Patent Publication Number: US-2013238056-A1

Title: Rf-powered communication for implantable device

Description:
TECHNICAL FIELD 
     This application relates generally to implantable devices and more specifically, but not exclusively, to RF-powered communication for implantable devices. 
     BACKGROUND 
     Implantable devices may be employed in various applications. For example, an implantable sensing device may perform functions such as sensing blood pressure, sensing cardiac signals, sensing neurological signals, and so on. 
     In practice, there may be a need to communicate with an implantable device after it has been implanted in a patient. For example, an external monitoring device located in a person&#39;s home, a doctor&#39;s office, a clinic, or some other suitable location may be used to retrieve information collected by and/or stored in an implanted device. In the case of an implanted blood pressure sensing device, blood pressure readings collected by the implanted device may occasionally (e.g., periodically) be uploaded to an external monitoring device. Similarly, an external programming device located in any of the above locations may be used by a treating physician to change the operating parameters of an implanted device. Such parameters may include, for example, sensing timing and/or thresholds to be used by the implanted device. 
     In a typical implementation, an implanted device utilizes radiofrequency (RF) telemetry to communicate with an external device. Consequently, the implanted device includes an RF transceiver that transmits and receives RF signals. In such an implementation, however, it is generally desirable to leave the transceiver in a powered-down or low power state as much as possible since the transceiver consumes a relatively large amount of power. Here, it should be appreciated that the replacement of the battery in an implanted device involves a surgical procedure. Hence, long battery life is an important aspect of such a device. 
     Some types of implanted devices employ a wakeup scheme whereby an implanted device will periodically turn on its transceiver (e.g., its receiver) to determine whether an external device is attempting to establish a communication session. For example, whenever an external device wishes to establish communication with an implanted device, the external device may transmit signals over one or more designated RF channels. Typically, these signals comprise information such as, for example, an identifier (e.g., key) that identifies the external device and/or the implanted device. 
     Thus, every time the transceiver of the implanted device is turned on (e.g., at defined intervals), the transceiver conducts an RF scan to determine whether an external device is attempting to establish communication with the implanted device. This may involve, for example, performing an identifier (ID) scan that checks each designated RF channel for any signals that comprise a specified identifier. In the event such a message is detected, the implanted device transmits one or more signals (e.g., in accordance with a handshake protocol) to establish communication with the external device. 
     For some types of implantable devices, low power consumption is of upmost importance. For example, it is desirable for very small implantable devices such as sensing devices and satellite pacing devices to remain implanted for as long as possible (e.g., many years). Due to size limitations, however, such devices have relatively small batteries. While the power required for the primary operations (e.g., sensing) performed by the devices may be kept very low, a relatively large amount of power is still used for RF communication with external devices. Consequently, a need exists for techniques for reducing the amount of battery power consumed by an implantable device during RF communication operations. 
     SUMMARY 
     A summary of several sample aspects of the disclosure follows. This summary is provided for the convenience of the reader and does not wholly define the breadth of the disclosure. For convenience, the term some aspects may be used herein to refer to a single aspect or multiple aspects of the disclosure. 
     The disclosure relates in some aspects to a communication scheme that facilitates reduced battery power consumption in an implantable device. To conserve battery power, a communication circuit of the implantable device is normally decoupled from a battery of the implantable device. The communication circuit is subsequently coupled to the battery (i.e., the communication circuit is powered-up) upon receipt of RF signals at the implantable device. Here, a circuit that controls whether the communication circuit is to be powered-up obtains its power from the received RF signals (e.g., power is magnetically coupled into the implantable device via an RF magnetic field). Consequently, the implantable device is able to perform RF signal monitoring (e.g., RF “sniffing”) without using battery power. Rather, battery power is only used for subsequent communication operations once it has been determined that the implantable device is receiving RF signals. 
     In some embodiments, a communication circuit is powered-up upon detection of RF signals. For example, upon detection of RF signals received via an antenna, a signal may be generated to control a circuit (e.g., a switching circuit) that selectively couples power from a power source to the communication circuit. Thus, the communication circuit will use battery power only after it has been determined that RF signals have been detected at the implantable device. Upon power-up, the communication circuit may then attempt to establish communication with an external device (e.g., in the event the external device was the source of the RF signals). 
     In some embodiments, a verification circuit obtains power based on (e.g., derived from) received RF signals. Upon power-up, the verification circuit processes received RF signals to determine whether to power-up a communication circuit. For example, in some embodiments, the verification circuit determines whether the received RF signals comprise a defined identifier (e.g., a communication key) that indicates that the RF signals are from a known type of external device. Based on this determination, the verification circuit provides a control signal to selectively couple power from a power source to the communication circuit. Advantageously, the verification circuit does not consume battery power and the communication circuit will be powered-up (and, hence, commence consuming battery power) only after it has been determined that RF signals have been received from a known type of external device. Upon power-up, the communication circuit may then conduct any needed communication with the external device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the disclosure will be more fully understood when considered with respect to the following detailed description, the appended claims, and the accompanying drawings, wherein: 
         FIG. 1  is a simplified block diagram of an embodiment of a communication system including RF-powered control circuitry in an implantable device; 
         FIG. 2  is a simplified block diagram illustrating several components of an embodiment of an implantable device that includes an RF-powered detection circuit; 
         FIG. 3  is a simplified block diagram illustrating several components of an embodiment of an implantable device that includes an RF-powered verification circuit; 
         FIG. 4  is a simplified diagram of an embodiment of a communication system including RF-powered control circuitry in an implantable device that provides sensing functionality; 
         FIG. 5  is a simplified flowchart of an embodiment of operations where power is coupled to a communication circuit upon detection of RF signals; 
         FIG. 6  is a simplified flowchart of an embodiment of operations where a verification circuit is powered by RF signals; 
         FIG. 7  is a simplified diagram of an embodiment of a communication system including an implantable cardiac device; 
         FIG. 8  is a simplified diagram of an embodiment of an implantable cardiac device in electrical communication with one or more leads implanted in a patient&#39;s heart for sensing conditions in the patient, delivering therapy to the patient, or providing some combination thereof; and 
         FIG. 9  is a simplified functional block diagram of an embodiment of an implantable cardiac device, illustrating basic elements that may be configured to sense conditions in the patient, deliver therapy to the patient, or provide some combination thereof. 
     
    
    
     In accordance with common practice the various features illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. In addition, some of the drawings may be simplified for clarity. Thus, the drawings may not depict all of the components of a given apparatus or method. Finally, like reference numerals may be used to denote like features throughout the specification and figures. 
     DETAILED DESCRIPTION 
     The description that follows sets forth one or more illustrative embodiments. It will be apparent that the teachings herein may be embodied in a wide variety of forms, some of which may appear to be quite different from those of the disclosed embodiments. Consequently, the specific structural and functional details disclosed herein are merely representative and do not limit the scope of the disclosure. For example, based on the teachings herein one skilled in the art should appreciate that the various structural and functional details disclosed herein may be incorporated in an embodiment independently of any other structural or functional details. Thus, an apparatus may be implemented or a method practiced using any number of the structural or functional details set forth in any disclosed embodiment(s). Also, an apparatus may be implemented or a method practiced using other structural or functional details in addition to or other than the structural or functional details set forth in any disclosed embodiment(s). 
       FIG. 1  depicts an embodiment of a communication system  100  where an implantable device  102  communicates via RF signaling with an external device  104 . In general, communication circuitry  106  and  108  (e.g., each comprising an RF transceiver, not shown) of the implantable device  102  and the external device  104 , respectively, send and receive RF signals via respective antennas  110  and  112 . 
