WO2018229716A1

WO2018229716A1 - Secure intra-cardiac pacemakers

Info

Publication number: WO2018229716A1
Application number: PCT/IB2018/054404
Authority: WO
Inventors: Yoram Palti
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-06-16
Filing date: 2018-06-15
Publication date: 2018-12-20
Anticipated expiration: 2019-12-16

Abstract

A plurality of intracardiac pacemakers are synchronized together in order to achieve multi- chamber pacing despite the fact that there is no physical connection between the devices. One of the intracardiac pacemakers acts as a master and the remaining intracardiac pacemakers act as slaves to that master. Synchronization of the pacing pulses generated by each of these intracardiac pacemakers is accomplished using ultrasound signals.

Description

SECURE INTRA-CARDIAC PACEMAKERS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the benefit of US Provisional Application 62/520,853 filed June 16, 2017, which is incorporated herein by reference in its entirety.

BACKGROUND

[0002] Multi-chamber pacing is a conventional solution for treating cardiac mechanical dyssynchrony, in order to obtain more efficient cardiac performance. It is currently in wide use with subcutaneously implanted pacemakers equipped with multiple leads that serve to stimulate different locations in the various cardiac ventricles and atria. This requires maintaining a synchronized and accurate timing of the various electrical stimuli that are applied at each location. This synchronization is typically achieved by a controller or processor in the pacemaker, which controls the timing of the pulses that are applied to each of the device's leads.

[0003] FIG. 1 is a simplified block diagram of the Medtronic Micra Pacemaker, which is a prior art leadless Intra-Cardiac Pacemaker (ICP) that is implanted directly into the right ventricle. This prior art ICP 100 includes anode and cathode terminals 11, and cardiac activity monitoring circuitry 12 that monitors cardiac activity by measuring millivolt level electrical signals that the heart imposes on to the anode and cathode. The output of the cardiac activity monitoring circuit 12 is provided to a controller 15. Based on the cardiac activity reported by the cardiac activity monitoring circuit 12, the controller 15 decides when a pacing pulse should be generated. The controller 15 instructs the pulse generator circuit 13 to generate a pacing pulse at the desired time with a desired amplitude and a desired pulse shape, and the pulse generator circuit 13 responds to those instructions and generates the requested pulse. The controller 15 can output data (e.g., to report the measured cardiac activity and any pacing events) to an external device via the RF communication circuit 16. And the operation of the pacemaker 100 can be controlled (e.g., to update pacing parameters or update firmware) by the external device via the RF communication circuit 16. A battery 14 provides power for components 11-16 in the pacemaker 100.

[0004] Although this type of leadless ICP provides significant benefits, it also has a number of disadvantages. First, it relies on RF for communication and control functions, which renders it susceptible to unauthorized manipulation/hacking of the device's performance and vulnerable to the capture of associated confidential medical data. Second, electromagnetic detectors (e.g., metal detectors, such as those used in airports) may also disrupt its operation. Third, unlike subcutaneously implanted pacemakers with multiple leads (which can provide appropriately synchronized electric stimulation at a plurality of cardiac locations), any given ICP can only apply a pacing pulse to a single location in the heart. As a result, conventional ICPs are not well-suited for implementing multi-chamber pacing.

SUMMARY OF THE INVENTION

[0005] One aspect of the invention is directed to a first system for implementing multi-chamber cardiac pacing. This first system comprises a first leadless intracardiac pacing module configured for implantation into one chamber of a subject's heart at a first position, and a second leadless intracardiac pacing module configured for implantation into an other chamber of the subject's heart at a second position.

[0006] The first pacing module includes a first anode, a first cathode, a first cardiac monitoring circuit configured to monitor cardiac activity based on signals arriving from at least one of the first anode and the first cathode and generate first data indicative of the monitored cardiac activity, and a first pulse generating circuit configured to generate a first pacing pulse between the first anode and the first cathode in response to receipt of a first command. The first pacing module also includes a first controller programmed to (a) determine, based on the first data, a first time at which the first pulse generating circuit should generate the first pacing pulse, (b) generate the first command so that the first pulse generating circuit will generate the first pacing pulse at the first time, (c) determine, based on the first data, a second time at which the second pacing module should generate a second pacing pulse, and (d) output control data that specifies the second time. The first pacing module also includes a first ultrasound communication module configured to accept the control data from the first controller and transmit the control data to the second pacing module using an ultrasound carrier signal.

[0007] The second pacing module includes a second anode, a second cathode, a second pulse generating circuit configured to generate a second pacing pulse between the second anode and the second cathode in response to receipt of a second command, a second ultrasound communication module configured to extract the control data from the ultrasound carrier signal, and a second controller programmed to input the extracted control data and, in response to the extracted control data, generate the second command so that the second pulse generating circuit will generate the second pacing pulse at the second time.

[0008] In some embodiments of the first system, the second pacing module further includes a second cardiac monitoring circuit configured to monitor cardiac activity based on signals arriving from at least one of the second anode and the second cathode and generate second data indicative of the monitored cardiac activity. In these embodiments, the second controller is programmed to ignore the extracted control data if the second controller determines, based on the second data, that generating the second command could pose a danger to the subject.

[0009] In some embodiments of the first system, the second pacing module further includes an ultrasound wake-up circuit that generates a wake-up signal in response to receipt of ultrasound energy. The wake-up signal is provided to the second ultrasound

communication module. In these embodiments, the second ultrasound communication module is configured to default to a powered-down state and to power up in response to the wake-up signal, the first ultrasound communication module is configured to generate a second ultrasound signal that is capable of activating the ultrasound wake-up circuit, and the first controller is further programmed to instruct the first ultrasound communication module to generate the second ultrasound signal prior to outputting the control data.

[0010] Some embodiments of the first system further comprise a third leadless intracardiac pacing module configured for implantation into the subject's heart at a third position. In these embodiments, the third pacing module includes a third anode, a third cathode, a third pulse generating circuit configured to generate a third pacing pulse between the third anode and the third cathode in response to receipt of a third command, a third ultrasound communication module configured to extract the control data from the ultrasound carrier signal, and a third controller programmed to input the extracted control data and, in response to the extracted control data, generate the third command so that the third pulse generating circuit will generate the third pacing pulse at a third time. The first controller is further programmed to (e) determine, based on the first data, a third time at which the third pacing module should generate a third pacing pulse, and (f) output control data that specifies the third time. [0011] Some embodiments of the first system further comprise an auxiliary apparatus that includes a fourth controller configured to generate commands for controlling the first pacing module and the second pacing module, and a fourth ultrasound communication module configured to transmit the commands generated by the fourth controller to the first pacing module and the second pacing module using an ultrasound carrier signal.

[0012] In some embodiments of the first system, the first ultrasound communication module is configured to receive ultrasound signals arriving from outside the subject's body, extract first incoming control data from the arriving ultrasound signals, and provide the first incoming control data to the first controller. In these embodiments, the first controller is programmed to respond to the first incoming control data. In some of these embodiments, the second ultrasound communication module is configured to receive ultrasound signals arriving from outside the subject's body, extract second incoming control data from the arriving ultrasound signals, and provide the second incoming control data to the second controller; and the second controller is programmed to respond to the second incoming control data.

