claim 92 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that intercommunicate with one or more other leadless cardiac pacemakers and optionally a non-

- 119 - implanted programmer via communication that has transmitting communication power requirements essentially met by power consumed in cardiac pacing.

95. A system according to Claim 92 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality comprising: a hermetic housing 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 that bidirectionally communicate with at least one other device within or outside the body.

96. The system according to Claim 92 further comprising: the at least two leadless electrodes communicating bidirectionally among the leadless cardiac pacemaker plurality 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.

97. The system according to Claim 92 further comprising: the at least two leadless electrodes e communicating bidirectionally among the leadless cardiac pacemaker plurality and transmitting 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.

98. The system according to Claim 92 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that deliver a coded pacing pulse with a code assigned according to pacemaker location and 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 responding to the message in a predetermined manner depending on type and location of the event.

99. The system according to Claim 92 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that communicate to at least one pacemaker of the leadless cardiac pacemaker plurality occurrence of a

- 120 - 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.

100. The system according to Claim 92 further comprising: the plurality of leadless cardiac pacemakers for co-implantation in a single patient and multiple-chamber pacing, the bidirectional communication among the leadless cardiac pacemaker plurality communicating 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 decoding the information and responding depending on location of the receiving pacemaker and predetermined system functionality.

101. The system according to Claim 92 further comprising: the leadless cardiac pacemaker plurality comprising an atrial leadless cardiac pacemaker implanted in electrical contact to an atrial cardiac chamber, the atrial leadless cardiac pacemaker that: waits 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; responds to the sensed atrial heartbeat by generating an atrial pacing pulse that signals to at least one pacemaker of the leadless cardiac pacemaker plurality 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; times a predetermined atrial-to-atrial (AA) escape interval; and responds 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.

102. The system according to Claim 101 further comprising: the atrial leadless cardiac pacemaker that times an atrial-to-atrial (AA) escape interval after generating an atrial pacing pulse whether triggered by atrial sensing or delivered in response to timeout of an escape interval.

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103. The system according to Claim 101 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 (AA) 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; in presence of a premature signal, restart a ventricle-to-atrial (VA) escape interval that is shorter than the atrial-to-atrial (AA) 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 the VA escape interval or the AA 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, and starting an AA escape interval and returning to a wait state.

104. The system according to Claim 103 further comprising: the 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.

105. The system according to Claim 92 further comprising: the leadless cardiac pacemaker plurality comprising a right-ventricular leadless cardiac pacemaker implanted in electrical contact to a right-ventricular cardiac chamber, 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 that a right-ventricular heartbeat has occurred and

- 122 - that encodes the right- ventricular pacing pulse with a code signifying a right-ventricular location and a sensed event type; restart a predetermined right ventricular-to-right ventricular (VV) escape interval after delivering a ventricular pacing pulse after either an atrial ventricular (AV) delay, a ventricular to ventricular (VV) delay, or a ventricular sensed event; 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.

106. The system according to Claim 105 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 (VV) escape interval since a most recent right-ventricular heartbeat; determine whether the signal originating from the co-implanted atrial leadless cardiac pacemaker is premature; in presence of a premature signal, wait for an event with no effect on ventricular pacing; 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 the VV escape interval or the AV 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, and starting a ventricular-to-ventricular (VV) escape interval and returning to a wait state.

107. The system according to Claim 106 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

- 123 - in case of failure of a triggered signal from a co-implanted atrial leadless cardiac pacemaker.

108. The system according to Claim 92 further comprising: the leadless cardiac pacemaker plurality comprising a left-ventricular leadless cardiac pacemaker implanted in electrical contact to a left- ventricular cardiac chamber, the left-ventricular leadless cardiac pacemaker that: waits 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 responds 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.

109. The system according to Claim 108 further comprising: the left-ventricular leadless cardiac pacemaker configured to: detect a signal originating from a co-implanted atrial leadless cardiac pacemaker; examine an elapsed amount of the left ventricular escape interval since a most recent left-ventricular heartbeat; determine whether the signal originating from the co-implanted atrial leadless cardiac pacemaker is premature; in presence of a premature signal, wait for an event with no effect on ventricular pacing; in absence of a premature signal, start a left atrium to left ventricular (AV) escape interval that is representative of a typical time from an atrial beat to a left ventricular beat in sinus rhythm; and respond to timeout of the AV escape interval by delivering a left ventricular pacing pulse, causing a left ventricular heartbeat, the left ventricular pacing pulse encoding paced type and left ventricular location of a left ventricular heartbeat event, and starting a ventricular- to-ventricular (VV) escape interval and returning to a wait state.

- 124 -

110. The system according to Claim 92 further comprising: a controller coupled to the at least two electrodes that examines output pulse duration from the at least one pacemaker of the leadless cardiac pacemaker plurality for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activates delivery of a pacing pulse following a predetermined delay of zero or more milliseconds, the predetermined delay being determined from a method in a group consisting of preset at manufacture, programmed via an external programmer, and adaptively monitoring and conforming to duration of a triggering signal.

111. The system according to Claim 92 further comprising: a controller coupled to the at least two electrodes that examines output pulse waveform from the at least one pacemaker of the leadless cardiac pacemaker plurality for usage as a signature for determining triggering information validity and, for a signature arriving within predetermined limits, activates delivery of a pacing pulse following a predetermined delay of zero or more milliseconds.

112. The system according to Claim 92 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality for operation in a particular location and a particular functionality at manufacture and/or at programming by an external programmer.

113. The system according to Claim 92 further comprising: the leadless cardiac pacemaker plurality comprising a right ventricular leadless cardiac pacemaker and a left ventricular leadless cardiac pacemaker that 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.

114. The system according to Claim 92 further comprising: the leadless cardiac pacemaker plurality formed in an arrangement for multi-site pacing that synchronizes depolarization for tachyarrhythmia prevention.

- 125 -

115. The system according to Claim 92 further comprising: the at least two leadless electrodes that 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.

116. The system according to Claim 92 further comprising: the at least two leadless electrodes that 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 binary-coded notches in a pacing pulse.

117. The system according to Claim 92 further comprising: a receiving amplifier/filter controlled by multiple controllable gain settings; and a processor that controls 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.

118. The system according to Claim 92 further comprising: the at least two leadless electrodes that 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 modulation of off-time between pacing pulses.

119. The system according to Claim 92 further comprising: a tank capacitor coupled across a pair of the at least two leadless electrodes that charge and discharge for generating a pacing pulse ; a charge pump circuit coupled to the tank capacitor that controls charging of the tank capacitor; and a processor that controls 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.

120. A method for multi-chamber cardiac pacing comprising: configuring a plurality of leadless cardiac pacemakers for implantation;

- 126 - configuring an atrial leadless cardiac pacemaker of the leadless cardiac pacemaker plurality for implantation in electrical contact to an atrial cardiac chamber; waiting 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 atrial-to-atrial (AA) escape interval; responding to the sensed atrial heartbeat by generating an atrial pacing pulse that signals to at least one pacemaker of the leadless cardiac pacemaker plurality 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 length AA escape interval; and responding to timeout of the AA escape interval by delivering an atrial pacing pulse, causing an atrial heartbeat, the atrial pacing pulse identifying paced type and/or atrial location of an atrial heartbeat event.