     In a typical implementation, the implantable device  102  is implanted in a patient (not shown) to provide a sensing function, a stimulation function, some other type of implant-related function, or a combination of these functions. The external device  104 , in turn, is used to download information to and/or upload information from the implantable device  102 . For example, the external device  104  may include programming functionality that programs one or more configuration parameters into the implantable device  102 . As another example, the external device  104  may include monitoring functionality that acquires sensed information and/or current parameter information from the implantable device  102 . 
     Processing circuitry  114  and input/output (I/O) circuitry  116  in the implantable device  102  conduct operations that draw power from a power source  118  (e.g., including a battery and a power conditioning circuit). For example, the processing circuitry  114  may operate on information generated by the I/O circuitry  116  (e.g., a sensing circuit) upon sensing a condition within the patient. As another example, the processing circuitry  114  may provide information that is used by the I/O circuitry  116  (e.g., a stimulation circuit) to stimulate patient tissue. 
     In a typical implementation, the implantable device  102  will not need to communicate with the external device  104  very often. For example, the system  100  may be configured such that the external device  104  initiates communication with the implantable device  102  on a daily, weekly, or some other relatively infrequent basis. Even though such communication does not occur often, this communication may still consume a relatively large amount of power at the implantable device  102  since the communication circuitry  106  may repeatedly process received RF signals to determine whether the external device  104  is attempting to establish communication. 
     To reduce the amount of power drawn from the power source  118  by communication circuitry  106 , the implantable device  102  includes RF-powered control circuitry  120  that keeps the communication circuitry  106  decoupled from the power source  118  most of the time. Here, the control circuitry  120  couples power to the communication circuitry  106  only when it is expected that the external device  104  is attempting to establish communication with the implantable device  102 . For example, the control circuitry  120  may be configured to couple the power source  118  to the communication circuitry  106  if RF signals are received at the implantable device  102 . 
     Advantageously, the control circuitry  120  is powered by received RF signals instead of the power source  118 . For example, the control circuitry  120  may include rectification circuitry (not shown in  FIG. 1 ) that rectifies RF signals received via the antenna  110 . The resulting rectified signal may then be used to power any circuitry that controls the coupling of the power source  118  to the communication circuitry  106 . 
     The received RF signals may be used in different ways in different implementations to control this coupling. As described in more detail below in conjunction with  FIG. 2 , in some embodiments, the received RF signals are used to control a circuit (e.g., a switching circuit) that selectively couples the power source  118  to the communication circuitry  106 . For example, upon detection of an RF signal (e.g., upon receipt of RF signals of sufficient magnitude), a switch may be turned on to couple the power source  118  to the communication circuitry  106 . 
     As described in more detail below in conjunction with  FIG. 3 , in some embodiments, the received RF signals are used to power a verification circuit that processes the received RF signals to determine whether an external device is attempting to establish communication with the implantable device  102 . Based on this determination, the verification circuit selectively couples the power source  118  to the communication circuitry  106 . For example, the verification circuit may determine whether the received RF signals comprise a defined identifier (e.g., a communication key). If so, the verification circuit couples the power source  118  to the communication circuitry  106 . 
     Through the use of a communication scheme as taught herein, a significant reduction in communication-related battery consumption may be achieved. For example, all RF “sniffing” operations may be performed using power obtained from received RF signals. Only when an RF signal is received (and, optionally, verification performed), is the communication circuitry  106  coupled to the power source  118  and powered-up. Thus, battery power that would otherwise be consumed to conduct periodic RF “sniffing” operations is not consumed in implantable devices that employ a communication scheme as disclosed herein. Moreover, in some embodiments, battery power that would otherwise be wasted processing signals from non-verified sources (e.g., RF interference) is not consumed in implantable devices that employ a communication scheme as disclosed herein. 
     Such a communication scheme may be advantageously employed in an implantable device that has relatively low power consumption. For example, the processing circuitry  114  and the I/O circuitry  116  may conduct periodic sensing operations or other operations that draw a relatively small amount of power from the power source  118 . In such a case, it is possible that the implantable device  102  may remain implanted in a patient for a very long time (e.g., 10-15 years or longer) provided that the communication circuitry  106  does not consume too much power from the power source  118 . As a specific example, the need for very small implantable medical devices (IMDs) such as sensors and satellite pacers is increasing as technology advances permit small, low power highly integrated electronics including high density, very low power integrated circuits (ICs). 
     Although the power used to operate such devices is very small, the power associated with transmitting data to or from such a device may be relatively high, limited in some aspects by physical laws and ambient electromagnetic noise. As discussed above, one source of power consumption relates to listening or “sniffing” for commands from external devices (e.g., interrogation devices and/or programming devices). Here, the greater the depth of implant, the greater the power required for the RF receiver circuit that “sniffs” (e.g., periodically monitors) for externally generated commands. In practice, implantable sensors or satellite pacing devices may need to be deeply implanted in a patient (in contrast with subcutaneously implanted conventional pacers that employ leads going to the heart). Such a deep implant results in a greater distance between the IMD and the external device and, consequently, more energy being used for communication between these devices. 
     A specific example of power consumption for sensing operations and conventional telemetry operations follows. This example involves a wireless pressure transducer that is placed inside a patient (e.g., inside the heart). Here, it is assumed that the transducer captures 30 seconds of stored pressure waveforms which will be downloaded once a day (e.g., at 8 Kbits/second) for 15 years. In a sample implementation, telemetry operations and recording the data uses 1 milliamp for approximately 4 seconds per day. Thus, approximately 6 milliamp-hours will be consumed to generate and upload this data over a period of 15 years. 
     The current consumption for RF “sniffing” operations will be calculated assuming that the RF receiver turns on for a duration of one millisecond every 30 seconds. In a sample implementation, 200 microamperes are consumed during each “sniffing” or listening operation. Accordingly, the total energy cost over 15 years is approximately 13 milliamp-hours. 
     For a crystal oscillator/counter that runs on 66 nanoamps, the total energy cost over 15 years is approximately 9 milliamp-hours. Thus, the total energy cost for the wireless pressure transducer is approximately 29 milliamp-hours. Assuming a self-discharge rate for the battery of 2%/year, the wireless pressure transducer would require a battery having a capacity on the order of 40 milliamp-hours. 
     The use of a communication scheme as taught herein reduces the amount of IMD battery power that is consumed for RF communication. An antenna (e.g., a coil) in the IMD receives AC magnetically coupled RF energy from the external device, and uses the received RF energy frequency and/or morphology as a command to, for example, change parameters of the IMD or to invoke data transmission from the IMD. Energy for transmission of a response and/or data to the external device may come, magnetically, from the external device or may come from the IMD battery. In either case, the IMD does not need to draw current from a battery to “sniff” for RF transmissions (e.g., commands) from an external device. 
     Continuing with the example set forth above, in a case where “sniffing” operations do not consume battery power, the energy requirements for the wireless pressure transducer would be approximately 15 milliamp-hours. Following adjustment for a self-discharge rate of 2%/year, the wireless pressure transducer would require a battery having a capacity on the order of 20 milliamp-hours. 
     Thus, a very small primary battery may be employed in this case. For example, the dimensions of such a battery may be on the order of 3 millimeters in diameter by 8 millimeters in length. 
     The use of a communication scheme as taught herein also provides benefits when using rechargeable batteries. For example, these batteries would not need to be recharged as often in this case as compared to a system where RF “sniffing” relied solely on battery power. 
       FIG. 2  illustrates several components of an embodiment of an implantable device that employs a detection circuit  202 . Most of the time (e.g., when the implantable device is not communicating with an external device), the detection circuit  202  is in a state that decouples a power source  204  from an RF communication circuit  206 . When the detection circuit  202  detects the reception of RF signals, however, the detection circuit  202  couples power from the power source  204  to the communication circuit  206 . In the example of  FIG. 2 , this involves coupling a power terminal  208  (e.g., power path) of the power source  204  with a power terminal  210  of the communication circuit  206 . 
