Abstract:
A system is provided for optically communicating with an implantable device. In one embodiment, the system includes an implantable device having a large memory and an external unit which downloads information from the memory for analysis and display. The implantable device includes a light-emitting diode (LED) and a modulator for driving the LED. Although various frequencies can be used, frequencies which experience relatively little attenuation through body tissue are presently preferred. The external device includes a photomultiplier tube (PMT) and a demodulator for equalizing and demodulating the detection signal produced by the PMT in response to detected light. A high bandwidth channel (perhaps as much as  500  Mbits/sec) is created by these components. This channel advantageously allows for a substantially reduced download time.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to wireless communication systems for devices implanted in the body, and more particularly to optical communication between an implanted device and a device external to the body.  
           [0003]    2. Description of the Related Art  
           [0004]    Implantable devices have become a standard method of treating various medical conditions, many of which relate to the heart. Examples of devices which are routinely implanted include pacemakers, defibrillators, and nerve stimulators. These devices and others which have not yet become routine (such as implanted personal identification chips) are being provided with large memories for storing vast amounts of data. In the case of medical devices, this data may include physiological data such as the electrogram (electrical waveform at the electrodes), instantaneous heart rate, blood pressure, volume pumped, body temperature, etc., and configuration data such as mode of operation, amplifier sensitivity, filter bandwidth, and error messages. Often the device stores data that has been collected over a period of hours or days. This data is periodically retrieved by a doctor to monitor the patient&#39;s condition and to monitor the device&#39;s status. In response, the doctor might re-program the device for a different mode of operation, sensitivity setting, etc..  
           [0005]    A method is needed to retrieve this data rapidly. The retrieval needs to be rapid so as to minimize the inconvenience to the patient who will usually have to remain in the doctor&#39;s office for the data retrieval process. To download four megabytes of medical device data, for example, at 20 Kbit/s would take nearly a half-hour—an undesirably long time for both the patient and medical professional or technician.  
           [0006]    One method for data retrieval is the use of electromagnetic coupling between a pair of coils. One coil is excited to induce a current in the other. Modulation of the excitation signal can be detected in the induced current, and so communication is achieved. The problem with this is bandwidth. The coils each have a self-inductance which acts to attenuate high frequency signals, so that the bandwidth of communications is limited.  
           [0007]    Another method for data retrieval is to provide a direct electrical connection. A wire connected to the implanted device is passed directly through the skin and coupled to the external device. Inherent with this technique is increased discomfort and increased risk of infection.  
           [0008]    Thus, another method is needed to transfer a large amount of data quickly from the implanted device to the external device with minimal discomfort.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, there is provided herein a system for communicating between an implantable device and an external device. In one embodiment, the system includes an implantable device having a large memory and an external unit which downloads information from the memory for analysis and display. The implantable device includes a light-emitting diode (LED) and a modulator for driving the LED. The LED emits a modulated light signal representing the data that is stored in memory. One light frequency range which may be used is 4.3×10 14 -5.0× 14  Hz, as body tissue exhibits good transmission in this range. The external device includes a photo-multiplier tube (PMT) for detecting and amplifying the modulated light signal, and a demodulator for equalizing and demodulating the detection signal produced by the PMT in response to modulated light.  
           [0010]    These components will support a high bandwidth optical channel capable of carrying as much as 500 Mbits or more, and thereby provide for a substantially reduced data retrieval time. The implantable device may further include a receiver coil which has currents induced in response to a communication signal from the external device. A power converter may be coupled to the receiver coil to convert the induced currents into energy for powering the LED.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 shows an implantable medical device having optical telemetry, implanted in an environment within which a high-bandwidth channel would be desirably employed;  
         [0013]    [0013]FIG. 2 is a block diagram of an implantable pacemaker/defibrillator;  
         [0014]    [0014]FIG. 3 is a schematic diagram illustrating communications between an implantable device and an external device;  
         [0015]    [0015]FIG. 4 is a block diagram of portions of an external device;  
         [0016]    [0016]FIG. 5 is a block diagram of a telemetry module which supports an optical communications link;  
         [0017]    [0017]FIG. 6 shows an exemplary configuration of the system; and  
         [0018]    [0018]FIG. 7 shows a second exemplary configuration of the system.  
         [0019]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of examples in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    The following description illustrates the principles of the invention with respect to an implantable pacemaker (“pacer”) and an external device (“programmer”). The invention, however, is directed to an improved telemetry link between any implantable device and any external device configurable to download information from the implantable device. Thus, the invention applies to implantable cardioverter/defibrillators (ICD&#39;s), nerve stimulators, drug delivery devices, or any other implantable device configured to transmit data to an external device.  
         [0021]    Turning now to the figures, FIG. 1 shows a human torso  102  having a heart  104  and an implanted pacer  106 . Also shown is a wand  108  which is an extensible portion of an external programmer  110 . Wand  108  is placed on an exterior surface of torso  102  near to the pacer  106 . In the embodiment shown, pacer  106  is a pacemaker coupled to heart  104  to assist in regulating its operation. In any case, pacer  106  includes a memory for storing data for later retrieval. In the case of a pacemaker, the data may represent measured physiological signals such as cardiac voltages (EKG signals), blood temperatures, oxygen levels, sugar levels, etc.  