121. The method according to Claim 120 further comprising: encoding an atrial pacing pulse that identifies synchronous pacing triggered by an atrial sensed event with a first code; and encoding an atrial pacing pulse that identifies atrial pacing following the AA escape interval with a second code distinct from the first code.

122. The method according to Claim 120 further comprising: delivering the atrial 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.

123. The method according to Claim 120 further comprising: upon delivery of an atrial pacing pulse, timing an atrial-to-atrial (AA) escape interval.

124. The method according to Claim 120 further comprising: detecting at the atrial leadless cardiac pacemaker a signal originating from a co-implanted ventricular leadless cardiac pacemaker; examining at the atrial leadless cardiac pacemaker an elapsed amount of the atrial-to- atrial (AA) escape interval since a most recent atrial heartbeat;

- 127 - determining 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; in presence of a premature signal, restarting a ventricle-to-atrium (VA) escape interval that is shorter than the atrial-to-atrial (AA) escape interval and representative of a typical time from a ventricular beat to a next atrial beat in sinus rhythm; and responding to timeout of the VA escape interval or the AA 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, and restarting an atrial- to-atrial (AA) escape interval.

125. A method for multi-chamber cardiac pacing comprising: configuring a plurality of leadless cardiac pacemakers for implantation; configuring a right-ventricular leadless cardiac pacemaker of the leadless cardiac pacemaker plurality for implantation in electrical contact to a right-ventricular cardiac chamber; waiting 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; responding 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 that a right-ventricular heartbeat has occurred and that encodes the right- ventricular pacing pulse with a code signifying a right- ventricular location and a sensed event type, restarting a predetermined ventricular-to-ventricular (VV) escape interval after ventricular pacing, and entering a wait state; and responding 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.

126. The method according to Claim 125 further comprising: encoding a right-ventricular pacing pulse that identifies synchronous pacing triggered by a right-ventricular sensed event with a first code; and

- 128 - encoding a right-ventricular pacing pulse that identifies right-ventricular pacing following a ventricular-to-ventricular (VV) escape interval with a second code distinct from the first code.

127. The method according to Claim 125 further comprising: upon delivery of a right-ventricular pacing pulse, timing a ventricular-to-ventricular (VV) escape interval.

128. The method according to Claim 125 further comprising: detecting at the right-ventricular leadless cardiac pacemaker a signal originating from a co-implanted atrial leadless cardiac pacemaker; examining at the right-ventricular leadless cardiac pacemaker an elapsed amount of the ventricular-to-ventricular (VV) escape interval since a most recent right- ventricular heartbeat; determining whether the signal originating from the co-implanted atrial leadless cardiac pacemaker is premature; in presence of a premature signal, waiting for an event with no effect on ventricular pacing; for 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; and responding to timeout of the AV escape interval or the VV 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.

129. A method for multi-chamber cardiac pacing comprising: configuring a plurality of leadless cardiac pacemakers for implantation; configuring a left-ventricular leadless cardiac pacemaker of the leadless cardiac pacemaker plurality for implantation in electrical contact to a left-ventricular cardiac chamber and for operation in cardiac resynchronization therapy (CRT-P); waiting 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

- 129 - responding 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.

130. The method according to Claim 129 further comprising: detecting a signal originating from a co-implanted atrial leadless cardiac pacemaker; examining an elapsed amount of the left ventricular escape interval since a most recent left-ventricular heartbeat; determining whether the signal originating from the co-implanted atrial leadless cardiac pacemaker is premature; in presence of a premature signal, waiting for an event with no effect on ventricular pacing; in absence of a premature signal, starting a left atrium to left ventricular (AV) escape interval that is representative of a typical time from an atrial beat to a left ventricular beat in sinus rhythm; and responding to timeout of the left ventricular escape interval or the AV escape interval by delivering a left ventricular pacing pulse, causing a left ventricular heartbeat, the left ventricular pacing pulse encoding paced type and left ventricular location of a left ventricular heartbeat event, and starting a ventricular-to-ventricular (VV) escape interval and returning to a wait state.

131. The method according to Claim 129 further comprising: configuring the left-ventricular leadless cardiac pacemaker for operation in cardiac resynchronization therapy (CRT-P).

132. A cardiac pacing system comprising: at least one leadless cardiac pacemaker that implants in electrical contact with a cardiac chamber and performs 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 for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicating with the co-implanted ICD.

- 130 -

133. The system according to Claim 132 further comprising: the at least one leadless cardiac pacemaker that intercommunicates and/or to communicates with a non-implanted programmer and/or the implanted ICD via the at least two electrodes that are also used for delivering pacing pulses.

134. The system according to Claim 132 further comprising: the at least one leadless cardiac pacemaker driving communication signals in absence of an antenna or telemetry coil dedicated to communication.

135. The system according to Claim 132 further comprising: the at least one leadless cardiac pacemaker that intercommunicates and/or to communicates with a non-implanted programmer via communication that has transmitted communication power requirements essentially met by power consumed in cardiac pacing.

136. A system according to Claim 132 further comprising: the at least one leadless cardiac pacemaker comprising: a hermetic housing 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 that perform bidirectional communication with at least one other device within or outside the body.

137. The system according to Claim 132 further comprising: the at least one leadless cardiac pacemaker that communicates 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.

138. The system according to Claim 132 further comprising: the at least one leadless cardiac pacemaker that communicates bidirectionally among the leadless cardiac pacemaker plurality and/or the implanted ICD to facilitate sensitivity and specificity of the ICD.

- 131 -

139. The system according to Claim 132 further comprising: the at least one leadless cardiac pacemaker that communicates 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.

140. The system according to Claim 132 further comprising: the at least two leadless electrodes that 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 issuing a unique code corresponding to an event type and a location of the individual pacemaker.

141. The system according to Claim 132 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that deliver a coded pacing pulse with a code assigned according to pacemaker location and 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.

142. The system according to Claim 132 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that deliver a coded pacing pulse with a code assigned according to pacemaker location and 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 that 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.

143. The system according to Claim 132 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that communicate to at least one pacemaker of the leadless cardiac pacemaker plurality occurrence of a

- 132 - 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.

144. The system according to Claim 132 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that receive conducted communication from a co-implanted cardioverter-defibrillator (ICD) that directs the individual pacemakers to deliver overdrive anti-tachycardia pacing in response to a detected tachyarrhythmia.

145. The system according to Claim 132 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality that operate in a particular location and a particular functionality at manufacture and/or at programming by an external programmer.

146. The system according to Claim 132 further comprising: the leadless cardiac pacemaker plurality comprising a right ventricular leadless cardiac pacemaker and a left ventricular leadless cardiac pacemaker that 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.

147. The system according to Claim 146 further comprising: the right-ventricular leadless cardiac pacemaker that: 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.