[0013] In some embodiments of the first system, the first pacing module further includes a first power source that provides power to the first controller and the first pulse generating circuit; and the second pacing module further includes a second power source that provides power to the second controller and the second pulse generating circuit. In some of these embodiments, the first power source comprises a first battery and the second power source comprises a second battery.

[0014] Another aspect of the invention is directed to a first leadless intracardiac pacing module that comprises a housing configured for implantation into one chamber of a subject's heart, an anode supported by the housing, a cathode supported by the housing, a cardiac monitoring circuit supported by the housing and configured to monitor cardiac activity based on signals arriving from at least one of the anode and the cathode and generate first data indicative of the monitored cardiac activity, and a pulse generating circuit supported by the housing and configured to generate a first pacing pulse between the anode and the cathode in response to receipt of a first command. The first leadless intracardiac pacing module also comprises a controller supported by the housing, wherein the controller is programmed to (a) determine, based on the first data, a first time at which the pulse generating circuit should generate the first pacing pulse, (b) generate the first command so that the pulse generating circuit will generate the first pacing pulse at the first time, (c) determine, based on the first data, a second time at which a second pacing module that has been implanted into an other chamber of the heart should generate a second pacing pulse, and (d) output control data that specifies the second time. The first leadless intracardiac pacing module also comprises an ultrasound communication module supported by the housing and configured to accept the control data output by the controller and to transmit the control data to the other chamber of the subject's heart using an ultrasound carrier signal.

[0015] In some embodiments of the first leadless intracardiac pacing module, the ultrasound communication module is configured to receive ultrasound signals arriving from outside the subject's body, extract incoming control data from the arriving ultrasound signals, and provide the incoming control data to the controller. In these embodiments, the controller is programmed to respond to the incoming control data.

[0016] Some embodiments of the first leadless intracardiac pacing module further comprise a battery that provides power to the controller and the pulse generating circuit.

[0017] Another aspect of the invention is directed to a second leadless intracardiac pacing module intended for use in cooperation with a remote master that generates a first pacing pulse at a first time. The second leadless intracardiac pacing module comprises a housing configured for implantation into one chamber of a subject's heart; an anode supported by the housing; a cathode supported by the housing; a pulse generating circuit supported by the housing and configured to generate a second pacing pulse between the anode and the cathode in response to receipt of a command; an ultrasound communication module supported by the housing and configured to extract control data from an incoming ultrasound carrier signal, wherein the control data specifies a second time at which the second pacing pulse should be generated; and a controller supported by the housing, wherein the controller is programmed to input the extracted control data and, in response to the extracted control data, generate the command so that the pulse generating circuit will generate the second pacing pulse at the second time.

[0018] Some embodiments of the second leadless intracardiac pacing module further comprise a cardiac monitoring circuit configured to monitor cardiac activity based on signals arriving from at least one of the anode and the cathode and generate second data indicative of the monitored cardiac activity. In these embodiments, the controller is programmed to ignore the extracted control data if the controller determines, based on the second data, that generating the command could pose a danger to the subject.

[0019] Some embodiments of the second leadless intracardiac pacing module further comprise an ultrasound wake-up circuit that generates a wake-up signal in response to receipt of ultrasound energy. The wake-up signal is provided to the ultrasound communication module. In these embodiments, the ultrasound communication module is configured to default to a powered-down state and to power up in response to the wake-up signal.

[0020] In some embodiments of the second leadless intracardiac pacing module, the ultrasound communication module is configured to receive ultrasound signals arriving from outside the subject's body, extract incoming control data from the arriving ultrasound signals, and provide the incoming control data to the controller; and the controller is programmed to respond to the incoming control data.

[0021] Some embodiments of the second leadless intracardiac pacing module further comprise a battery that provides power to the controller and the pulse generating circuit.

[0022] Another aspect of the invention is directed to a second system for

synchronizing a plurality of implantable modules. The second system comprises a first module configured for implantation into a subject's body at a first position; and a second module configured for implantation into the subject's body at a second position.

[0023] In these embodiments, the first module includes a first monitoring circuit configured to monitor a time-varying parameter in the subject's body and generate first data indicative of the monitored parameter, a first circuit configured to generate a first output in response to receipt of a first command, and a first controller programmed to (a) determine, based on the first data, a first time at which the first circuit should generate the first output, (b) generate the first command so that the first circuit will generate the first output at the first time, (c) determine, based on the first data, a second time at which the second module should generate a second output, and (d) output control data that specifies the second time. The first module also includes a first ultrasound communication module configured to accept the control data from the first controller and transmit the control data to the second module using an ultrasound carrier signal. [0024] In these embodiments, the second module includes a second circuit configured to generate a second output in response to receipt of a second command, a second ultrasound communication module configured to extract the control data from the ultrasound carrier signal, and a second controller programmed to input the extracted control data and, in response to the extracted control data, generate the second command so that the second circuit will generate the second output at the second time.

[0025] In some embodiments of the second system, the second module further includes an ultrasound wake-up circuit that generates a wake-up signal in response to receipt of ultrasound energy, and the wake-up signal is provided to the second ultrasound communication module. In these embodiments, the second ultrasound communication module is configured to default to a powered-down state and to power up in response to the wake-up signal, the first ultrasound communication module is configured to generate a second ultrasound signal that is capable of activating the ultrasound wake-up circuit, and the first controller is further programmed to instruct the first ultrasound communication module to generate the second ultrasound signal prior to outputting the control data.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a simplified block diagram of a prior art leadless implantable pacemaker.

[0027] FIG. 2 depicts a system in which two implantable pacing modules are implanted into different chambers of a subject's heart.

[0028] FIG. 3 is a block diagram that depicts additional details for implementing the implanted devices in the FIG. 2 embodiment.

[0029] FIG. 4. is a block diagram of one embodiment of the ultrasound

communication module depicted in FIG. 3.

[0030] FIG. 5 is a schematic diagram of a circuit that may be used to implement the ultrasound wake-up circuit depicted in FIG. 3.

[0031] FIG. 6 is a flowchart of the processes that are implemented in the master device depicted in FIG. 3. [0032] FIG. 7 is a flowchart of the processes that are implemented in the slave device depicted in FIG. 3.

[0033] FIG. 8 depicts an external unit that is capable of communicating with the implanted devices shown in FIG. 3.

[0034] FIGS. 9A and 9B are flowcharts of two processes that may be used by an external unit to communicate with the implanted devices shown in FIG. 3.

[0035] Various embodiments are described in detail below with reference to the accompanying drawings, wherein like reference numerals represent like elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] FIG. 2 depicts a system in which a first implantable pacing module 20M

(which acts as master) and a second implantable pacing module 20S (which acts as a slave) are implanted into different chambers of a subject's heart. This system overcomes the disadvantages of ICPs described above by (a) using ultrasound wave communication between the implanted devices 20M and 20S and devices that are external to the subject's body in place of the conventional RF communication approach; and (b) using ultrasound wave communication between the multiple ICPs 20M and 20S that are implanted into a single body to implement and/or maintain synchronization between those ICPs, so that they can generate appropriate signals in synchrony with one another.