     The detection of RF signals may be accomplished in different ways in different implementations. In some cases, detection involves generating an output signal in the event the received RF signal is of a certain magnitude (e.g., large enough to cause conduction through a rectifier diode). In some cases, detection is indicated when a received RF signal is of a sufficient magnitude to cause activation of a switch (e.g., a semiconductor switch comprising a transistor such as a FET). In a typical implementation, a received RF signal is coupled to a rectifier, and the output of a rectifier is coupled to a comparator associated with a reference level. Detection of the RF signal is then indicated by a change in the output of the comparator (e.g., where the output controls a FET switch to selectively couple power from the power source  204 ). 
     The detection circuit  202  is depicted as including a rectification circuit  212  and a switching circuit  214 . It should be appreciated, however, that in some implementations the rectification and switching operations may be performed (fully or in part) by a common circuit. 
     The rectification circuit  212  (e.g., comprising a rectifier) rectifies RF signals received via an antenna  216  and provides a rectified output signal on a signal path  232 . In some implementations, the rectification involves some form of power conditioning (e.g., simple capacitive filtering or more robust filtering) to generate a substantially DC rectified signal. 
     In general, in the absence of a rectified signal on the signal path  232 , the switching circuit  214  is in an “off” state, thereby presenting an “open circuit” (e.g., a high resistance path) between the power terminals  208  and  210 . Once a rectified signal is induced on the signal path  232 , the switching circuit  214  switches to an “on” state, thereby presenting a “closed circuit” (e.g., a low resistance path) between the power terminals  208  and  210 . 
     The communication circuit  206  is powered-up as a result of being coupled to the power source  204 . Upon power-up, the communication circuit  206  attempts to establish communication with an external device (not shown in  FIG. 2 ). For example, in some implementations, the communication circuit  206  may monitor for communication signals (RF signals) received via the antenna  216  and attempt to verify whether a known type of external device (e.g., an authorized programmer device) is sending the RF signals. This verification may involve, for example, determining whether the RF signals comprise a defined identifier (e.g., key) and, in some cases, conducting a communication session commencement handshaking operation. 
     If a communication session is established with the external device, the communication circuit  206  may cooperate with other components of the implantable device to conduct additional communication-related operations with the external device. For example, the communication circuit may cooperate with a processing circuit  218  (with an associated memory circuit  220  and clocking circuitry, not shown) to upload information to the external device and/or download information (e.g. commands and/or parameters) to the implantable device. 
       FIG. 2  illustrates an embodiment where the implantable device includes a sensing circuit  222  including at least one sensor  224  (e.g., a blood pressure sensor, a cardiac signal sensor, a neurological signal sensor, etc.) for sensing one or more conditions of a patient. In this example, an analog-to-digital converter (ADC) circuit  230  converts analog signals generated by the sensor  224  into digital information to provide sensed information that is sent to the processing circuit  218  for subsequent processing and/or that is stored in the memory circuit  220 . At some point in time, the sensed information (e.g., after processing) may be sent to the communication circuit  206  for transmission to the external device. 
     The example of  FIG. 2  also illustrates that in some embodiments at least one circuit is directly powered (e.g., continually powered) by the power source  204  while at least one other circuit is selectively powered (e.g., power is dynamically enabled and disabled) by the power source  204 . Specifically, the processing circuit  218 , the memory circuit  220 , and the sensing circuit  222  are directly powered by the power source  204  as indicated by a power terminal  226  from the power source  204 . Thus, the coupling of the power source  204  to these components does not include the detection circuit  202 . Conversely, the communication circuit  206  is selectively powered by the power source  204  under the control of the detection circuit  202 . It should be appreciated that in other embodiments other types of circuits may be directly powered or selectively powered as taught herein. 
     At some point in time, the switching circuit  214  will revert back to its “off” state. As represented by a signal path  228 , the communication circuit  206  (or optionally the processing circuit  218  via another signal path, not shown) sends a control signal to the detection circuit  202  to cause the switching circuit  214  to decouple the power source  204  from the communication circuit  206 . For example, the switching circuit  214  may have a latching characteristic whereby, once the switching circuit  214  is turned “on,” it remains “on” until it is reset. The control signal may be sent, for example, upon determining that the communication with the external device has terminated. 
     In the embodiment of  FIG. 2 , in some cases RF signals (e.g., interference) will be detected at the implantable device and result in the communication circuit  206  being powered-up even though the RF signals were not sent by an external device that is authorized to communicate with the implantable device. In such a case, battery power may be wasted. Such RF signals may be generated by, for example, an RFID system, an air conditioner, an implantable device programmer that is not authorized to communicate with the implantable device, or some other RF signal source. 
       FIG. 3  illustrates several components of an embodiment of an implantable device that employs a verification circuit  302  to control whether a power source  304  is coupled to an RF communication circuit  306 . In a typical implementation, the verification circuit  302  determines whether RF signals are being received from an external device that is attempting to communicate with the implantable device (e.g., a programmer that is authorized to communicate with the implantable device). Based on this determination, the verification circuit  302  generates a control signal on a signal path  308  to control a switching circuit  310 . 
     Most of the time (e.g., when the implantable device is not communicating with an external device), this control signal is in a state that causes the switching circuit  310  to decouple the power source  304  from the communication circuit  306 . For example, the control signal may set the switching circuit  310  to an “off” state in this case. When the verification circuit  302  verifies the received RF signals, however, the control signal is set to a state that causes the switching circuit  310  to couple power from the power source  304  to the communication circuit  306 . For example, the control signal may switch the switching circuit  310  to an “on” state in this case. In the example of  FIG. 3 , this involves coupling a power terminal  312  of the power source  304  with a power terminal  314  of the communication circuit  306 . 
     Advantageously, the verification circuit  302  is powered by received RF signals. Thus, battery power is not consumed during the verification operation. Accordingly, the embodiment of  FIG. 3  may be less susceptible to interference as compared to the embodiment of  FIG. 2  in that the embodiment of  FIG. 3  may not consume as much battery power when subjected to RF interference. 
     The verification circuit  302  is depicted as including a rectification circuit  316 , a power conditioning circuit  318 , and a verification processing circuit  320 . It should be appreciated that in some implementations two or more of these operations may be performed (fully or in part) by a common circuit. 
     The rectification circuit  316  (e.g., comprising a rectifier) rectifies RF signals received via an antenna  322  and provides a rectified output signal on a signal path  324 . The power conditioning circuit  318  performs power conditioning (e.g., simple capacitive filtering or more robust filtering) on the rectified signal to provide DC power for the verification processing circuit  320  via a power path  330  (e.g., a conductive trace). In some implementations, the power conditioning circuit  318  includes an energy storage circuit (e.g., a capacitor and/or a rechargeable battery) that stores energy that may be used to provide power for one or more circuits when RF signals are not being received at the implantable device. 
     Upon obtaining power from the received RF signals, the verification processing circuit  320  is powered-up (e.g., activated) and commences processing RF signals received via the antenna  322  to determine whether the RF signals are from an external device that is attempting to communicate with the implantable device. These received RF signals may comprise the rectified signal provided on the signal path  324 , RF signals received from the antenna  322  via another signal path (e.g., a signal path directly from the antenna  322 , not shown), or RF signals received in some other manner (e.g., via another antenna, not shown). 
     Thus, in different implementations or under different operating conditions, the RF signals processed by the verification processing circuit  320  may or may not be the same RF signals (e.g., temporally) that are used to generate power for the verification processing circuit  320 . In some cases, the verification processing circuit  320  processes the same RF signals that were used to generate power for the verification processing circuit  320  (e.g., the rectified signal on signal path  324 ). Thus, in some cases, some of the signal power in an RF signal comprising a key may be used to power the verification circuit that verifies the key. In some cases, the verification processing circuit  320  processes a portion (e.g., a short burst) of the RF signals that were used to generate power for the verification processing circuit  320 . In some cases, the verification processing circuit  320  processes different RF signals (e.g., RF signals that arrived later in time) than those that were used to generate power for the verification processing circuit  320 . For example, the verification circuit  302  may use a first portion of the received RF signals to obtain power and then process a second portion of the received RF signals to provide the control signal. Here, the circuitry that provides the control signal may use (i.e., may be powered by) stored power that was obtained from the first portion of the received RF signal. 