148. The system according to Claim 132 further comprising: the at least two leadless electrodes that 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.

- 133 -

149. The system according to Claim 132 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.

150. The system according to Claim 132 further comprising: an implanted cardioverter-defibrillator (ICD) comprising a case and pair of electrodes mounted on or near the case, that receives and transmits conducted communication using a pulse modulated or frequency modulated carrier signal whereby the ICD detects communication pulses from co-implanted leadless cardiac pacemakers and transmits programming information to the co-implanted leadless cardiac pacemakers.

151. The system according to Claim 132 further comprising: an implanted cardioverter-defibrillator (ICD) that receives conducted communication using two implantable electrodes.

152. The system according to Claim 132 further comprising: individual pacemakers of the leadless cardiac pacemaker plurality comprising: a receiving amplifier/filter adapted for multiple controllable gain settings; and a processor that controls 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.

153. The system according to Claim 132 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 that charges and discharges wherein a pacing pulse is generated; a charge pump circuit coupled to the tank capacitor that controls charging of the tank capacitor; and a processor that controls recharging of the tank capacitor wherein recharging is discontinued when a battery terminal voltage falls below a predetermined

- 134 - value to ensure sufficient voltage for powering the leadless cardiac pacemaker.

154. A cardiac pacing system comprising: an implantable cardioverter-defibrillator (ICD); and at least one leadless cardiac pacemaker 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 that leadlessly intercommunicate by information conduction through body tissue.

155. The system according to Claim 154 further comprising: the at least one leadless cardiac pacemaker comprising at least two leadless electrodes that deliver cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and bidirectionally communicate with the co-implanted ICD.

156. The system according to Claim 154 further comprising: the at least one leadless cardiac pacemaker that intercommunicates and/or to communicates with a non-implanted programmer via communication that has transmitted communication power requirements essentially met by power consumed in cardiac pacing.

157. The system according to Claim 154 further comprising: the at least one leadless cardiac pacemaker that intercommunicates and/or to communicates with a non-implanted programmer via communication that has negligible transmission power requirements in addition to power consumed in cardiac pacing.

158. 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 electrodes

- 135 - driving communication signals in absence of an antenna or telemetry coil dedicated to communication.

159. The system according to Claim 158 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.

160. The system according to Claim 158 further comprising: the at least one leadless cardiac pacemaker that intercommunicates and/or communicates with a non-implanted programmer via communication that has transmitted communication power requirements essentially met by power consumed in cardiac pacing.

161. The system according to Claim 158 further comprising: the at least one leadless cardiac pacemaker that intercommunicates and/or communicates with a non-implanted programmer via communication that has negligible transmission power requirements in addition to power consumed in cardiac pacing.

162. A cardiac pacing system comprising: an implantable cardioverter-defibrillator (ICD); and at least one leadless cardiac pacemaker that implants in electrical contact with a cardiac chamber and performs 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 that communicates a code that signifies occurrence of sensed cardiac electrical signals and/or delivered pacing pulses and identifies an event type and/or location.

163. The system according to Claim 162 further comprising: at least two electrodes coupled to the ICD that transmit and/or receive conducted communication using a pulse-modulated or frequency-modulated carrier signal, the ICD c that detects communication pulses from at least one co-implanted

- 136 - leadless cardiac pacemaker and transmits programming information to the at least one co-implanted leadless cardiac pacemaker.

164. The system according to Claim 162 further comprising: the at least one leadless cardiac pacemaker that broadcasts information to the co- implanted ICD and/or at least one pacemaker of a plurality of leadless cardiac pacemakers.

165. The system according to Claim 162 further comprising: the at least one leadless cardiac pacemaker that receives the code and reacts based on the code, location of the receiving leadless cardiac pacemaker, and predetermined system functionality.

166. The system according to Claim 162 further comprising: the at least one leadless cardiac pacemaker that performs 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.

167. The system according to Claim 162 further comprising: the plurality of leadless cardiac pacemakers for co-implantation in a single patient and for 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.

168. The system according to Claim 162 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

- 137 - 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.

169. The system according to Claim 168 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.

170. The system according to Claim 162 further comprising: the leadless cardiac pacemaker plurality comprising a right-ventricular leadless cardiac pacemaker implanted in electrical contact to a right-ventricular cardiac chamber

- 138 - 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.

171. The system according to Claim 170 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.

172. The system according to Claim 162 further comprising: the leadless cardiac pacemaker plurality comprising a left-ventricular leadless cardiac pacemaker implanted in electrical contact to a left- ventricular cardiac chamber

- 139 - 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.

173. The system according to Claim 162 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.

174. A cardiac pacing system comprising: at least one leadless cardiac pacemaker that implants in electrical contact with a cardiac chamber and performs cardiac pacing functions in combination with a co- implanted implantable cardioverter-defibrillator (ICD).

- 140 -

175. A cardiac pacing system comprising: at least one leadless cardiac pacemaker that implants in electrical contact with a cardiac chamber and performs 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 for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and transmitting information to the co-implanted ICD.

176. A cardiac pacing system comprising: at least one leadless cardiac pacemaker that implants in electrical contact with a cardiac chamber and performs 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 for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and receiving information from the co-implanted ICD.

177. 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.

178. The method according to Claim 177 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;

- 141 - 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.

179. The method according to Claim 178 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.

180. The method according to Claim 177 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

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

181. The method according to Claim 180 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.

182. The method according to Claim 177 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.

183. The method according to Claim 177 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;

- 143 - 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.

184. A pacemaker comprising: the pacemaker configured as a leadless cardiac pacemaker comprising: a housing; a plurality of electrodes coupled on, within, or within two centimeters of the housing; a pulse delivery system hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse delivery system sourcing energy internal to the housing, and generating and delivering electrical pulses to the electrode plurality; an activity sensor hermetically contained within the housing that senses activity; and a processor hermetically contained within the housing and communicatively coupled to the pulse delivery system, the activity sensor, and the electrode plurality, the processor in combination controlling electrical pulse delivery at least partly based on the sensed activity.

185. (Canceled).

186. The pacemaker according to Claim 184 further comprising: the processor that controls electrical pulse delivery and application of the activity sensor according to at least one programmable parameter, the processor being programmable by communication signals transmitted via the electrode plurality.

187. The pacemaker according to Claim 184 further comprising: the activity sensor adapted for controlling rate-responsive pacing and selected from a group consisting of an accelerometer, a temperature sensor, and a pressure sensor.

- 144 -

188. The pacemaker according to Claim 184 further comprising: the activity sensor comprising an accelerometer and accelerometer amplifier that detect patient activity for rate-responsive pacing.

189. The pacemaker according to Claim 188 further comprising: the processor that samples signals from the accelerometer synchronously with a cardiac cycle determined by the pulse delivery system, compares acceleration signals acquired at relative times in multiple cardiac cycles, and distinguishes acceleration signals resulting from activity from signals resulting from cardiac wall motion.

190. The pacemaker according to Claim 184 further comprising: the activity sensor comprising a temperature transducer, the temperature transducer comprising a thermistor and a signal conditioning amplifier connected to the processor.