[0037] When ultrasound communication is used to communicate with the implanted devices 20M and 20S from outside the subject's body, external access and control of the relevant device and data transfer can only be achieved when direct contact with the implant wearer's body is made. More specifically, the acoustic impedance of the human body for ultrasound at frequencies above 100-150 kHz is very different from that of the ambient air. This impedance mismatch prevents the penetration of ultrasound waves at those frequencies from the ambient air into the body. Thus, if a system is designed such that communication with the implanted device can only be achieved via ultrasound at those frequencies, direct contact of a transducer with the subject's body must be achieved in order to communicate with the implanted device. This requirement for body contact protects the implants from manipulation, hacking, and electromagnetic detectors. [0038] Communication via ultrasound may be accomplished, for example, using piezoelectric ceramic transducers driven by ultrasound audio frequency circuits. This provides a number of advantages. First, it provides the ability to control ICP devices from outside the body without the risk of unauthorized control, hacking, or accidental undesired effects. Second, it provides the ability for a plurality of individual implanted device to communicate from device to device. Such communication may be used, for example, for coordination and data exchange between units, and for synchronization. This inter-device communication is possible because no air gaps exist between devices that are all implanted within the same body.

[0039] In the case of pacemakers, it is important to synchronize stimulation of the different cardiac elements, for example: right ventricle, right atrium, and left ventricle, to ensure optimal cardiac performance. This requires the ability to provide the various appropriate stimuli at proper times. Moreover, the timing of the various stimuli may need readjustment from time to time as the physiological and biophysical properties of the cardiac tissues change (e.g., due to cardiac physiological changes or changes in clinical needs), or the required functionality changes. Ultrasound signals are also used to implement inter-device communication and synchronization between all of the implanted devices 20M and 20S.

[0040] FIG. 3 is a block diagram that depicts additional details of one preferred embodiment for implementing the implanted devices 20M and 20S shown in FIG. 2. In this embodiment, the first implanted device 20M is implanted into a first chamber of the subject heart (e.g., the right ventricle) and acts as a master, and the second implanted device 20S is implanted into a second chamber of the subject's heart (e.g., the right atrium) and acts as a slave. Each of these implanted devices 20M and 20S is an Intra-Cardiac Pacemaker (ICP) that relies on Ultrasound Communication.

[0041] The master device 20M uses hardware that is similar to the conventional ICP

100 depicted in FIG. 1, but replaces the RF communication circuitry 16 of the conventional ICPs with ultrasound communication module 26M. Operation of the components 11-14 within the master device 20M is the same as in the conventional ICP 100, and the master controller 25M interacts with those components 11-14 in the same way that the controller 15 interacts with components 11-14 in the conventional ICP 100. [0042] The operation of the master controller 25M differs from the operation of the conventional controller 15 in two ways: first, communication with everything outside the master device 20M is implemented via the ultrasound communication module 26M instead of via RF; and second, the master controller 25M is programmed to control the operation of slave device 20S by issuing commands that are transmitted to the slave via ultrasound communications. The nature of these commands is described below.

[0043] The ultrasound communication module 26M may be used to implement oneway or two-way communications, with capabilities for both intra-body communication (e.g., for communicating with the slave device 20S) and communication with an external ultrasound unit 200 described below in connection with FIG. 8 (at such times when the external unit 200 is in contact with the subject's body).

[0044] In some embodiments, the ultrasound communication module 26M is implemented using the architecture depicted in FIG. 4. In these embodiments, incoming communication and outgoing communication are handled by separate hardware.

[0045] Incoming communications are handled by a first ultrasound transducer 52 and an ultrasound receiver 50. The first ultrasound transducer 52 (e.g., a piezoelectric element) is positioned with respect to the housing of the master device 20M so that incoming ultrasound signals traveling through the subject's body will arrive at the first ultrasound transducer 52. The first ultrasound transducer 52 generates a first electrical output signal in response to these first incoming ultrasound signals. The first ultrasound frequency receiver 50 generates, based on the first electrical output signal, first data corresponding to commands that have been encoded onto the first incoming ultrasound signal. The ultrasound receiver 50 includes whatever components are necessary to extract the first data that is encoded in the first electrical output signal that it receives. Examples include amplification, signal shaping, demodulation, analog to digital conversion, and other functions that will be apparent to persons skilled in the relevant arts. The output of the ultrasound receiver is provided to the controller 25M.

[0046] Outgoing communications are handled by an ultrasound transmitter 60 and a second ultrasound transducer 62. Data from the controller 25M arrives at the ultrasound transmitter 60, and the ultrasound transmitter 60 modulates that data onto an ultrasound- frequency electrical carrier signal. This electrical signal is provided to the ultrasound transducer 62 (e.g., a piezoelectric element), which converts the electrical signal into an ultrasound wave. The second ultrasound transducer 62 is positioned with respect to the housing of the master device 20M so that outgoing ultrasound signals will be coupled into the subject's body.

[0047] A wide variety of alternative architectures for implementing the ultrasound communication module 26M will be apparent to persons skilled in the relevant art. In one such variation, a single ultrasound transducer (not shown) can be used in place of the two ultrasound transducer 52, 62 if a conventional transmit/receive switch is added. In another variation, a single ultrasound transceiver (not shown) may be used in place of the separate ultrasound receiver 50 and ultrasound transmitter 60 depicted in FIG. 4. When piezoelectric elements are used to implement the ultrasound transducers 52, 62, a suitable size for those elements is between 0.3 and 3 mm in diameter.

[0048] Returning to FIGS. 2 and 3, these figures depict an embodiment that uses a plurality of ICPs 20M and 20S that are synchronized together in order to achieve multi- chamber pacing despite the fact that there is no physical connection between the devices 20M and 20S that are implanted within the subject's body. In this example, a first device 20M is implanted in the right ventricle, and a second device 20S is implanted in the right atrium. Optionally, additional devices (not shown) may be implanted at other locations in the heart (e.g., the left ventricle). Synchronization between all the implanted devices 20M and 20S is accomplished using ultrasound communication. This is advantageous because ultrasound communication is resistant to hacking and environmental interference, as explained above. In addition, intrabody ultrasound communication is more energy efficient than intrabody RF communication. As a result, particularly in situations when there is a need for frequent synchronization, systems that rely on ultrasound synchronization can provide significant advantages.

[0049] The slave device 20S uses hardware that is similar to the master device 20M described above. The operation of the components 11-14 within the slave device 20S is similar to that in the master device 20M, and the slave controller 25 S interacts with those components 11-14. The ultrasound communication module 26S may be implemented using any of the approaches described above in connection with the ultrasound communication module 25M. [0050] The operation of the slave controller 25 S is similar to the operation of the master controller 25M in that it receives input from the cardiac activity monitor 12, controls the pulse generator circuit 13 by issuing commands, and communicates with the ultrasound communication module 26S. The slave controller 25 S can also communicate with devices that are outside the subject's body via ultrasound communication (carried out using the ultrasound communication module 26S). The main distinction is that while the master controller 25M is programmed to issue commands that will ultimately be carried out by the slave device 20S, the slave controller 25 S is programmed to respond to incoming commands that arrive from the master controller 25M.