     The verification operation performed by the verification processing circuit  302  may take different forms in different implementations. In some implementations, the verification operation involves determining whether the received RF signals comprise a defined identifier. Such an identifier may take the form of, for example, a communication key that is associated with an authorized external device (e.g., a programmer device) and/or the implantable device. Thus, upon receipt of this key, the verification processing circuit  320  is able to make a determination that the RF signals are from an authorized external device and that communication is to be established with that external device. 
     In some implementations, the received RF signal takes the form of a series of pulses (e.g., generated using on-off keying or some other technique). For example, the identifier (or key) may be indicated by a defined sequence of pulse positions (in time) in the received RF signals. Thus, in some embodiments, the verification process (i.e., the processing of the RF signals) involves determining whether the received RF signals comprise a defined pulse position sequence. 
     Based on the results of the verification process, the verification processing circuit  320  controls (e.g., adjusts, if applicable) the state (e.g., value) of the control signal provided on the signal path  308  to selectively enable the coupling of power from the power source  304  to the communication circuit  306 . 
     Thus, if the received RF signals pass verification, the communication circuit  306  is powered-up as a result of being coupled to the power source  304 . Upon power-up, the communication circuit  306  attempts to establish communication with an external device (not shown in  FIG. 3 ). For example, in some implementations, the communication circuit  306  may monitor for communication signals (RF signals) received via the antenna  322  and further attempt to verify whether a known type of external device (e.g., an authorized programmer device) is sending the RF signals. This verification may involve, for example, conducting a communication session commencement handshaking operation. In some implementations, the communication circuit  306  may simply commence transmissions to the external device (e.g., sensed information may be automatically uploaded to the external device). 
     If a communication session is established with the external device, the communication circuit  306  may cooperate with other components of the implantable device to conduct additional communication-related operations with the external device. For example, the communication circuit  306  may cooperate with a processing circuit  326  (with an associated memory circuit  328  and clock circuitry) to upload information to the external device and/or download information (e.g. commands and/or parameters) to the implantable device. 
       FIG. 3  illustrates an embodiment where the implantable device includes a stimulation circuit  332  including a signal generator circuit  334  (e.g., at least one pulse generator, etc.) for stimulating tissue of a patient. In this example, a digital-to-analog converter (DAC) circuit  336  converts stimulation data provided by the processing circuit  326  and/or the memory circuit  328  to an analog signal and provides the analog signal to the signal generator circuit  334 . Here, one or more parameters that specified the stimulation data may have been provided by the external device via the communication circuit  306 . It should be appreciated that in some implementations the embodiment of  FIG. 3  may instead, or in addition, employ sensing circuitry as described herein (e.g., at  FIG. 2 ). Similarly, in some implementations, the embodiment of  FIG. 2  may instead, or in addition, employ stimulation circuitry as described herein (e.g., at  FIG. 3 ). 
     The example of  FIG. 3  also illustrates that in some embodiments at least one circuit is directly powered (e.g., continually powered) by the power source  304  while at least one other circuit is selectively powered (e.g., power is dynamically enabled and disabled) by the power source  304 . Specifically, the processing circuit  326 , the memory circuit  328 , and the stimulation circuit  332  are directly powered by the power source  304  as indicated by a power terminal  338  from the power source  304 . It should be appreciated that in other embodiments other types of circuits may be directly powered or selectively powered as taught herein. 
     At some point in time, the switching circuit  310  will revert back to its “off” state. As represented by a signal path  340 , the communication circuit  306  (or optionally the processing circuit  326  via another signal path, not shown) sends a control signal to the verification circuit  302  to cause the verification circuit  302  to change the value of the control signal provided on the signal path  308 . In this way, the switching circuit  310  will decouple the power source  304  from the communication circuit  306 . As above, the control signal provided on the signal path  340  may be sent, for example, upon determining that the communication with the external device has terminated. 
     As mentioned above, the teachings herein may be employed to provide an implantable device of a very small size.  FIG. 4  illustrates several components of an implantable blood pressure sensor device  402  that is implanted in a patient P and communicates with an external device  404 . 
     The sensor device  402  includes a battery  406 , control circuitry  408 , an antenna  410 , a pressure transducer  412 , a thin protruding tube  414 , and a biocompatible housing  416 . In the example, of  FIG. 4 , the tube  414  is inserted into a blood vessel V (e.g., a vein, an artery, a chamber, etc.) of the patient P to measure blood pressure at that location. The sensor device  402  may fixed in place within the patient P by means of sutures, an active fixation device, passive fixation, or some suitable fixation technique (fixation means not shown in  FIG. 4 ). 
     In some implementations, the sensor device  402  has dimensions on the order of: 7-8 millimeters in diameter or less and 10 millimeters in length or less. Thus, the sensor device  402  may be implanted into a patient using so-called minimally invasive techniques (e.g., subcutaneous injection techniques or venous techniques). In some implementations, the sensor device  402  has dimensions on the order of: 5-6 millimeters in diameter thus facilitating implant into a patient using a so-called venous approach. Typically, the battery of an implantable device takes up most of the space in the device. The above described device may accommodate, for example, dimensions for the battery  406  on the order of: 3-5 millimeters in diameter or less and 8 millimeters in length or less. Accordingly, as discussed herein, through the use of a communication scheme taught herein, the sensor device  402  may remain implanted for a very long period of time (e.g., on the order of 10-15 years). Moreover, the control circuitry  408  may wake up several times a day (e.g., once every 2 hours) to take measurements, store the sensed information in a memory circuit (in the control circuit  408 ), and occasionally upload the sensed information to an external device (e.g., on a daily, weekly, monthly, or some other basis). Thus, very detailed blood pressure profiles may be obtained over a long period of time after performing only a single surgical procedure. 
     The control circuitry  408  provides functionally such as, for example, the communication, control, detection, power conditioning, processing, memory, conversion, verification, clock, and switching functionality described above. In a typical implementation the control circuitry  408  is implemented as an application-specific integrated circuit (ASIC). 
     In a typical implementation, the antenna  410  comprises a coil. Such an antenna may have dimensions on the order of, for example: 3-4 millimeters or less in diameter and 3-5 millimeters or less in length. 
     The pressure transducer  412  includes a flexible diaphragm  418  or some other suitable component that is able to detect pressure that is transferred via the tube  414  (or some other suitable structure). For example, the flexible diaphragm  418  may be in fluid communication with the interior space of the tube  414  (e.g., which may be filled with a gel) such that pressure induced in the vessel V is imparted via the interior space of the tube  414  to the flexible diaphragm  418 . The flexible diaphragm  418  may be constructed of various types of biocompatible materials including, for example, silicone, polyurethane, titanium, and so on. For example, the pressure transducer  412  may comprise a thin titanium diaphragm with an attached strain gauge that is coupled to a bridge circuit that generates signals representative of the deflection of the flexible diaphragm  418 . 
     The external device includes an antenna  420  (e.g., a coil) that is much larger than the antenna  410 . For example, the antenna  420  may have dimensions on the order of 12-20 centimeters in diameter. In this way, an RF transceiver  420  of the external device  404  is able to more effectively exchange information with an RF transceiver (e.g., implemented in the control circuitry  408 ) of the sensor device  402 . Here, RF signals from the transceiver  420  are coupled to the antenna  420  thereby generating a relatively strong RF magnetic field that projects into the body of the patient P. Due in part to the large size of the antenna  420 , the RF magnetic field is strong enough to induce a voltage on the relatively small antenna  410 , thereby providing RF signals at the control circuitry  408  (e.g., including detection circuitry or verification circuitry as taught herein). Conversely, RF signals from the control circuitry  408  are coupled to the antenna  410  thereby generating a RF magnetic field that projects out of the body of the patient P. Due in part to the large size of the antenna  420 , the corresponding voltage induced on the antenna  420  is strong enough to be detected by the transceiver  420 . 