191. The pacemaker according to Claim 184 further comprising: the activity sensor comprising a pressure transducer and a signal conditioning amplifier connected to the processor.

192. The pacemaker according to Claim 184 further comprising: the activity sensor that operates with a power requirement of no more than 10 microwatts.

193. The pacemaker according to Claim 184 further comprising: the pulse delivery system that generates and delivers electrical energy with the stimulation pulse interrupted by at least one notch that conveys information to a device external to the pacemaker, the conveyed information comprising data selected from a group consisting of programmable parameter settings, event counts, power-supply voltage, power-supply current, rate -response control parameters adapted for converting an activity sensor signal to a rate-responsive pacing parameter.

194. The pacemaker according to Claim 184 further comprising: the pulse delivery system that generates and delivers electrical energy with the stimulation pulse that conveys information to a device external to the pacemaker

- 145 - in designated codes encoding the information in pacing pulse width, the conveyed information comprising data selected from a group consisting of programmable parameter settings, event counts, power-supply voltage, power-supply current, rate -response control parameters adapted for converting an activity sensor signal to a rate -responsive pacing parameter.

195. The pacemaker according to Claim 184 further comprising: the pulse delivery system that generates and delivers electrical energy with the stimulation pulse that conveys information to a device external to the pacemaker in designated codes encoding the information in modulation of off- time between pacing pulses, the conveyed information comprising data selected from a group consisting of programmable parameter settings, event counts, power-supply voltage, power-supply current, rate-response control parameters adapted for converting an activity sensor signal to a rate-responsive pacing parameter.

196. The pacemaker according to Claim 184 further comprising: a receiving amplifier/filter with multiple controllable gain settings; and the processor that controls gain setting for the receiving amplifier/filter, invokes a low- gain setting for normal operation and detecting presence of a cardiac pacing pulse, and invokes a high-gain setting for detecting and decoding information encoded in the detected cardiac pacing pulse.

197. The pacemaker according to Claim 184 further comprising: a tank capacitor coupled across a pair of the electrode plurality that charges and discharges for generating a pacing pulse ; a charge pump circuit coupled to the tank capacitor that controls charging of the tank capacitor; and the processor that controls 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.

198. A pacemaker comprising: the pacemaker configured as a leadless cardiac pacemaker comprising: a housing; a plurality of electrodes coupled to the housing;

- 146 - a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator generating and delivering electrical cardiac stimulation pulses to the electrode plurality; at least one amplifier hermetically contained within the housing and electrically coupled to the electrode plurality, the at least one amplifier amplifying signals received from the electrode plurality and amplifying signals for cardiac sensing; a power supply hermetically contained within the housing and coupled to the pulse generator, the power supply sourcing energy for the electrical pulses from internal to the housing; an activity sensor hermetically contained within the housing that senses activity; and a processor hermetically contained within the housing and communicatively coupled to the pulse generator, the amplifier, the activity sensor, and the electrode plurality, the processor receiving amplifier output signals from the amplifier and control electrical pulse delivery at least partly based on the sensed activity.

199. The pacemaker according to Claim 198 further comprising: the plurality of electrodes coupled on, within, or within two centimeters of the housing.

200. The pacemaker according to Claim 198 further comprising: the processor that controls electrical pulse delivery and application of the activity sensor according to at least one programmable parameter, the processor being programmable by communication signals transmitted via the electrode plurality.

201. The pacemaker according to Claim 198 further comprising: the activity sensor adapted for controlling rate-responsive pacing and selected from a group consisting of an accelerometer, a temperature sensor, and a pressure sensor.

202. The pacemaker according to Claim 198 further comprising: the activity sensor comprising an accelerometer and accelerometer amplifier that detect patient activity for rate-responsive pacing.

- 147 -

203. The pacemaker according to Claim 202 further comprising: the processor that samples signals from the accelerometer synchronously with a cardiac cycle determined by the pulse delivery system, compares acceleration signals acquired at relative times in multiple cardiac cycles, and distinguishes acceleration signals resulting from activity from signals resulting from cardiac wall motion.

204. The pacemaker according to Claim 198 further comprising: the activity sensor comprising a temperature transducer, the temperature transducer comprising a thermistor and a signal conditioning amplifier connected to the processor.

205. The pacemaker according to Claim 198 further comprising: the activity sensor comprising a pressure transducer and a signal conditioning amplifier connected to the processor.

206. The pacemaker according to Claim 198 further comprising: the activity sensor that operates with a power requirement of no more than approximately

10 microwatts.

207. A pacemaker comprising: the pacemaker operative as a leadless cardiac pacemaker comprising: a housing; a plurality of electrodes coupled to the housing; a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator generating and delivering electrical cardiac stimulation pulses to the electrode plurality and causing cardiac contractions, the pulse generator further conveying information to at least one device external to the pacemaker; at least one amplifier hermetically contained within the housing and electrically coupled to the electrode plurality, the at least one amplifier amplifying signals received from the electrode plurality and to detect cardiac contractions, the at least one amplifier further receiving information from the at least one external device;

- 148 - a power supply hermetically contained within the housing and coupled to the pulse generator, the power supply sourcing energy for the electrical pulses from internal to the housing; an activity sensor hermetically contained within the housing and adapted to sense activity; and a processor hermetically contained within the housing and communicatively coupled to the pulse generator, the at least one amplifier, the activity sensor, and the electrode plurality, the processor receiving amplifier output signals from the amplifier and controlling electrical pulse delivery at least partly based on the sensed activity.

208. The pacemaker according to Claim 207 further comprising: the plurality of electrodes coupled on, within, or within two centimeters of the housing.

209. The pacemaker according to Claim 207 further comprising: the activity sensor adapted for controlling rate-responsive pacing and selected from a group consisting of an accelerometer, a temperature sensor, and a pressure sensor.

210. The pacemaker according to Claim 207 further comprising: the at least one amplifier comprising a cardiac sensing amplifier that consumes no more than 5 microwatts, a communications amplifier that consumes no more than 25 microwatts, and a rate-response sensor amplifier that consumes no more than 10 microwatts.

211. The pacemaker according to Claim 207 further comprising: the power supply that consumes electrical power of no more than 2 microwatts and supplies electrical power of no more than 74 microwatts.

212. The pacemaker according to Claim 207 further comprising: the processor that consumes electrical power of no more than 5 microwatts averaged over one cardiac cycle.

213. A pacemaker comprising: the pacemaker operative as a leadless cardiac pacemaker comprising: a housing; a plurality of electrodes coupled to the housing;

- 149 - a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator generating and delivering electrical pulses to the electrode plurality powered from a source contained entirely within the housing; an activity sensor hermetically contained within the housing and adapted to sense activity; and a logic hermetically contained within the housing and communicatively coupled to the pulse generator, the activity sensor, and the electrode plurality, the logic controlling electrical pulse delivery at least partly based on the sensed activity.

214. The pacemaker according to Claim 213 further comprising: the plurality of electrodes coupled on, within, or within two centimeters of the housing.