[0051] Another distinction is that the inclusion of the cardiac activity monitoring circuit 12 in the slave device 20S is optional. When the cardiac activity monitoring circuit 12 is omitted from the slave device 20S, the slave controller 25 S should be programmed to only initiate pulses when a request for a pulse arrives from the master device 20M, and never initiate pulses under other circumstances.

[0052] A comparison of the hardware of the master device 20M and the slave device

20S in FIG. 3 reveals that those two devices share many common elements. In some embodiments of the system, the hardware that is used to implement the master device 20M and the slave device 20S is different, and actually includes the distinctions shown in FIG. 3. In alternative embodiments, two copies of a single hardware device that includes all the functionality of both the master device 20M and the slave device 20S can serve as both those devices, as long as one copy is configured in software or firmware to act as a master and the other copy is configured in software or firmware to act as the slave.

[0053] Taken together, the master device 20M (which is a first leadless intracardiac pacing module configured for implantation into one chamber of a subject's heart at a first position) and the slave device 20M (which is a second leadless intracardiac pacing module configured for implantation into an other chamber of the subject's heart at a second position) form a system for implementing multi-chamber cardiac pacing.

[0054] The master device 20M includes a first anode and a first cathode 11, and a first cardiac monitoring circuit 12 configured to monitor cardiac activity based on signals arriving from at least one of the first anode and the first cathode 11 and generate first data indicative of the monitored cardiac activity. A first pulse generating circuit 13 is configured to generate a first pacing pulse between the first anode and the first cathode 11 in response to receipt of a first command. This command could be applied in a variety of ways that will be apparent to persons skilled in the relevant arts including but not limited to a hardwired control input or writing data to a particular address.

[0055] The master device 20M also includes a first controller 25M programmed to (a) determine, based on the first data, a first time at which the first pulse generating circuit 13 should generate the first pacing pulse, (b) generate the first command so that the first pulse generating circuit 13 will generate the first pacing pulse at the first time, (c) determine, based on the first data, a second time at which the second pacing module 20S should generate a second pacing pulse, and (d) output control data that specifies the second time. The master device 20M also includes a first ultrasound communication module 26M configured to accept the control data from the first controller 25M and transmit the control data to the second pacing module 20S using an ultrasound carrier signal. The master device 20M also includes a power source 14 (e.g., a battery) that provides power to the first controller and the first pulse generating circuit.

[0056] The slave device 20S includes a second anode and a second cathode 11 and a second pulse generating circuit 13 configured to generate a second pacing pulse between the second anode and the second cathode 11 in response to receipt of a second command. A second ultrasound communication module 26S is configured to extract the control data from the ultrasound carrier signal that arrives from the master device 20M. The slave device 20S also includes a second controller 25 S programmed to input the extracted control data and, in response to the extracted control data, generate the second command so that the second pulse generating circuit 13 will generate the second pacing pulse at the second time. The slave device 20S also includes a power source 14 (e.g., a battery) that provides power to the second controller and the second pulse generating circuit.

[0057] Optionally, the slave device 20S may further include a second cardiac monitoring circuit 12 configured to monitor cardiac activity based on signals arriving from at least one of the second anode and the second cathode 11 and generate second data indicative of the monitored cardiac activity. In these embodiments, the second controller 25 S is programmed to ignore the extracted control data if the second controller determines, based on the second data, that generating the second command could pose a danger to the subject. [0058] Optionally, the slave device 20S may further include an ultrasound wake-up circuit 27 that generates a wake-up signal in response to receipt of ultrasound energy. In these embodiments, the wake-up signal is provided to the second ultrasound communication module 26S, and the second ultrasound communication module 26S is configured to default to a powered-down state and to power up in response to the wake-up signal. In addition, when the slave device 20S includes the wake-up circuit 27, the first ultrasound

communication module 26M in the master device 20M should be configured to generate a second ultrasound signal that is capable of activating the ultrasound wake-up circuit 27, and the first controller 25M in the master device 20M should be further programmed to instruct the first ultrasound communication module to generate the second ultrasound signal prior to outputting the control data.

[0059] Although primary batteries are preferred, rechargeable batteries may be used in alternative embodiments to power the master device 20M and the slave device 20S (including the ultrasound communication modules 26M, 26S contained therein). Recharging can be done using the various available technologies, for example, by induction, capacity charging, etc. In alternative embodiments, power to the various components in each of the devices 20M, 20M may be provided by a plurality of power sources.

[0060] FIG. 5 is a schematic diagram of a circuit that may be used to implement the ultrasound wake-up circuit 27 depicted in FIG. 3. The FIG. 5 wake-up circuit relies on a passive voltage multiplier 42 to process the electrical signal that is output from an ultrasound transducer 41. The passive voltage multiplier circuit is configured so that the output voltage will rise above a predetermined threshold when ultrasound waves arrive at the ultrasound transducer 41 from either the master device 20M (depicted in FIG. 3) or the external device 200 (depicted in FIG. 8) positioned externally, but in contact with the subject's body.

[0061] The ultrasound waves that are used to implement wake up may be applied in pulse mode at clinically used intensities and frequencies, e.g., at 1-5 MHz. The ultrasound waves that arrive at the ultrasound transducer 41 can be expected to generate AC pulses in the range of mV to 1 V. Assume that we start with a value of 0.1 V. Connecting the ultrasound transducer 41 to a passive voltage multiplier 42 having a cascade of 5-10 capacitor-rectifier pairs will produce about a IV output pulse in response to a corresponding AC pulse (for example a 1 MHz signal, in the range of 10-50 μ8). When the ultrasound wake-up signal is applied for a sufficient amount of time, the output of the passive voltage multiplier 42 will cause the voltage on the capacitor 43 to exceed a threshold (e.g., IV), which will switch the ultrasound control module 26S (shown in FIG. 3) from the off state to the on state.

[0062] Returning to FIG. 3, once the ultrasound control module 26S has been turned on, it will accept ultrasound control signals from either the external device 200 or the master device 20M. The ultrasound control module 26S can stay on for a preset period of time or remain on until it is turned off by a subsequent ultrasound command.

[0063] The system depicted in FIG. 3 is not limited to the situation when only a single slave device 20S is used. To the contrary, one or more additional slave devices may be included in the system. These additional slave devices are similar to the slave device 20S depicted on the bottom half of FIG. 3. When an additional slave device (not shown) is included in the system, the first controller 25M in the master device 20M is further programmed to (e) determine, based on the first data, a third time at which the additional slave device should generate a third pacing pulse, and (f) output control data that specifies the third time.