     As discussed herein, in some embodiments, the voltage induced at the antenna  410  is used to trigger detection circuitry (e.g., a semiconductor switch) of the control circuitry  408  and thereby connect the battery  406  to communication circuitry (including the transceiver) of the control circuitry  408 . Upon power-up, this transceiver commences communication with the transceiver  420 . 
     In some embodiments, the voltage induced at the antenna  410  is used to power verification circuitry (e.g., a receiver circuit) of the control circuitry  408 , whereby the verification circuitry demodulates the received signal and confirms a communication link. Upon confirmation of the communication link, the battery  406  is coupled to the communication circuitry of the control circuitry  408  to enable the communication circuitry to communicate with the external device  404 . As mentioned above, this embodiment is used in some aspects to prevent interference signals (e.g., stray electromagnetic radiation) from potentially triggering unnecessary power-ups of the communication circuitry of the sensor device  402 . 
     In the embodiments described herein, power and/or information may be coupled via a given antenna. For example, the antenna  410  receives both power and commands from the antenna  420 . Conversely, the antenna  420  receives information (e.g., sensed information) from the antenna  410 . 
     Different technologies may be employed to provide the RF signaling between the sensor device  402  and the external device  404  in different embodiments. For example, some embodiments employ a reflective impedance scheme whereby, for example, a transmitted RF signal may be modulated (e.g., by turning the RF signal on and off) by the information (e.g. commands or sensed information) being transmitted. Such a scheme may provide an efficient way (e.g., from a battery power consumption standpoint) for the sensor device  402  to transmit RF signals. In some implementations, a frequency in the range of 100 KHz to 2 MHz is used for the RF transmissions. 
     With the above in mind, sample operations that may be performed to provide a communication scheme as taught herein will be described with reference to the flowcharts of  FIGS. 5 and 6 .  FIG. 5  illustrates sample operations that may be performed by an implantable device that power a communication circuit upon detection of a received RF signal (e.g., as in  FIG. 2 ).  FIG. 6  illustrates sample operations that may be performed by an implantable device where a received RF signal powers a verification circuit (e.g., as in  FIG. 3 ). 
     For purposes of illustration, the operations of  FIGS. 5 and 6  (or any other operations discussed or taught herein) may be described as being performed by specific components (e.g., components of  FIG. 1 ,  2 ,  3 ,  4 ,  7 ,  8 , or  9 ). It should be appreciated, however, that these operations may be performed by other types of components and may be performed using a different number of components. It also should be appreciated that one or more of the operations described herein may not be employed in a given implementation. 
     Referring initially to block  502  of  FIG. 5 , when the implantable device is initialized upon implant, the RF communication circuit of the implantable device is decoupled from the internal power source (e.g., battery). However, other circuitry (e.g., sensing circuits) that is continually powered by the internal power source will commence normal operations at this time. 
     As represented by block  504 , a detection circuit of the implantable device passively waits for the arrival RF signals. Once RF signals of sufficient magnitude arrive at the implantable device, the RF signals are detected and a signal indicative of this detection is generated. As discussed herein, the detection of the RF signals and the generation of the detection indication signal do not use power from the internal power source. 
     As represented by block  506 , as a result of the detection of the RF signals, power from the internal power source is coupled to the RF communication circuit. For example, the signal generated at block  504  may control a switch that selectively couples the power source to the RF communication circuit. 
     As represented by block  508 , upon power-up, the RF communication circuit commences processing received RF signals. For example, the RF communication circuit may “sniff” for additional RF signals and, if applicable, verify a communication link to an external device (e.g., upon verification of a received key). 
     As represented by block  510 , in the event it is determined that an external device (e.g., an authorized programmer) is attempting to communicate with the implantable device, the implantable device commence communication with the external device to, for example, upload information (e.g., sensed information, parameters, etc.) and/or download information (e.g., commands, parameters, etc.). 
     As represented by block  512 , at some point in time, power is decoupled from the RF communication circuit. For example, upon termination of the communication between the external device and the implantable device, the implantable device may return to the state where it passively waits for the arrival of RF signals (as represented by the arrow from block  512  to block  504 ). 
     Referring now to  FIG. 6 , as represented by block  602 , the RF communication circuit of the implantable device is decoupled from the internal power source (e.g., battery) at initialization. Again, however, other circuitry (e.g., sensing circuits) that is continually powered by the internal power source will commence normal operations. 
     As represented by block  604 , a verification circuit of the implantable device passively waits for the arrival of RF signals. Once RF signals of sufficient magnitude arrive at the implantable device, power is obtained from these RF signals to power-up the verification circuit. 
     As represented by block  606 , upon power-up, the verification circuit commences processing received RF signals to determine whether an external device (e.g., an authorized programmer) is attempting to communicate with the implantable device. For example, the verification circuit may “sniff” for additional RF signals and, if applicable, verify a communication link to an external device (e.g., upon verification of a received key). Based on the results of this processing (e.g., the verification), the verification circuit provides a control signal that control whether the RF communication circuit is to be coupled to the internal power source. 
     As represented by block  608 , depending on the current state (e.g., value) of the control signal, power from the internal power source is selectively coupled to the RF communication circuit. 
     As represented by block  610 , in the event the RF communication circuit is powered-up, the RF communication circuit commences communication with the external device. As discussed herein, this communication may involve, for example, uploading information (e.g., sensed information, parameters, etc.) and/or downloading information (e.g., commands, parameters, etc.). 
     As represented by block  612 , at some point in time, power is decoupled from the RF communication circuit and the verification circuit. For example, upon termination of the communication between the external device and the implantable device, the implantable device may return to the state where it passively monitors for RF signals (as represented by the arrow from block  612  to block  604 ). 
     As mentioned above, in some embodiments the teachings herein may be incorporated into an implantable cardiac device. An example of such an implantable cardiac device will be described in more detail in conjunction with  FIGS. 7-9 .  FIG. 7  illustrates a sample communication system from a high-level perspective.  FIGS. 8 and 9  illustrate sample components of an implantable cardiac device. 
       FIG. 7  is a simplified diagram of a device  702  (implanted within a patient P) that communicates with a device  704  that is located external to the patient P. The implanted device  702  and the external device  704  communicate with one another via a wireless communication link  706  (as represented by the depicted wireless symbol). 
     In the illustrated example, the implanted device  702  is an implantable cardiac device including one or more leads  708  that are routed to the heart H of the patient P. For example, the implanted device  702  may be a pacemaker, an implantable cardioverter defibrillator, or some other similar device. It should be appreciated, however, that the implanted device  702  may take other forms. 
     The external device  704  also may take various forms. For example, the external device  704  may be a base station, a programmer, a home safety monitor, a personal monitor, a follow-up monitor, a wearable monitor, or some other type of device that is configured to communicate with the implanted device  702 . 
     The communication link  706  may be used to transfer information between the devices  702  and  704  in conjunction with various applications such as remote home-monitoring, clinical visits, data acquisition, remote follow-up, and portable or wearable patient monitoring/control systems. For example, when information needs to be transferred between the devices  702  and  704 , the patient P moves into a position that is relatively close to the external device  704 , or vice versa. 
     The external device  704  may send information it receives from an implanted device to another device (e.g., that may provide a more convenient means for a physician to review the information). For example, the external device  704  may send the information to a web server (not shown). In this way, the physician may remotely access the information (e.g., by accessing a website). The physician may then review the information uploaded from the implantable device to determine whether medical intervention is warranted. 