215. A pacemaker comprising: the pacemaker operative as a leadless cardiac pacemaker comprising: a housing; a plurality of electrodes coupled to the housing; a pulse generator hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse generator for generating and delivering electrical pulses to the electrode plurality; an activity sensor hermetically contained within the housing and adapted to sense activity; and a processor hermetically contained within the housing and communicatively coupled to the pulse generator, the activity sensor, and the electrode plurality, the processor controlling electrical pulse delivery at least partly based on the sensed activity and communicating with at least one device external to the pacemaker via the electrode plurality.

216. The pacemaker according to Claim 215 further comprising: the plurality of electrodes coupled on, within, or within two centimeters of the housing.

217. The pacemaker according to Claim 215 further comprising: the processor that controls electrical pulse delivery and application of the activity sensor according to at least one programmable parameter, the processor being programmable by communication signals transmitted via the electrode plurality.

- 150 -

218. The pacemaker according to Claim 215 further comprising: the processor that communicates to the at least one device external to the pacemaker by communication signals transmitted via the electrode plurality.

219. A method for operating a pacemaker comprising: sensing electrical signals conducted through a body from a co-implanted cardioverter- defibrillator (ICD); decoding and storing information encoded in the electrical signals; sensing an activity sensor signal within the pacemaker; converting the activity sensor signal to a rate -responsive pacing parameter as a function of the stored information; and controlling pacing pulse delivery as a function of the rate-responsive pacing parameter.

220. The method according to Claim 219 further comprising: encoding information in the electrical signals comprising: generating and delivering electrical energy with a stimulation pulse interrupted by at least one notch that conveys information.

221. The method according to Claim 219 further comprising: encoding information in the electrical signals comprising: generating and delivering electrical energy with a stimulation pulse that conveys information in designated codes encoding the information in pacing pulse width.

222. The method according to Claim 219 further comprising: encoding information in the electrical signals comprising: generating and delivering electrical energy with a stimulation pulse that conveys information in designated codes encoding the information in modulation of off-time between pacing pulses.

223. A method for operating a pacemaker comprising: implanting a leadless cardiac pacemaker to cardiac muscle, the leadless cardiac pacemaker further comprising a rate-response sensor; measuring an activity signal using the rate-response sensor, the activity signal including an artifact signal resulting from cardiac muscle motion;

- 151 - sampling the activity signal synchronously with a cardiac cycle; monitoring the activity signal at identified points in the cardiac cycle; and removing the artifact signal from the activity signal based on the monitoring.

224. The method according to Claim 223 further comprising: measuring the activity signal using an accelerator.

225. The method according to Claim 223 further comprising: measuring the activity signal using a thermistor or pressure sensor.

226. A delivery system configured for implanting a biostimulation device comprising: a stylet extending along an axis from knob end to a threaded end that engages an internally threaded nut of the biostimulation device; and a catheter tube configured to axially contain the stylet, the catheter tube comprising a feature that engages a corresponding feature on the biostimulation device whereby the stylet can be rotated relative to the catheter tube for disengagement of the stylet threaded end from the biostimulation device threaded end.

227. The system according to Claim 226 further comprising: a sliding sheath assembly that axially slides over the stylet and engaged biostimulation device, the sliding sheath assembly comprising a feature that engages a corresponding feature on the biostimulation device whereby the stylet can be rotated relative to the sliding sheath for disengagement of the stylet threaded end from the biostimulation device threaded end.

228. The system according to Claim 227 further comprising: the sliding sheath that protects patient tissue from damage during insertion of the biostimulation device.

229. The system according to Claim 227 further comprising: the sliding sheath further comprising a feature that engages and rotates the biostimulation device and affixes a fixation member coupled to the biostimulation device into patient tissue.

- 152 -

230. The system according to Claim 227 further comprising: the sliding sheath assembly engaging feature configured to enable rotational counter traction for stylet disengagement.

231. The system according to Claim 226 further comprising: the sliding sheath assembly that axially retracts from the stylet a predetermined distance whereby electrodes of the biostimulation device are exposed, enabling threshold testing while the biostimulation device remains engaged to the stylet.

232. The system according to Claim 226 further comprising: the catheter tube comprising a sheath, the sheath comprising a sheath slot that aligns with an alignment pin of the biostimulation device that prevents rotation of the biostimulation device with respect to the sheath; and the stylet extending along the axis from a knob coupled to a proximal end to a screw coupled to the distal end, the stylet configured whereby for the knob fully retracted the biostimulation device is fully contained within the sheath and a fixation member coupled to the biostimulation device is protected, and whereby for the knob fully depressed the alignment pin is contained within the sheath and electrodes of the biostimulation device are fully exposed, enabling threshold testing before disengagement.

233. The system according to Claim 226 further comprising: a catheter tube assembly extending from a knob end to a socket end and enclosing an internal lumen configured to axially slide over the stylet and engage the biostimulation device, the catheter tube assembly comprising a socket at the socket end that engages a corresponding nut on the biostimulation device whereby the stylet can be rotated relative to the catheter tube assembly for disengagement of the stylet threaded end from the biostimulation device threaded end.

234. The system according to Claim 233 further comprising: the stylet that screws into a multiple-sided nut attached to the biostimulation device; and the socket configured as a locking hex socket that engages the multiple-sided nut on the biostimulation device whereby the stylet and socket are configured to enable counter-rotation of the stylet relative to the biostimulation device.

- 153 -

235. The system according to Claim 233 wherein: the catheter tube assembly is a sheath-less catheter; and the biostimulation device is a leadless cardiac pacemaker and the delivery system is configured to deploy the leadless cardiac pacemaker with active or passive fixation using the sheath-less catheter.

236. The system according to Claim 233 further comprising: the sheath-less catheter socket comprising the socket configured to enable rotational counter traction for stylet disengagement.

237. The system according to Claim 236 further comprising: a biocompatible soluble protective covering that covers a fixation member coupled to the biostimulation device during insertion of the biostimulation device into a patient's body whereby the patient's body tissue is protected.

238. The system according to Claim 237 further comprising: the biocompatible soluble protective covering comprising mannitol, polyvinylpyrrolidone, or a protective salt.

239. The system according to Claim 233 further comprising: the biocompatible soluble protective covering comprising a material that forms a protective capsule at room temperature, dissolves when implanted, and has no toxic side effects.

240. The system according to Claim 233 further comprising: the biocompatible soluble protective covering that dissolves in a selected time whereby a fixation device coupled to the biostimulation device is exposed, the fixation device being advanced by rotating the catheter tube assembly and the stylet until the biostimulation device is anchored.

241. The system according to Claim 233 further comprising: the catheter tube assembly circumferentially enclosing an inner lumen and the stylet extending through the lumen, the catheter tube assembly comprising a catheter knob at a proximal end and a multiple-sided nut socket at a distal end of the catheter tube assembly, the multiple-sided nut socket configured to receive

- 154 - external features of a multiple-sided nut coupled to a biostimulation device, the multiple-sided nut socket and biostimulation device multiple-sided nut configured to prevent counter- rotation of the biostimulation device and the catheter tube assembly when engaged; and the stylet comprising a stylet knob at a proximal end and a threaded screw at a distal end of the stylet, the threaded screw configured for engaging the internally threaded nut of the biostimulation device.