[0064] FIG. 6 is a flowchart of the processes that are implemented in the master device 20M. In this discussion, reference numbers that begin with the letter "S" refer to FIG. 6, and reference numbers that begin with a numeral refer to FIG. 3. In step S10, cardiac activity is monitored by the cardiac activity monitoring circuit 12, and this activity is reported to the master controller 25M. Next, in step S20, the master controller 25M decides, if dual chamber pacing is called for based on the reported cardiac activity. If dual chamber pacing is called for, processing proceeds to step S24, where the master controller sends control data to the slave device 20S requesting the slave to generate a pacing pulse in the second chamber of the heart at time t2. The master controller 25M accomplishes this by outputting data to the ultrasound communication module 26M. In response to receipt of this data, the ultrasound communication module 26M modulates the data onto an ultrasound carrier signal that is coupled into the subject's body. The ultrasound carrier signal arrives at the slave device 20S, which is implanted into the second chamber of the heart. The ultrasound communication module 26S in the slave device 20S extracts the data from the ultrasound carrier signal and forwards the data (which is a request for a pacing pulse) to the slave controller 25S. The slave controller 25 S sends appropriate commands to the pulse generator circuit 13 in the slave device 20S, and the pulse generator circuit 13 responds by generating the requested pulse at time t2. [0065] After the master controller 25M sends the control data to the slave in step S24, the master controller 25M can expect the slave device 20S to generate the requested pulse in the second chamber of the heart at time t2. Dual chamber pacing requires coordination of the timing between the pulses that are generated in each chamber. The master controller 25M then sends instructions (in step S26) to the pulse generator circuit 13 located in the master device 20M in order to cause the pulse generator circuit 13 to generate the appropriate pacing pulse in the first chamber at the appropriate time tl . Note that the master controller 25M decides what the appropriate times are for generating the pacing pulses in the first and second chambers based on the report of the cardiac activity that it received from the cardiac activity monitor 12 in step S10. The master controller 25M then issues the commands described above to generate the pulses in the first and second chambers at tl and t2 respectively.

[0066] Returning to step S20, if the master controller 25M decided that the reported cardiac activity does not call for dual chamber pacing, processing proceeds to step S30. In step S30, a determination is made by the master controller 25M whether the cardiac activity calls for a pulse in the first chamber only. This decision is made based on the monitoring of cardiac activity described above in connection with step S10. If the cardiac activity calls for a pulse in the first chamber only, processing proceeds to step S34. In step S34, the master controller 25M sends instructions to the pulse generator circuit 13 located in the master device 20M in order to cause the pulse generator circuit 13 to generate the appropriate pacing pulse in the first chamber at the appropriate time tl .

[0067] If it is determined in step S30 that the cardiac activity does not call for a pulse in the first chamber only, processing proceeds to step S40. In step S40, a determination is made by the master controller 25M whether the cardiac activity calls for a pulse in the second chamber only. Once again, this decision is made based on the monitoring of cardiac activity described above in connection with step S10. If the cardiac activity calls for a pulse in the second chamber only, processing proceeds to step S44. In step S44, the master controller sends control data to the slave device 20S requesting the slave to generate a pacing pulse in the second chamber of the heart at time t2. The master controller 25M accomplishes this as described above in connection with step S24.

[0068] In those embodiments that include more than one slave device, the approach described above for commanding a single slave device 20S to generate a pulse can be extended to a plurality of slave devices. In this situation, the master controller 25M would send appropriate control data to each of the slave devices to command each of the slave devices to generate a pacing pulse at the appropriate time, as determined by the master controller 25M.

[0069] FIG. 7 is a flowchart of the processes that are implemented in the slave device

20S. In this discussion, reference numbers that begin with the letter "S" refer to FIG. 7, and reference numbers that begin with a numeral refer to FIG. 3. In step S60, the slave controller 25 S waits for control data to arrive from the master device 20M. Note that the path of the control data from the master device 20M will include the ultrasound carrier signal (with the control data modulated thereon) arriving at the ultrasound communication module 26S. The ultrasound communication module 26S will extract the control data from the ultrasound carrier signal and forward the data to the slave controller 25 S. The slave controller 25 S examines the control data. Next, in step S70, the slave controller 25 S checks if the control data is a request for a pulse. If the control data is a request for a pulse, processing continues in step S74, where the slave controller 25 S sends instructions to the pulse generator circuit 13 in the slave device 20S to instruct the pulse generator circuit 13 to generate a pulse that matches the characteristics described in the control data. The pulse generator circuit 13 response to these instructions and generates the requested pulse.

[0070] Returning to step S70, it the control data is not a request for a pulse, processing proceeds to step S80. In step S80, the slave controller 25 S checks if the control data is a synchronization command. If the control data is a synchronization command, processing continues in step S84, where a synchronization routine is executed in order to synchronize the slave device 20S with the master device 20M. Returning to step S80, if the control data is not a synchronization command, processing proceeds at step S90, for the processing of other commands.

[0071] As noted above, dual chamber pacing requires coordination of the timing between the pulses that are generated in each chamber, and the master controller 25M issues the commands described above to generate the pulses in the first and second chambers at tl and t2 respectively. In order for the pulse that is ultimately generated by the slave device 20M to occur at the correct time, the master device 20M and the slave device 20M are preferably synchronized. A wide variety of approaches for synchronizing the master device 20M and the slave device 20S will be readily apparent to persons skilled in the relevant arts. In one example of such an approach that relies on relative timing, the command that instructs the slave device 20S to generate a pulse at time t2 uses relative timing with respect to the time that the command is issued. In this situation, for example, the command from the master device 20M could instruct the slave device 20S to generate its pulse at a specified time (e.g., 5 mS) from the end of the transmission that includes the command.

[0072] In an example of an approach that relies on absolute timing, the master controller 25 and the slave controller 25 each include their own internal clock, and each keep track of time using an absolute timing system. Periodically, the time value of the master controller's internal clock is transmitted from the master controller 25M to the slave controller 25 S. the slave controller 25 S examines the time value that it receives from the master controller. If the received time value does not match the slave controller's internal clock, the slave controller will update its internal clock in order to synchronize that clock with the master controller's clock. With this arrangement, the command that instructs the slave device 20S to generate a pulse at time t2 uses absolute timing with respect to the internal clocks. In this situation, for example, the command from the master device 20M could instruct the slave device 20S to generate its pulse at a specified absolute time (e.g., at 122 mS after 1 :03 :05 PM). When an absolute timing approach is used, it is preferable for the internal clocks in the master controller 25M and the slave controller 25 S to be as stable as possible. When clocks with very high stability are used, synchronization signals need only be generated and transmitted at relatively long intervals (e.g., every 10-500 minutes). This factor is also useful to reduce energy consumption.

[0073] FIG. 8 depicts an external unit 200 that is capable of communicating with the master device 20M and/or the slave device 20S (both shown in FIG. 3) after those devices have been implanted into the subject's body. The controllers 25M and 25 S in the master and slave devices, respectively, are preferably programmed to accommodate communication with the external unit 200 depicted in FIG. 8. (Note that while FIG. 8 only shows the ultrasound communication module 26M of the master device, the exact same approach may be used for communicating with the corresponding ultrasound communication module in the slave device.)

[0074] The external unit 200, also referred to herein as an auxiliary apparatus, includes an external controller 225 that is configured to generate commands for controlling the master device 20M and the slave device 20S (both shown in FIG. 3) in response to user input received via the user interface 210. The external unit 200 also includes a communication module 240, 242 configured to transmit the commands generated by the external controller 225 to the master device 20M and the slave device 20S using an ultrasound carrier signal.