     Exemplary Cardiac Device 
       FIGS. 8 and 9  describe exemplary components of an implantable cardiac device that may be used in connection with the various embodiments that are described herein. For example, in some implementations, the disclosed implantable cardiac device may incorporate RF-powered circuitry as discussed herein. As another example, in some implementations, an implantable cardiac device that incorporates RF-powered circuitry as discussed herein may include one or more of the components described in  FIGS. 8 and 9 . It is to be appreciated and understood that other devices, including those that are not necessarily implantable, may be used in various implementations and that the description below is given, in its specific context, to assist the reader in understanding, with more clarity, the embodiments described herein. 
       FIG. 8  shows an exemplary implantable cardiac device  800  in electrical communication with a patient&#39;s heart H by way of three leads  804 ,  806 , and  808 , suitable for delivering multi-chamber stimulation and shock therapy. Bodies of the leads  804 ,  806 , and  808  may be formed of silicone, polyurethane, plastic, or similar biocompatible materials to facilitate implant within a patient. Each lead includes one or more conductors, each of which may couple one or more electrodes incorporated into the lead to a connector on the proximal end of the lead. Each connector, in turn, is configured to couple with a complimentary connector (e.g., implemented within a header) of the device  800 . 
     To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, the device  800  is coupled to an implantable right atrial lead  804  having, for example, an atrial tip electrode  820 , which typically is implanted in the patient&#39;s right atrial appendage or septum.  FIG. 8  also shows the right atrial lead  804  as having an optional atrial ring electrode  821 . 
     To sense left atrial and ventricular cardiac signals and to provide left chamber pacing therapy, the device  800  is coupled to a coronary sinus lead  806  designed for placement in the coronary sinus region via the coronary sinus for positioning one or more electrodes adjacent to the left ventricle, one or more electrodes adjacent to the left atrium, or both. As used herein, the phrase “coronary sinus region” refers to the vasculature of the left ventricle, including any portion of the coronary sinus, the great cardiac vein, the left marginal vein, the left posterior ventricular vein, the middle cardiac vein, the small cardiac vein or any other cardiac vein accessible by the coronary sinus. 
     Accordingly, an exemplary coronary sinus lead  806  is designed to receive atrial and ventricular cardiac signals and to deliver left ventricular pacing therapy using, for example, a left ventricular tip electrode  822  and, optionally, a left ventricular ring electrode  823 ; provide left atrial pacing therapy using, for example, a left atrial ring electrode  824 ; and provide shocking therapy using, for example, a left atrial coil electrode  826  (or other electrode capable of delivering a shock). For a more detailed description of a coronary sinus lead, the reader is directed to U.S. Pat. No. 5,466,254, “Coronary Sinus Lead with Atrial Sensing Capability” (Helland), which is incorporated herein by reference. 
     The device  800  is also shown in electrical communication with the patient&#39;s heart H by way of an implantable right ventricular lead  808  having, in this implementation, a right ventricular tip electrode  828 , a right ventricular ring electrode  830 , a right ventricular (RV) coil electrode  832  (or other electrode capable of delivering a shock), and a superior vena cava (SVC) coil electrode  834  (or other electrode capable of delivering a shock). Typically, the right ventricular lead  808  is transvenously inserted into the heart H to place the right ventricular tip electrode  828  in the right ventricular apex so that the RV coil electrode  832  will be positioned in the right ventricle and the SVC coil electrode  834  will be positioned in the superior vena cava. Accordingly, the right ventricular lead  808  is capable of sensing or receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle. 
     The device  800  is also shown in electrical communication with a lead  810  including one or more components  844  such as a physiologic sensor. The component  844  may be positioned in, near or remote from the heart. 
     It should be appreciated that the device  800  may connect to leads other than those specifically shown. In addition, the leads connected to the device  800  may include components other than those specifically shown. For example, a lead may include other types of electrodes, sensors or devices that serve to otherwise interact with a patient or the surroundings. 
       FIG. 9  depicts an exemplary, simplified block diagram illustrating sample components of the device  800 . The device  800  may be adapted to treat both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. While a particular multi-chamber device is shown, it is to be appreciated and understood that this is done for illustration purposes. Thus, the techniques and methods described below can be implemented in connection with any suitably configured or configurable device. Accordingly, one of skill in the art could readily duplicate, eliminate, or disable the appropriate circuitry in any desired combination to provide a device capable of treating the appropriate chamber(s) with, for example, cardioversion, defibrillation, and pacing stimulation. 
     A housing  900  for the device  800  is often referred to as the “can”, “case” or “case electrode”, and may be programmably selected to act as the return electrode for all “unipolar” modes. The housing  900  may further be used as a return electrode alone or in combination with one or more of the coil electrodes  826 ,  832  and  834  for shocking purposes. The housing  900  may be constructed of a biocompatible material (e.g., titanium) to facilitate implant within a patient. 
     The housing  900  further includes a connector (not shown) having a plurality of terminals  901 ,  902 ,  904 ,  905 ,  906 ,  908 ,  912 ,  914 ,  916  and  918  (shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the terminals). The connector may be configured to include various other terminals (e.g., terminal  921  coupled to a sensor or some other component) depending on the requirements of a given application. 
     To achieve right atrial sensing and pacing, the connector includes, for example, a right atrial tip terminal (AR TIP)  902  adapted for connection to the right atrial tip electrode  820 . A right atrial ring terminal (AR RING)  901  may also be included and adapted for connection to the right atrial ring electrode  821 . To achieve left chamber sensing, pacing, and shocking, the connector includes, for example, a left ventricular tip terminal (VL TIP)  904 , a left ventricular ring terminal (VL RING)  905 , a left atrial ring terminal (AL RING)  906 , and a left atrial shocking terminal (AL COIL)  908 , which are adapted for connection to the left ventricular tip electrode  822 , the left ventricular ring electrode  823 , the left atrial ring electrode  824 , and the left atrial coil electrode  826 , respectively. 
     To support right chamber sensing, pacing, and shocking, the connector further includes a right ventricular tip terminal (VR TIP)  912 , a right ventricular ring terminal (VR RING)  914 , a right ventricular shocking terminal (RV COIL)  916 , and a superior vena cava shocking terminal (SVC COIL)  918 , which are adapted for connection to the right ventricular tip electrode  828 , the right ventricular ring electrode  830 , the RV coil electrode  832 , and the SVC coil electrode  834 , respectively. 
     At the core of the device  800  is a programmable microcontroller  920  that controls the various modes of stimulation therapy. As is well known in the art, microcontroller  920  typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy, and may further include memory such as RAM, ROM and flash memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, microcontroller  920  includes the ability to process or monitor input signals (data or information) as controlled by a program code stored in a designated block of memory. The type of microcontroller is not critical to the described implementations. Rather, any suitable microcontroller  920  may be used that carries out the functions described herein. The use of microprocessor-based control circuits for performing timing and data analysis functions are well known in the art. 
     Representative types of control circuitry that may be used in connection with the described embodiments can include the microprocessor-based control system of U.S. Pat. No. 4,940,052 (Mann et al.), the state-machine of U.S. Pat. Nos. 4,712,555 (Thornander et al.) and 4,944,298 (Sholder), all of which are incorporated by reference herein. For a more detailed description of the various timing intervals that may be used within the device and their inter-relationship, see U.S. Pat. No. 4,788,980 (Mann et al.), also incorporated herein by reference. 
       FIG. 9  also shows an atrial pulse generator  922  and a ventricular pulse generator  924  that generate pacing stimulation pulses for delivery by the right atrial lead  804 , the coronary sinus lead  806 , the right ventricular lead  808 , or some combination of these leads via an electrode configuration switch  926 . It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial and ventricular pulse generators  922  and  924  may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The pulse generators  922  and  924  are controlled by the microcontroller  920  via appropriate control signals  928  and  930 , respectively, to trigger or inhibit the stimulation pulses. 