242. The system according to Claim 241 further comprising: the catheter tube assembly extruded from polyurethane, polytetrafluoroethylene, polyolefm, polyvinyl chloride or silicone and having a diameter smaller than or equal to the biostimulation device; the catheter knob constructed from rigid plastic or metal; the stylet constructed from stainless steel; the stylet knob constructed from rigid plastic or metal.

243. The system according to Claim 241 further comprising: the biostimulation device is a leadless cardiac pacemaker.

244. The system according to Claim 226 further comprising: the stylet and the catheter tube comprise the delivery system with only two concentric members forming a catheter.

245. The system according to Claim 226 further comprising: the stylet and the catheter tube configured for implanting the biostimulation device adapted for active or passive fixation to patient tissue.

246. The system according to Claim 226 further comprising: a radio-opaque marker adapted for coupling to the stylet, the catheter tube, and/or the biostimulation device for identification under fluoroscopy and facilitation of positioning.

247. A method for implanting a biostimulation device in patient body tissue comprising: screwing a threaded end of a stylet into an internally threaded nut of the biostimulation device;

- 155 - positioning the catheter tube whereby the biostimulation device is located adjacent a selected patient body tissue; engaging a feature of the catheter tube against a corresponding feature on the biostimulation device whereby rotation of the catheter tube rotates the biostimulation device relative to the patient body tissue; rotating the catheter tube whereby fixation device on the biostimulation device affixes to the patient body tissue; and counter-rotating the stylet relative to the catheter tube whereby the threaded end of a stylet is unscrewed from the internally threaded nut of the biostimulation device.

248. The method according to Claim 247 further comprising: prior to positioning the catheter tube, axially sliding a catheter tube over the stylet and the biostimulation device whereby the stylet and biostimulation device are internally contained within the catheter tube.

249. The method according to Claim 248 further comprising: prior to locating the biostimulation device adjacent the selected patient body tissue, partially withdrawing the catheter tube whereby the patient body tissue is exposed to the fixation device on the biostimulation device for affixation.

250. The method according to Claim 248 further comprising: prior to unscrewing the stylet from the biostimulation device, partially withdrawing the catheter tube whereby the patient body tissue is exposed to electrodes of the biostimulation device; and measuring signal amplitudes and pacing thresholds of the biostimulation device.

251. The method according to Claim 247 further comprising: withdrawing the stylet from the catheter tube; and withdrawing the catheter tube from the patient's body.

252. The method according to Claim 247 wherein: the biostimulation device is a leadless cardiac pacemaker; and the patient body tissue is cardiac tissue.

- 156 -

253. The method according to Claim 247 further comprising: axially sliding a sheath over the stylet and a leadless cardiac pacemaker whereby the stylet and leadless cardiac pacemaker with active fixation member are internally contained within the sheath; inserting the sheath and stylet combination intravenously either in a cephalic, subclavian, or femoral vein of a patient; moving the sheath and stylet combination progressively towards patient cardiac tissue until a distal end of the sheath reaches a selected site of the cardiac tissue; at the selected site gently advancing a stylet knob into the sheath to expose the active fixation member coupled to the leadless cardiac pacemaker; rotating the sheath whereby the leadless cardiac pacemaker is rotated and the active fixation member is attached to the cardiac tissue; upon sufficient attachment of the active fixation member retracting the sheath relative to the stylet knob whereby electrodes of the leadless cardiac pacemaker are exposed; holding the sheath while rotating the stylet knob whereby the stylet is unscrewed and disengaged from the leadless cardiac pacemaker; retracting the sheath and stylet combination from the patient's cephalic, subclavian, or femoral vein.

254. The method according to Claim 253 further comprising: after exposure of leadless cardiac pacemaker leads, testing implanted leadless cardiac pacemaker pacing thresholds and signal amplitudes; and positioning the leadless cardiac pacemaker until the pacing thresholds and signal amplitudes meet selected criteria.

255. The method according to Claim 247 further comprising: axially sliding a sheath over the stylet and a leadless cardiac pacemaker whereby the stylet and leadless cardiac pacemaker with passive fixation member are internally contained within the sheath; inserting the sheath and stylet in combination to a selected location of a patient's body tissue; at the selected site gently retracting the sheath relative to the stylet knob position to expose the leadless cardiac pacemaker and the electrodes; rotating stylet knob while holding the sheath stationary whereby the stylet is disengaged from the leadless cardiac pacemaker; and

- 157 - retracting the sheath and stylet combination from the patient.

256. The method according to Claim 255 further comprising: after exposure of leadless cardiac pacemaker leads, testing implanted leadless cardiac pacemaker pacing thresholds and signal amplitudes; and positioning the leadless cardiac pacemaker until the pacing thresholds and signal amplitudes meet selected criteria.

257. The method according to Claim 255 further comprising: inserting and positioning a guide wire under fluoroscopy to a selected location in a patient's coronary sinus using a coronary sinus introducer system; advancing a dilator and introducer over the guide wire to the selected location; withdrawing the guide wire followed by the dilator whereby the introducer remains positioned at the selected location; advancing the leadless cardiac pacemaker into the coronary sinus for positioning at the selected location and leadless cardiac pacemaker electrodes are exposed; testing the leadless cardiac pacemaker at the selected location; optionally repositioning and testing the leadless cardiac pacemaker to a predetermined test result; and retracting the introducer from the patient.

258. A delivery apparatus for implanting a leadless cardiac pacemaker comprising: a bi-concentric-axial element catheter configured for engaging to and disengaging from the leadless cardiac pacemaker via relative motion of an internal stylet element to an externally circumferential tube element; and the tube element comprising a feature that engages a corresponding feature on the leadless cardiac pacemaker whereby the stylet element can be rotated relative to the tube element for disengagement of a stylet threaded end from a leadless cardiac pacemaker threaded end.

259. (Canceled).

260. The apparatus according to Claim 258 further comprising: the tube element comprising a sliding sheath that axially slides over the stylet and engaged leadless cardiac pacemaker and protects patient tissue from damage during insertion of the leadless cardiac pacemaker, the sliding sheath comprising

- 158 - a feature that engages a corresponding feature on the leadless cardiac pacemaker whereby the stylet can be rotated relative to the sliding sheath for disengagement of the stylet threaded end from the leadless cardiac pacemaker threaded end.

261. The apparatus according to Claim 258 further comprising: the tube element comprising a sheath-less catheter extending axially from a knob end to a socket end and enclosing an internal lumen configured to axially slide over the stylet and engage the leadless cardiac pacemaker, the sheath-less catheter comprising a socket at the socket end that engages a corresponding nut on the leadless cardiac pacemaker whereby the stylet can be rotated relative to the sheath-less catheter for disengagement of the stylet threaded end from the leadless cardiac pacemaker threaded end; and a biocompatible soluble protective covering configured to cover a fixation member coupled to the leadless cardiac pacemaker during insertion of the biostimulation device into a patient's body whereby the patient's body tissue is protected.