[0075] In order to communicate with the external unit 200, The master device 20M and the slave device 20S (both shown in FIG. 3) must be designed to communicate with that unit. For the master device 20M, this may be accomplished, for example, by configuring the first ultrasound communication module 26M to receive ultrasound signals arriving from outside the subject's body, extract first incoming control data from the arriving ultrasound signals, and provide the first incoming control data to the first controller 25M. The first controller 25M is programmed to respond to the first incoming control data. For the slave device 20S, this may be accomplished, for example, by configuring the second ultrasound communication module 26S to receive ultrasound signals arriving from outside the subject's body, extract second incoming control data from the arriving ultrasound signals, and provide the second incoming control data to the second controller 25 S. The second controller 25 S is programmed to respond to the second incoming control data.

[0076] One situation in which communication with implanted pacemakers is desirable is when the cardiac stimulating pulse parameters (voltage, duration, etc.) need to be changed based on examination of the patient and the performance of the master and slave devices 20M, 20S. Such interactions usually require two-way communication to provide data from those devices to the care provider or another unit, as well as to transmit the control instructions back to the master and slave devices 20M, 20S.

[0077] External control over the master device 20M and/or the slave device 20S relies on an external ultrasound communication system 200 that must be positioned on the patient's body and make good acoustic contact with the skin surface before communication can occur. Optionally, conventional ultrasound gels may be used to improve the acoustic coupling, and to ensure that there is no intervening air gap between the external unit 200 and the subject's skin.

[0078] The master controller 25M controls outbound communication from the master device 20M to the external unit 200 using ultrasound. This may be accomplished, for example, by using bidirectional ultrasound communication hardware in both the ultrasound communication module 26M and the external unit 200. [0079] The external unit 200, which is positioned outside the subject's body controls the implanted pacemakers 20M by coupling ultrasound waves into the body so that those ultrasound waves can travel through the subject's body and arrive at the implanted device 20M. After they reach the ultrasound communication module 26M, the ultrasound waves are converted to an electrical signal by the ultrasound transducer 52, and that electrical signal is received by the ultrasound receiver 50. The output of the ultrasound receiver 50 is provided to the controller 25M. The external unit 200 is preferably housed in an appropriate housing that makes it possible to bring the ultrasound transducer 242 into contact with the surface of the subject's body. This ultrasound transducer 242 (e.g., a piezoelectric element) is driven by an ultrasound transmitter 240 which, in turn, is controlled by the external controller 225 within the external unit 200. When piezoelectric elements are used to implement the ultrasound transducers 242, 272, a suitable size for those elements is between 0.3 and 3 mm in diameter.

[0080] The external controller 225 is configured to generate commands for controlling the implanted device 20M. Control of the external unit 200 may be effectuated using any appropriate user interface 210, the details of which will be apparent to persons skilled in the relevant arts.

[0081] When the external unit 200 is placed against the surface of the subject's body so that the ultrasound transducer 242 is in acoustic contact with the surface of the subject's body, the external unit 200 can transmit commands into the subject's body via ultrasound. The external unit 200 may be positioned so that the ultrasound transducer 242 touches the surface of the subject's body at a location that is close to the location of the implanted device 20M. Optionally, ultrasound gel (e.g., similar to the gels used for medical sonograms) may be used to enhance the acoustic coupling between the ultrasound transducer 242 and the surface of the subject's body.

[0082] The signal path from the external controller 225 to the ultrasound

communication module 26M includes an ultrasound frequency transmitter 240 and an ultrasound frequency receiver 50. Although the exact nature of the transmitter 240 and the receiver 50 is not critical, the receiver 50 should be designed to be the counterpart of the transmitter 240. For example, if the transmitter 240 uses a particular approach to encode the commands that it receives from the second controller 125, the receiver 50 should use the counterpart of that approach to decode the electrical signals that it receives. Examples of suitable approaches for encoding and decoding include digital modulation/demodulation techniques (including but not limited to amplitude-shift keying, phase-shift keying, pulse- position modulation, etc.) and analog modulation/demodulation techniques (including but not limited to amplitude modulation, frequency modulation, phase modulation, etc.). Optionally, differential pulse position modulation may be used for implementing synchronization. In some embodiments, the external controller 225 implements framing of the data prior to transmission. In some embodiments, the external controller 225 encodes the data prior to transmission in the ultrasound pulse intervals and/or durations, and/or position, etc.

[0083] Any suitable communication protocol may be used. For example, the message, data, or command may be determined by the specific protocol. Optionally, the controller 25M in the master device 20M may be configured to check all incoming data for integrity using any of a variety of techniques. The protocol may also implement an error detection or error correction logic (e.g. simple parity, checksums, to more complex Hanning code, or other). This can help the receiver side to understand if the message / command / data detected is valid or might be corrupted.

[0084] Optionally, a secure communication protocol may be employed by the system e.g., by having the external controller 225 encrypt the data that it sends to the ultrasound transmitter 240 and by having the controller 25M in the master device 20M decrypt the data that it receives from the ultrasound receiver 50. A wide variety of approaches for

implementing this encryption/decryption or another security protocol can be used, the details of which will be apparent to persons skilled in the relevant arts.

[0085] Notably, whenever the external unit 200 is not touching the surface of the subject's body and is not in acoustic contact with the surface of the subject's body, the external unit 200 will not be able to send its commands into the subject's body via

ultrasound. This renders the implanted device 150 immune from external control.

[0086] The configuration depicted in FIG. 8 may also be used to communicate data in the reverse direction (i.e., from the implanted device back up to the external unit 200).

Communication in this direction relies on the ultrasound transmitter 60 and the ultrasound transducer 62 within the ultrasound communication module 26M to transmit outbound data, and these components operate in a manner similar to the ultrasound transmitter 240 and ultrasound transducer 242 described above. Similarly, reception of the ultrasound data by the external unit 200 relies on the ultrasound transducer 272 and the ultrasound receiver 270. And these components operate in a manner similar to the ultrasound transducer 52 and the ultrasound receiver 50 described above.

[0087] In alternative embodiments, particularly where data security/privacy (in contrast to secured control) is not a concern, a one-way ultrasound communication system may be used to provide communication from the external unit 200 into the ultrasound communication module 26M, and outbound communication (i.e., communication from the implanted device 20M to the outside world) may be implemented using a conventional RF based setup.

[0088] Note that while this portion of the discussion describes ultrasound

communication with the ultrasound communication module 26M within the master device 20M, the same approach may be used for communication with the ultrasound communication module 26S within the slave device 20S.

[0089] FIGS. 9 A and 9B depict flowcharts of two processes that may be used by the external unit 200 to communicate with the implanted device 20M, 20S, so that the external unit 200 can control or monitor the operation of the implanted device 20M, 20S. This can be useful, for example, for performing control functions, modifying pacing pulse parameters, stimulating parameter changes, data acquisition, synchronization, etc. The starting point of the FIG. 9A process is based on the assumption that the implanted device 20M, 20S is always ready to receive communication signals, and that communication can be initiated by the external unit 200 at any time. This approach makes it easier to initiate a communication session, but requires the expenditure of additional power to maintain the ready state.