     Microcontroller  920  further includes timing control circuitry  932  to control the timing of the stimulation pulses (e.g., pacing rate, atrio-ventricular (A-V) delay, atrial interconduction (A-A) delay, or ventricular interconduction (V-V) delay, etc.) or other operations, as well as to keep track of the timing of refractory periods, blanking intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc., as known in the art. 
     Microcontroller  920  further includes an arrhythmia detector  934 . The arrhythmia detector  934  may be utilized by the device  800  for determining desirable times to administer various therapies. The arrhythmia detector  934  may be implemented, for example, in hardware as part of the microcontroller  920 , or as software/firmware instructions programmed into the device  800  and executed on the microcontroller  920  during certain modes of operation. 
     Microcontroller  920  may include a morphology discrimination module  936 , a capture detection module  937  and an auto sensing module  938 . These modules are optionally used to implement various exemplary recognition algorithms or methods. The aforementioned components may be implemented, for example, in hardware as part of the microcontroller  920 , or as software/firmware instructions programmed into the device  800  and executed on the microcontroller  920  during certain modes of operation. 
     The electrode configuration switch  926  includes a plurality of switches for connecting the desired terminals (e.g., that are connected to electrodes, coils, sensors, etc.) to the appropriate I/O circuits, thereby providing complete terminal and, hence, electrode programmability. Accordingly, switch  926 , in response to a control signal  942  from the microcontroller  920 , may be used to determine the polarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) by selectively closing the appropriate combination of switches (not shown) as is known in the art. 
     Atrial sensing circuits (ATR. SENSE)  944  and ventricular sensing circuits (VTR. SENSE)  946  may also be selectively coupled to the right atrial lead  804 , coronary sinus lead  806 , and the right ventricular lead  808 , through the switch  926  for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial and ventricular sensing circuits  944  and  946  may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. Switch  926  determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. The sensing circuits (e.g., circuits  944  and  946 ) are optionally capable of obtaining information indicative of tissue capture. 
     Each sensing circuit  944  and  946  preferably employs one or more low power, precision amplifiers with programmable gain, automatic gain control, bandpass filtering, a threshold detection circuit, or some combination of these components, to selectively sense the cardiac signal of interest. The automatic gain control enables the device  800  to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation. 
     The outputs of the atrial and ventricular sensing circuits  944  and  946  are connected to the microcontroller  920 , which, in turn, is able to trigger or inhibit the atrial and ventricular pulse generators  922  and  924 , respectively, in a demand fashion in response to the absence or presence of cardiac activity in the appropriate chambers of the heart. Furthermore, as described herein, the microcontroller  920  is also capable of analyzing information output from the sensing circuits  944  and  946 , a data acquisition system  952 , or both. This information may be used to determine or detect whether and to what degree tissue capture has occurred and to program a pulse, or pulses, in response to such determinations. The sensing circuits  944  and  946 , in turn, receive control signals over signal lines  948  and  950 , respectively, from the microcontroller  920  for purposes of controlling the gain, threshold, polarization charge removal circuitry (not shown), and the timing of any blocking circuitry (not shown) coupled to the inputs of the sensing circuits  944  and  946  as is known in the art. 
     For arrhythmia detection, the device  800  utilizes the atrial and ventricular sensing circuits  944  and  946  to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. It should be appreciated that other components may be used to detect arrhythmia depending on the system objectives. In reference to arrhythmias, as used herein, “sensing” is reserved for the noting of an electrical signal or obtaining data (information), and “detection” is the processing (analysis) of these sensed signals and noting the presence of an arrhythmia. 
     Timing intervals between sensed events (e.g., P-waves, R-waves, and depolarization signals associated with fibrillation) may be classified by the arrhythmia detector  934  of the microcontroller  920  by comparing them to a predefined rate zone limit (e.g., bradycardia, normal, low rate VT, high rate VT, and fibrillation rate zones) and various other characteristics (e.g., sudden onset, stability, physiologic sensors, and morphology, etc.) in order to determine the type of remedial therapy that is needed (e.g., bradycardia pacing, anti-tachycardia pacing, cardioversion shocks or defibrillation shocks, collectively referred to as “tiered therapy”). Similar rules may be applied to the atrial channel to determine if there is an atrial tachyarrhythmia or atrial fibrillation with appropriate classification and intervention. 
     Cardiac signals or other signals may be applied to inputs of an analog-to-digital (A/D) data acquisition system  952 . The data acquisition system  952  is configured (e.g., via signal line  956 ) to acquire intracardiac electrogram (“IEGM”) signals or other signals, convert the raw analog data into a digital signal, and store the digital signals for later processing, for telemetric transmission to an external device  954 , or both. For example, the data acquisition system  952  may be coupled to the right atrial lead  804 , the coronary sinus lead  806 , the right ventricular lead  808  and other leads through the switch  926  to sample cardiac signals across any pair of desired electrodes. 
     The data acquisition system  952  also may be coupled to receive signals from other input devices. For example, the data acquisition system  952  may sample signals from a physiologic sensor  970  or other components shown in  FIG. 9  (connections not shown). 
     The microcontroller  920  is further coupled to a memory  960  by a suitable data/address bus  962 , wherein the programmable operating parameters used by the microcontroller  920  are stored and modified, as required, in order to customize the operation of the device  800  to suit the needs of a particular patient. Such operating parameters define, for example, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each shocking pulse to be delivered to the patient&#39;s heart H within each respective tier of therapy. One feature of the described embodiments is the ability to sense and store a relatively large amount of data (e.g., from the data acquisition system  952 ), which data may then be used for subsequent analysis to guide the programming of the device  800 . 
     Advantageously, the operating parameters of the implantable device  800  may be non-invasively programmed into the memory  960  through a telemetry circuit  964  in telemetric communication via communication link  966  with the external device  954 , such as a programmer, transtelephonic transceiver, a diagnostic system analyzer or some other device. The microcontroller  920  activates the telemetry circuit  964  with a control signal (e.g., via bus  968 ). The telemetry circuit  964  advantageously allows intracardiac electrograms and status information relating to the operation of the device  800  (as contained in the microcontroller  920  or memory  960 ) to be sent to the external device  954  through an established communication link  966 . 
     The device  800  can further include one or more physiologic sensors  970 . In some embodiments the device  800  may include a “rate-responsive” sensor that may provide, for example, information to aid in adjustment of pacing stimulation rate according to the exercise state of the patient. One or more physiologic sensors  970  (e.g., a pressure sensor) may further be used to detect changes in cardiac output, changes in the physiological condition of the heart, or diurnal changes in activity (e.g., detecting sleep and wake states). Accordingly, the microcontroller  920  responds by adjusting the various pacing parameters (such as rate, A-V Delay, V-V Delay, etc.) at which the atrial and ventricular pulse generators  922  and  924  generate stimulation pulses. 
     While shown as being included within the device  800 , it is to be understood that a physiologic sensor  970  may also be external to the device  800 , yet still be implanted within or carried by the patient. Examples of physiologic sensors that may be implemented in conjunction with the device  800  include sensors that sense respiration rate, pH of blood, ventricular gradient, oxygen saturation, blood pressure and so forth. Another sensor that may be used is one that detects activity variance, wherein an activity sensor is monitored diurnally to detect the low variance in the measurement corresponding to the sleep state. For a more detailed description of an activity variance sensor, the reader is directed to U.S. Pat. No. 5,476,483 (Bornzin et al.), which patent is hereby incorporated by reference. 