262. An external programmer that communicates with an implanted biostimulator system comprising: an interface that couples to at least two electrodes that make electrical contact with body skin for communicating with at least one implanted biostimulator; and bidirectional communication pathways coupled to the interface configured for bidirectional communication with the at least one implanted biostimulator and comprising a receiving pathway that decodes information encoded on stimulation pulses generated by the at least one implanted biostimulator and conducted through body tissue.

263. The programmer according to Claim 262 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the programmer to the at least one implanted biostimulator by conduction through the body tissue by modulation that avoids skeletal muscle stimulation.

264. The programmer according to Claim 262 further comprising: a processor coupled to the bidirectional communication pathways that manages communication with at least one leadless cardiac pacemaker.

- 159 -

265. The programmer according to Claim 262 further comprising: a processor coupled to the bidirectional communication pathways that manages communication with at least one leadless cardiac pacemaker implanted adjacent to an inside or an outside wall of a cardiac chamber.

266. The programmer according to Claim 262 further comprising: a processor coupled to the bidirectional communication pathways that performs at least one operation of operations comprising electrocardiogram sensing, retrieving status information from implanted pacemakers, and modifying configuration parameters of multiple implanted pacemakers within a single or multiple cardiac cycles in information passed through a common electrode set; and a display interface coupled to the processor that displays an electrocardiogram sensed from the at least two electrodes.

267. The programmer according to Claim 262 further comprising: a processor coupled to the bidirectional communication pathways that performs at least one operation of operations comprising electrocardiogram sensing, retrieving status information from implanted pacemakers, and modifying configuration parameters of multiple implanted pacemakers within a single or multiple cardiac cycles in information passed through a common electrode set; and a secondary link coupled to the processor that communicates by wireless or cable transmission to a remote display device to display an electrocardiogram sensed from the at least two electrodes.

268. The programmer according to Claim 262 further comprising: the bidirectional communication pathways for communication with a plurality of leadless cardiac pacemakers via the at least two electrodes and conduction through body tissue.

269. The programmer according to Claim 262 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the programmer to the at least one implanted biostimulator by direct conduction using modulated signals at a frequency in a range from approximately 10 kHz to 100 kHz.

- 160 -

270. The programmer according to Claim 262 further comprising: the at least two electrodes and the bidirectional communication pathways operating in combination for bidirectional information signal communication and for sensing an electrocardiogram, the bidirectional communication pathways comprising a receiving pathway including a low-pass filter adapted to separate the electrocardiogram from the information signals.

271. The programmer according to Claim 262 further comprising: the at least two electrodes and the bidirectional communication pathways operating in combination for bidirectional information signal communication and for sensing an electrocardiogram, the bidirectional communication pathways comprising a receiving pathway including a low-pass filter adapted to separate the electrocardiogram from the information signals and a blanking controller configured to blank noise and unwanted signals from the electrocardiogram when the communication channel is active.

272. The programmer according to Claim 262 wherein: the at least two electrodes comprise more than two electrodes enabling an electrocardiogram (ECG) to be sensed at multiple vectors and selection from among the multiple vectors for conducted communication with the at least one leadless cardiac pacemaker whereby system signal-to-noise ratio can be maximized.

273. The programmer according to Claim 262 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the programmer to a plurality of implanted devices in a common communication event.

274. The programmer according to Claim 262 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the programmer to a plurality of implanted devices in a common communication event whereby information specific to a single implanted device or a subset of implanted devices have a unique address assigned to the single implanted device or the subset of implanted devices and encoded in the information.

- 161 -

275. The programmer according to Claim 262 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the programmer to a plurality of implanted devices in a common communication event whereby information designates a specific function that is executed by a particular implanted device or a particular subset of implanted devices, the information being passed to the implanted device plurality without individual address information for activating execution by the particular implanted device or the particular subset of implanted devices alone.

276. The programmer according to Claim 262 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the programmer to a plurality of implanted devices in a common communication event whereby information designates a specific function that is executed by a particular implanted device or a particular subset of implanted devices that comprise programming specific to the function adapted to recognize the received information is relevant to the function.

277. The programmer according to Claim 262 further comprising: the at least two electrodes forming a conductive communication path between the programmer and a skin surface; and the receiving pathway comprising: an electrocardiogram (ECG) amplifier/filter comprising a differential band-pass amplifier that selects and amplifies signals in a frequency range from approximately 1 Hz to approximately 100 Hz; an analog-to-digital converter (ADC) that digitizes the filtered and amplified signal; and a processor coupled to the ADC that receives and optionally displays ECG data, and decodes information encoded into cardiac pacing pulses.

278. The programmer according to Claim 262 further comprising: the at least two electrodes forming a conductive communication path between the programmer and a skin surface; and the bidirectional communication pathways further comprising a transmitting pathway comprising:

- 162 - a processor that communicates information to at least one implanted leadless cardiac pacemaker; a command/message encoder coupled to the processor via a parallel interface that encodes and serializes data into a bit stream, the encoding being selected from a group of encoding techniques consisting of on-off keying, frequency-shift keying, frequency modulation, and amplitude shift keying; a modulator coupled to the command/message encoder that receives and modulates the serialized data using a frequency in a range from approximately 10 kHz to approximately 100 kHz; and an amplifier coupled to the modulator that increases signal amplitude to a level suitable for robust conducted communication.

279. The programmer according to Claim 262 further comprising: the receiving pathway that decodes information in a group consisting of encoded using pacing pulse width, encoded using binary-coded notches in a pacing pulse, and encoded using modulation of off-time between pacing pulses.

280. A biostimulator system comprising: at least one implantable device; and an external programmer that communicates with the at least one implantable device via bidirectional communication pathways comprising a receiving pathway that decodes information encoded on stimulation pulses generated by ones of the at least one implantable device and conducted through body tissue to the external programmer.

281. The system according to Claim 280 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the external programmer to ones of the at least one implantable device by direct conduction through the body tissue by modulation that avoids skeletal muscle stimulation using modulated signals at a frequency in a range from approximately 10 kHz to 100 kHz.

282. The system according to Claim 280 further comprising: the bidirectional communication pathways further comprising a transmitting pathway that passes information from the programmer to the at least one implantable device in

- 163 - a common communication event whereby information is sent to at least one target device of the at least one implantable device using a technique selected from a group consisting of: information specific to a single implantable device or a subset of implantable devices having a unique address assigned to the single implantable device or the subset of implantable devices and encoded in the information; information designating a specific function that is executed by a particular implantable device or a particular subset of implantable devices, the information being passed to the at least one implantable device without individual address information for activating execution by the particular implantable device or the particular subset of implantable devices alone; and information designates a specific function that is executed by a particular implantable device or a particular subset of implantable devices that comprise programming specific to the function adapted to recognize the received information is relevant to the function.