[0090] The FIG. 9 A process begins at step SI A in which the user provides instructions to the external unit 200 via the user interface 210 (after positioning the external unit against the subject's skin so that the ultrasound transducers (e.g., 242, 270) make contact with the subject skin). Next, in step S4, the external controller 225 initiates a synchronization and handshaking routine to establish communication with the implanted device 20M/20S. Next, in step S5, any data that the implanted device 20M/20S has previously stored for export is received from the implanted device 20M/20S. Next, in step S6, instructions are transferred to the implanted device 20M/20S. In step S7, new data is received from the implanted device 20M/20S, and this new data is tested in step S8 to determine if the functional modifications to the implanted device 20M/20S were correctly made. If the functional modifications were correctly made the communication session stops at step S9. If, on the other hand, the functional modifications were not correctly made, processing returns to step S5 for another attempt.

[0091] In the FIG. 9B process, the external communication hardware portion of the implanted device is asleep most of the time, and wakes up periodically to transmit a synchronization signal. As a result, the external unit 200 cannot communicate with the implanted device 20M/20S 100% of the time. The external unit (where power consumption is not a limiting factor) waits for a synchronization signals from the slave device 20S. Once a synchronization signal arrives, indicating that the implanted device 20M, 20S has waken up from its sleep state, the external unit 200 can initiate communication with the implanted device 20M, 20S. Here, the process begins in step SIB, which is similar to step SI A described above. However, in this FIG. 9B process, communication does not begin immediately after the external unit 200 has been activated. Instead, the external unit 200 waits for signals from the implanted device 20M/20S to arrive in step S2. As soon as this sync signal arrives at the external unit 200, processing continues at steps S4-S9 as described above in connection with FIG. 9A. Note that in the FIG. 9B embodiment, the periodicity of the sync signals may depend on the accuracy of the internal clock of the implanted device 20M, 20S. The higher the clock stability the longer the interval between communication and the larger the energy savings.

[0092] Note that ultrasound attenuation in body tissues is considerably lower than that of RF, and this characteristic can be relied on to provide improved battery life. Further energy saving can be achieved using the apparatus and technology described in US patent application 14/454,858 filed August 8, 2014 and published as US 2015/0045669, which is incorporated herein by reference.

[0093] In any of the embodiments described herein, the frequency of the ultrasound used for communication between the various components 20M 20S, and 200 is preferably between 0.5-20 MHz, and more preferably between 1-5 MHz or between 1-3 MHz. In some preferred embodiments, ultrasound with a frequency of around 2 MHz is used. These frequency ranges are preferred because low frequency ultrasound (e.g. 20-100 kHz) can cross the air/body interface with relatively low losses, and therefore may not provide the desired level of security. In contrast, the corresponding losses for higher frequency ultrasound (e.g. on the order of 1-5 MHz) are large enough to provide the desired level of security. As for the upper limit, the frequency of the ultrasound is preferably below 10 MHz, because higher frequencies will undergo significant attenuation as they pass through tissue in the body, to the point where the signal may not be able to reach its destination.

[0094] The ultrasound power is preferably within the allowed range, preferably less than one tenth the maximal allowed power. The depth of penetration of 2 MHz signals is sufficient for any intra-body location. The ultrasound beam generated by the external device 200 should preferably be relatively wide such that there is no need to point the beam axis exactly at the implanted device 20M/20S. Examples of suitable transducers for this purpose include single element, small diameter (2-10 mm) Piezo electric elements.

[0095] Note that while the embodiments described above are described in the context of intracardiac pacemakers, the invention is not limited to that context. To the contrary, the concepts described herein may be generalized and applied to any type of implanted device that requires security and wireless synchronization between a plurality of implanted devices. In this more generalized context, the system would include a plurality of implantable modules that are synchronized together. A first module is implanted into a subject's body at a first position, and a second module is implanted into the subject's body at a second position.

[0096] The first module includes a first monitoring circuit configured to monitor a time-varying parameter in the subject's body and generate first data indicative of the monitored parameter. Examples of time varying parameters that can be monitored by the first module include blood pressure, the level of a compound in the subject's blood (e.g., glucose), etc. The first module also includes (1) a first circuit configured to generate a first output in response to receipt of a first command; (2) a first controller programmed to (a) determine, based on the first data, a first time at which the first circuit should generate the first output, (b) generate the first command so that the first circuit will generate the first output at the first time, (c) determine, based on the first data, a second time at which the second module should generate a second output, and (d) output control data that specifies the second time; and (3) a first ultrasound communication module configured to accept the control data from the first controller and transmit the control data to the second module using an ultrasound carrier signal. [0097] The second module includes (1) a second circuit configured to generate a second output in response to receipt of a second command; (2) a second ultrasound communication module configured to extract the control data from the ultrasound carrier signal; and (3) a second controller programmed to input the extracted control data and, in response to the extracted control data, generate the second command so that the second circuit will generate the second output at the second time.

[0098] Optionally, the second module in this generalized system may further include an ultrasound wake-up circuit that generates a wake-up signal in response to receipt of ultrasound energy. The wake-up signal is provided to the second ultrasound communication module. The second ultrasound communication module is configured to default to a powered- down state and to power up in response to the wake-up signal. The first ultrasound communication module is configured to generate a second ultrasound signal that is capable of activating the ultrasound wake-up circuit. And the first controller is further programmed to instruct the first ultrasound communication module to generate the second ultrasound signal prior to outputting the control data.

[0099] While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations, and changes to the described

embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A system for implementing multi-chamber cardiac pacing, the system comprising: a first leadless intracardiac pacing module configured for implantation into one chamber of a subject's heart at a first position; and

a second leadless intracardiac pacing module configured for implantation into an other

chamber of the subject's heart at a second position,

wherein the first pacing module includes

a first anode,

a first cathode,

a first cardiac monitoring circuit configured to monitor cardiac activity based on

signals arriving from at least one of the first anode and the first cathode and generate first data indicative of the monitored cardiac activity, a first pulse generating circuit configured to generate a first pacing pulse between the first anode and the first cathode in response to receipt of a first command, a first controller programmed to (a) determine, based on the first data, a first time at which the first pulse generating circuit should generate the first pacing pulse, (b) generate the first command so that the first pulse generating circuit will generate the first pacing pulse at the first time, (c) determine, based on the first data, a second time at which the second pacing module should generate a second pacing pulse, and (d) output control data that specifies the second time, and

a first ultrasound communication module configured to accept the control data from the first controller and transmit the control data to the second pacing module using an ultrasound carrier signal, and

wherein the second pacing module includes

a second anode,

a second cathode,

a second pulse generating circuit configured to generate a second pacing pulse

between the second anode and the second cathode in response to receipt of a second command,

a second ultrasound communication module configured to extract the control data from the ultrasound carrier signal, and a second controller programmed to input the extracted control data and, in response to the extracted control data, generate the second command so that the second pulse generating circuit will generate the second pacing pulse at the second time.