     The one or more physiologic sensors  970  may optionally include one or more of components to help detect movement (via, e.g., a position sensor or an accelerometer) and minute ventilation (via an MV sensor) in the patient. Signals generated by the position sensor and MV sensor may be passed to the microcontroller  920  for analysis in determining whether to adjust the pacing rate, etc. The microcontroller  920  may thus monitor the signals for indications of the patient&#39;s position and activity status, such as whether the patient is climbing up stairs or descending down stairs or whether the patient is sitting up after lying down. 
     The device  800  additionally includes a battery  976  that provides operating power to all of the circuits shown in  FIG. 9 . For a device  800  which employs shocking therapy, the battery  976  is capable of operating at low current drains (e.g., preferably less than 10 μA) for long periods of time, and is capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse (e.g., preferably, in excess of 2 A, at voltages above 200 V, for periods of 10 seconds or more). The battery  976  also desirably has a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, the device  800  preferably employs lithium or other suitable battery technology. 
     The device  800  can further include magnet detection circuitry (not shown), coupled to the microcontroller  920 , to detect when a magnet is placed over the device  800 . A magnet may be used by a clinician to perform various test functions of the device  800  and to signal the microcontroller  920  that the external device  954  is in place to receive data from or transmit data to the microcontroller  920  through the telemetry circuit  964 . 
     The device  800  further includes an impedance measuring circuit  978  that is enabled by the microcontroller  920  via a control signal  980 . The known uses for an impedance measuring circuit  978  include, but are not limited to, lead impedance surveillance during the acute and chronic phases for proper performance, lead positioning or dislodgement; detecting operable electrodes and automatically switching to an operable pair if dislodgement occurs; measuring respiration or minute ventilation; measuring thoracic impedance for determining shock thresholds; detecting when the device  800  has been implanted; measuring stroke volume; and detecting the opening of heart valves, etc. The impedance measuring circuit  978  is advantageously coupled to the switch  926  so that any desired electrode may be used. 
     In the case where the device  800  is intended to operate as an implantable cardioverter/defibrillator (ICD) device, it detects the occurrence of an arrhythmia, and automatically applies an appropriate therapy to the heart aimed at terminating the detected arrhythmia. To this end, the microcontroller  920  further controls a shocking circuit  982  by way of a control signal  984 . The shocking circuit  982  generates shocking pulses of low (e.g., up to 0.5 J), moderate (e.g., 0.5 J to 10 J), or high energy (e.g., 11 J to 40 J), as controlled by the microcontroller  920 . Such shocking pulses are applied to the patient&#39;s heart H through, for example, two shocking electrodes and as shown in this embodiment, selected from the left atrial coil electrode  826 , the RV coil electrode  832  and the SVC coil electrode  834 . As noted above, the housing  900  may act as an active electrode in combination with the RV coil electrode  832 , as part of a split electrical vector using the SVC coil electrode  834  or the left atrial coil electrode  826  (i.e., using the RV electrode as a common electrode), or in some other arrangement. 
     Cardioversion level shocks are generally considered to be of low to moderate energy level (so as to minimize pain felt by the patient), be synchronized with an R-wave, pertain to the treatment of tachycardia, or some combination of the above. Defibrillation shocks are generally of moderate to high energy level (i.e., corresponding to thresholds in the range of 5 J to 40 J), delivered asynchronously (since R-waves may be too disorganized), and pertaining to the treatment of fibrillation. Accordingly, the microcontroller  920  is capable of controlling the synchronous or asynchronous delivery of the shocking pulses. 
     As mentioned above, the device  800  may include one or more components that provide RF-powered functionality as taught herein. For example, the telemetry circuit  964  may include a communication circuit along with a detection circuit or a verification circuit as taught herein. Here, the power to be coupled to the communication circuit may be obtained from the battery  976  via a power path  940 . Also, one or more of the sense circuit  944 , the sense circuit  946 , the sensors  970 , or the data acquisition system  952  may acquire information that is to be sent through the use of an RF-powered communication circuit. The data described above may be stored in the data memory  960 . In addition, the microcontroller  920  (e.g., a processor providing signal processing functionality) also may implement or support at least a portion of the processing functionality discussed herein. 
     It should be appreciated that various modifications may be incorporated into the disclosed embodiments based on the teachings herein. For example, the structure and functionality taught herein may be incorporated into types of devices other than the specific types of devices described above. In addition, different types of detection circuits or verification circuits may be employed in different embodiments. 
     Also, the circuitry discussed herein may be implemented in different ways in different embodiments. For example, in some embodiments, a rectification circuit is implemented using at least one conventional diode rectifier. In other embodiments, a rectification circuit is implemented using a synchronous rectifier. In a typical case, a synchronous rectifier employs field effect transistors (FETs) to rectify the RF alternating current (AC) signal by steering the alternating current into direct current (DC). The resulting signal may then be filtered using conventional filtering. 
     It should be appreciated from the above that the various structures and functions described herein may be incorporated into a variety of apparatuses (e.g., a stimulation device, a lead, a monitoring device, a satellite device, etc.) and implemented in a variety of ways. Different embodiments of such an apparatus may include a variety of hardware and software processing components. In some embodiments, hardware components such as processors, controllers, state machines, logic, or some combination of these components, may be used to implement the described components or circuits. 
     In some embodiments, code including instructions (e.g., software, firmware, middleware, etc.) may be executed on one or more processing devices to implement one or more of the described functions or components. The code and associated components (e.g., data structures and other components used by the code or used to execute the code) may be stored in an appropriate data memory that is readable by a processing device (e.g., commonly referred to as a computer-readable medium). 
     Moreover, some of the operations described herein may be performed by a device that is located externally with respect to the body of the patient. For example, an implanted device may send raw data or processed data to an external device that then performs the necessary processing. 
     The components and functions described herein may be connected or coupled in many different ways. The manner in which this is done may depend, in part, on whether and how the components are separated from the other components. In some embodiments some of the connections or couplings represented by the lead lines in the drawings may be in an integrated circuit, on a circuit board or implemented as discrete wires or in other ways. 
     The signals discussed herein may take various forms. For example, in some embodiments a signal may comprise electrical signals transmitted over a wire, light pulses transmitted through an optical medium such as an optical fiber or air, or RF waves transmitted through a medium such as air, and so on. In addition, a plurality of signals may be collectively referred to as a signal herein. The signals discussed above also may take the form of data. For example, in some embodiments an application program may send a signal to another application program. Such a signal may be stored in a data memory. 
     Moreover, the recited order of the blocks in the processes disclosed herein is simply an example of a suitable approach. Thus, operations associated with such blocks may be rearranged while remaining within the scope of the present disclosure. Similarly, the accompanying method claims present operations in a sample order, and are not necessarily limited to the specific order presented. 
     Also, it should be understood that any reference to elements herein using a designation such as “first,” “second,” and so forth does not generally limit the quantity or order of those elements. Rather, these designations may be used herein as a convenient method of distinguishing between two or more different elements or instances of an element. Thus, a reference to first and second elements does not mean that only two elements may be employed there or that the first element must precede the second element in some manner. Also, unless stated otherwise a set of elements may comprise one or more elements. In addition, terminology of the form “at least one of A, B, or C” or “one or more of A, B, or C” or “at least one of the group consisting of A, B, and C” used in the description or the claims means “A or B or C or any combination of these elements.” 
     As used herein, the term “determining” encompasses a wide variety of actions. For example, “determining” may include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining, and the like. Also, “determining” may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory), and the like. Also, “determining” may include resolving, selecting, choosing, establishing, and the like. 
     While certain embodiments have been described above in detail and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive of the teachings herein. In particular, it should be recognized that the teachings herein apply to a wide variety of apparatuses and methods. It will thus be recognized that various modifications may be made to the illustrated embodiments or other embodiments, without departing from the broad scope thereof. In view of the above it will be understood that the teachings herein are intended to cover any changes, adaptations or modifications which are within the scope of the disclosure.