283. The system according to Claim 280 further comprising: the bidirectional communication pathways further comprising a receiving pathway including a low-pass filter adapted to separate the electrocardiogram from the information signals.

284. The system according to Claim 280 further comprising: the bidirectional communication pathways further comprising a receiving pathway including a low-pass filter adapted to separate the electrocardiogram from the information signals and a blanking controller configured to blank noise and unwanted signals from the electrocardiogram when the communication channel is active.

285. The system according to Claim 280 further comprising: the external programmer further configured to perform at least one operation of operations comprising electrocardiogram sensing, retrieving status information from implanted pacemakers, modifying configuration parameters of multiple implanted pacemakers simultaneously in information passed through a common electrode set, displaying electrocardiograms, and displaying information received from the at least one implantable device.

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286. The system according to Claim 280 further comprising: the at least one implantable device comprising at least one leadless cardiac pacemaker.

287. The system according to Claim 280 further comprising: the at least one implantable device comprising at least one leadless cardiac pacemaker implanted adjacent to an inside or an outside wall of a cardiac chamber.

288. The system according to Claim 280 wherein: the bidirectional communication pathways are configured for communication with a plurality of leadless cardiac pacemakers via at least two electrodes and conduction through body tissue.

289. The system according to Claim 280 further comprising: the at least one implantable device comprising at least one leadless cardiac pacemaker that generates cardiac pacing pulses, encodes information onto the generated cardiac pacing pulses by selective alteration of pacing pulse morphology that is benign to therapeutic effect and energy cost of the pacing pulse, and conducts the cardiac pacing pulses into body tissue via electrodes for driving communication signals in absence of an antenna or telemetry coil dedicated to communication.

290. The system according to Claim 289 further comprising: the at least one leadless cardiac pacemaker configured to detect a natural cardiac depolarization, time a selected delay interval, and deliver an information-encoded pulse during a refractory period following the natural cardiac depolarization.

291. The system according to Claim 289 wherein: the encoded information selected from a group consisting of pacemaker state information, battery voltage, lead impedance, sensed cardiac signal amplitude, pacemaker current drain, and programmed parameters.

292. The system according to Claim 289 further comprising: the at least one leadless cardiac pacemaker configured to charge a tank capacitor in preparation for stimulation pulse generation, time one or more windows between pulse generation, disable charging of the tank capacitor during the one or more

- 165 - timed windows, and enable a receive amplifier in the implanted biostimulator while the tank capacitor is disabled.

293. The system according to Claim 292 further comprising: the external programmer further configured to detect a stimulation pulse from the at least one leadless cardiac pacemaker and transmit data after a selected delay to coincide with a window that the at least one leadless cardiac pacemaker's receive amplifier is enabled.

294. A method for communicating in an implantable biostimulator system comprising: monitoring, at an external programmer, electrical signals conducted through body tissue to body surface electrodes; detecting pulses generated by a body-implanted biostimulator; and decoding information encoded into the generated pulse by the body-implanted biostimulator.

295. The method according to Claim 294 further comprising: generating cardiac pacing pulses at an implanted leadless cardiac pacemaker; encoding information onto generated cardiac pacing pulses at the implanted leadless cardiac pacemaker by selective alteration of pacing pulse morphology that is benign to therapeutic effect and energy cost of the pacing pulse; and conducting the cardiac pacing pulses into body tissue via electrodes for driving communication signals in absence of an antenna or telemetry coil dedicated to communication.

296. The method according to Claim 294 further comprising: generating cardiac pacing pulses at an implanted leadless cardiac pacemaker; and encoding information onto generated cardiac pacing pulses at the implanted leadless cardiac pacemaker comprising at least one of pacemaker state information, battery voltage, lead impedance, sensed electrocardiogram amplitude, pacemaker current drain, and programmed parameters.

297. The method according to Claim 294 further comprising: generating cardiac pacing pulses at an implanted leadless cardiac pacemaker;

- 166 - encoding information onto generated cardiac pacing pulses at the implanted leadless cardiac pacemaker by selective alteration of pacing pulse morphology that is benign to therapeutic effect and energy cost of the pacing pulse; detecting at the implanted leadless cardiac pacemaker a natural cardiac depolarization; inhibiting cardiac pacing pulse delivery with delay for delivery during a refractory period following the natural cardiac depolarization; and conducting the cardiac pacing pulses into body tissue via electrodes for driving communication signals in absence of an antenna or telemetry coil dedicated to communication.

298. The method according to Claim 294 further comprising: generating stimulation pulses on stimulating electrodes of an implanted biostimulator; and encoding information onto generated stimulation pulses comprising gating the stimulation pulses for selected durations at selected timed sections in the stimulation pulses whereby gating removes current flow through the stimulating electrodes and timing of the gated sections encodes the information.

299. The method according to Claim 294 further comprising: generating stimulation pulses on stimulating electrodes of an implanted biostimulator; and encoding information in pulse width of generated stimulation pulses.

300. The method according to Claim 294 further comprising: generating stimulation pulses on stimulating electrodes of an implanted biostimulator; and encoding information onto generated stimulation pulses comprising selectively varying timing between consecutive stimulation pulses.

301. The method according to Claim 294 further comprising: distinguishing a generated cardiac pacing pulse from a natural cardiac depolarization in an electrocardiogram comprising comparative pattern recognition of a pacing pulse and an R-wave produced during a cardiac cycle.

302. The method according to Claim 294 further comprising: charging a tank capacitor in preparation for stimulation pulse generation; generating stimulation pulses on stimulating electrodes of an implanted biostimulator; encoding information onto generated stimulation pulses; timing one or more windows between pulse generations;

- 167 - disabling charging of the tank capacitor during the one or more timed windows; and enabling a receive amplifier in the implanted biostimulator while the tank capacitor is disabled.

303. The method according to Claim 302 further comprising: sensing, at the external programmer, stimulation pulses generated by the implanted biostimulator; transmitting information from the external programmer to the implanted biostimulator to coincide with the one or more timed windows.

304. The method according to Claim 302 further comprising: detecting at the external programmer a stimulation pulse from the implanted biostimulator; timing a selected delay; and transmitting data after the selected delay to coincide with a window that the implanted biostimulator' s receive amplifier is enabled.

305. The method according to Claim 294 further comprising: physically connecting the external programmer to a body surface via at least two body surface electrodes; communicating information among the external programmer and at least two implanted leadless cardiac pacemakers; transmitting encoded information from the external programmer to the implanted leadless cardiac pacemakers via the at least two body surface electrodes using a modulated signal at a frequency in a range of approximately 10 kHz to approximately 100 kHz; receiving information at the external programmer via the at least two body surface electrodes from ones of the at least two implanted leadless cardiac pacemakers comprising detecting information encoded into generated pacing pulses; and receiving surface electrocardiogram at the external programmer via the at least two body surface electrodes for display and analysis.

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306. The method according to Claim 294 further comprising: encoding and/or decoding information in a group consisting of encoded using pacing pulse width, encoded using binary-coded notches in a pacing pulse, and encoded using modulation of off-time between pacing pulses.

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