2. The system of claim 1, wherein the second pacing module further includes a second cardiac monitoring circuit configured to monitor cardiac activity based on signals arriving from at least one of the second anode and the second cathode and generate second data indicative of the monitored cardiac activity, and

wherein the second controller is programmed to ignore the extracted control data if the

second controller determines, based on the second data, that generating the second command could pose a danger to the subject.

3. The system of claim 1, wherein the second pacing module further includes an

ultrasound wake-up circuit that generates a wake-up signal in response to receipt of ultrasound energy, and wherein the wake-up signal is provided to the second ultrasound communication module,

wherein the second ultrasound communication module is configured to default to a powered- down state and to power up in response to the wake-up signal,

wherein the first ultrasound communication module is configured to generate a second

ultrasound signal that is capable of activating the ultrasound wake-up circuit, and wherein the first controller is further programmed to instruct the first ultrasound

communication module to generate the second ultrasound signal prior to outputting the control data.

4. The system of claim 1, further comprising a third leadless intracardiac pacing module configured for implantation into the subject's heart at a third position,

wherein the third pacing module includes

a third anode,

a third cathode,

a third pulse generating circuit configured to generate a third pacing pulse between the third anode and the third cathode in response to receipt of a third command, a third ultrasound communication module configured to extract the control data from the ultrasound carrier signal, and

a third controller programmed to input the extracted control data and, in response to the extracted control data, generate the third command so that the third pulse generating circuit will generate the third pacing pulse at a third time, wherein the first controller is further programmed to (e) determine, based on the first data, a third time at which the third pacing module should generate a third pacing pulse, and

(f) output control data that specifies the third time.

5. The system of claim 1, further comprising an auxiliary apparatus that includes a fourth controller configured to generate commands for controlling the first pacing module and the second pacing module, and

a fourth ultrasound communication module configured to transmit the commands generated by the fourth controller to the first pacing module and the second pacing module using an ultrasound carrier signal.

6. The system of claim 1, wherein the first ultrasound communication module is

configured to receive ultrasound signals arriving from outside the subject's body, extract first incoming control data from the arriving ultrasound signals, and provide the first incoming control data to the first controller, and

wherein the first controller is programmed to respond to the first incoming control data.

7. The system of claim 6, wherein the second ultrasound communication module is configured to receive ultrasound signals arriving from outside the subject's body, extract second incoming control data from the arriving ultrasound signals, and provide the second incoming control data to the second controller, and

wherein the second controller is programmed to respond to the second incoming control data.

8. The system of claim 1, wherein the first pacing module further includes a first power source that provides power to the first controller and the first pulse generating circuit and wherein the second pacing module further includes a second power source that provides power to the second controller and the second pulse generating circuit.

9. The system of claim 8, wherein the first power source comprises a first battery and wherein the second power source comprises a second battery.

10. A leadless intracardiac pacing module comprising:

a housing configured for implantation into one chamber of a subject's heart,

an anode supported by the housing;

a cathode supported by the housing;

a cardiac monitoring circuit supported by the housing and configured to monitor cardiac activity based on signals arriving from at least one of the anode and the cathode and generate first data indicative of the monitored cardiac activity;

a pulse generating circuit supported by the housing and configured to generate a first pacing pulse between the anode and the cathode in response to receipt of a first command; a controller supported by the housing, wherein the controller is programmed to (a) determine, based on the first data, a first time at which the pulse generating circuit should generate the first pacing pulse, (b) generate the first command so that the pulse generating circuit will generate the first pacing pulse at the first time, (c) determine, based on the first data, a second time at which a second pacing module that has been implanted into an other chamber of the heart should generate a second pacing pulse, and (d) output control data that specifies the second time; and

an ultrasound communication module supported by the housing and configured to accept the control data output by the controller and to transmit the control data to the other chamber of the subject's heart using an ultrasound carrier signal.

11. The pacing module of claim 10, wherein the ultrasound communication module is configured to receive ultrasound signals arriving from outside the subject's body, extract incoming control data from the arriving ultrasound signals, and provide the incoming control data to the controller, and

wherein the controller is programmed to respond to the incoming control data.

12. The pacing module of claim 10, further comprising a battery that provides power to the controller and the pulse generating circuit.

13. A leadless intracardiac pacing module intended for use in cooperation with a remote master that generates a first pacing pulse at a first time, the pacing module comprising: a housing configured for implantation into one chamber of a subject's heart, an anode supported by the housing;

a cathode supported by the housing;

a pulse generating circuit supported by the housing and configured to generate a second pacing pulse between the anode and the cathode in response to receipt of a command, an ultrasound communication module supported by the housing and configured to extract control data from an incoming ultrasound carrier signal, wherein the control data specifies a second time at which the second pacing pulse should be generated; and a controller supported by the housing, wherein the controller is programmed to input the extracted control data and, in response to the extracted control data, generate the command so that the pulse generating circuit will generate the second pacing pulse at the second time.

14. The pacing module of claim 13, further comprising:

a cardiac monitoring circuit configured to monitor cardiac activity based on signals arriving from at least one of the anode and the cathode and generate second data indicative of the monitored cardiac activity, and

wherein the controller is programmed to ignore the extracted control data if the controller determines, based on the second data, that generating the command could pose a danger to the subject.

15. The pacing module of claim 13, further comprising:

an ultrasound wake-up circuit that generates a wake-up signal in response to receipt of

ultrasound energy, and wherein the wake-up signal is provided to the ultrasound communication module,

wherein the ultrasound communication module is configured to default to a powered-down state and to power up in response to the wake-up signal.

16. The pacing module of claim 13, wherein the ultrasound communication module is configured to receive ultrasound signals arriving from outside the subject's body, extract incoming control data from the arriving ultrasound signals, and provide the incoming control data to the controller, and

wherein the controller is programmed to respond to the incoming control data.

17. The pacing module of claim 13, further comprising a battery that provides power to the controller and the pulse generating circuit.

18. A system for synchronizing a plurality of implantable modules, the system

comprising:

a first module configured for implantation into a subject's body at a first position; and a second module configured for implantation into the subject's body at a second position, wherein the first module includes

a first monitoring circuit configured to monitor a time-varying parameter in the

subject's body and generate first data indicative of the monitored parameter, a first circuit configured to generate a first output in response to receipt of a first command,

a first controller programmed to (a) determine, based on the first data, a first time at which the first circuit should generate the first output, (b) generate the first command so that the first circuit will generate the first output at the first time, (c) determine, based on the first data, a second time at which the second module should generate a second output, and (d) output control data that specifies the second time, and

a first ultrasound communication module configured to accept the control data from the first controller and transmit the control data to the second module using an ultrasound carrier signal, and

wherein the second module includes

a second circuit configured to generate a second output in response to receipt of a second command,

a second ultrasound communication module configured to extract the control data from the ultrasound carrier signal, and

a second controller programmed to input the extracted control data and, in response to the extracted control data, generate the second command so that the second circuit will generate the second output at the second time.

19. The system of claim 18, wherein the second module further includes an ultrasound wake-up circuit that generates a wake-up signal in response to receipt of ultrasound energy, and wherein the wake-up signal is provided to the second ultrasound communication module, wherein the second ultrasound communication module is configured to default to a powered- down state and to power up in response to the wake-up signal,