         [0022]    Illustratively, programmer  110  is a programmer/analyzer for use by a physician. The programmer/analyzer operates to download information stored in pacer  106  by transmitting signals which place the pacer in a mode for downloading, and thereafter detecting signals sent by the device. Then, under control of the physician or other medical professional, the programmer/analyzer operates to analyze and display the information in a format which allows the physician to diagnose any problems. After performing an analysis, the physician may instruct the programmer/analyzer to adjust operating parameters of pacer  106 . If this is the case, the programmer/analyzer provides new operating parameters to pacer  106 .  
         [0023]    [0023]FIG. 2 is a block diagram of an exemplary pacer  106 . Pacer  106  has a power supply  202  coupled to a microprocessor  204 . Power supply  202  provides support to all the devices shown in FIG. 2 through connections not shown. Microprocessor  204  is coupled to a memory  206 , a first interval timer  208 , and a second interval timer  210  via an I/O (input/output) bus  211 . Microprocessor  204  is also coupled to control an atrium sensor/stimulator  212  and a ventricle sensor/stimulator  214 , each of which may be coupled to the heart by flexible leads. Finally, microprocessor  204  is coupled to a telemetry module  218  to communicate with programmer  106 .  
         [0024]    Microprocessor  204  preferably is programmable and operates according to a program stored in a nonvolatile memory. The program often is parameterized—i.e. one or more of the operations the microprocessor performs is alterable by setting a parameter. For example, the microprocessor may be programmed to periodically trigger atrium stimulator  212 . One of the parameters for this operation might be a value specifying the rate at which the stimulator is triggered. The parameters may be provided to microprocessor  204  via telemetry module  218  and stored in memory  206 .  
         [0025]    Pacer  106  in FIG. 2 uses first interval timer  208  to determine the delay between trigger signals applied to atrium stimulator  212  and ventricle stimulator  214 . Further, second interval timer  210  measures the time since the last heartbeat sensed by the atrium sensor/stimulator  212  or ventricle sensor/stimulator  214 . When either timer elapses, the elapsed timer asserts an interrupt to microprocessor  204  to notify microprocessor  204  that the set amount of time has passed. Microprocessor  204  determines the source of the interrupt and takes the appropriate action. For example, if a maximum time has elapsed since the last heartbeat, microprocessor  204  might trigger atrium sensor/stimulator  212 .  
         [0026]    Microprocessor  204  preferably also monitors one or more physiological signals. For example, microprocessor  204  may detect cardiac voltage signals via atrium sensor  212  and/or ventricle sensor  214 . Blood pressure, body temperature, and adaptive configuration data may also be monitored. These signals preferably are logged in memory  206  for later retrieval by programmer  110 . Memory  206  preferably is large enough to store a variety of physiological signals that are monitored over a period of several days. This amount of data may comprise several megabytes of data. Memory  206  preferably is implemented as dynamic random access memory (DRAM) or other suitable memory type.  
         [0027]    Atrium sensor/stimulator  212  is an interface circuit between microprocessor  204  and a heart lead coupled to an atrium of the heart. Similarly, ventricle sensor/stimulator  214  is an interface circuit between microprocessor  204  and a heart lead that is coupled to a ventricle of the heart. These interface circuits are configured to apply a customized electrical energy pulse to the respective region of the heart in response to a trigger signal from microprocessor  204 . Interface circuits  212 ,  214  may also be configured to measure cardiac voltage signals from the electrodes so that microprocessor  204  can monitor the performance of the heart. The microprocessor  204  may store the cardiac waveforms (or “electrograms”) in memory for subsequent retrieval by a medical technician.  
         [0028]    Telemetry module  218  may be designed to be activated by programmer  110  when wand  108  enters into proximity with pacer  106 . This event causes telemetry module  218  to be activated and to notify microprocessor  204  of an incoming communication. Microprocessor  204  monitors the incoming communication from programmer  110  and stores programming data or parameters, and responds to any requests. For example, one request might be to transfer the data from memory  206  to programmer  110 . In this case, microprocessor  204  provides the data from memory  206  to telemetry module  218  for transferal to programmer  110 .  
         [0029]    [0029]FIG. 3 is a schematic diagram of the communications channels employed by pacer  106  and programmer  110 . A wand transmitter  302  provides a communication signal which is transmitted to a pacer receiver  304  through body tissues  306 . This communication signal, for example, might represent a programmer request for the pacer  106  to transmit data. This technique using a pair of coils is well known to those of ordinary skill in the art. An example of this technique is illustrated in U.S. Pat. No. 5,314,453, which is hereby incorporated by reference as though completely set forth herein.  
         [0030]    To provide a download of a substantial amount of data in as short a time as possible from pacer  106  to programmer  110 , a high bandwidth connection in the reverse direction (i.e. from the pacer to the programmer) is desired. This high-bandwidth connection comprises a pacer transmitter  308  which transmits a modulated light signal to a wand receiver  310  through body tissues  306 . It is contemplated that wand transmitter  302  and implant receiver  304  are coils that communicate via a shared inductive coupling. Thus one embodiment uses an inductive coupling communications link for programmer  110  to transmit data and commands to pacer  106 , and an optical communications link to transmit data and status information from pacer  106  to programmer  110 . Alternatively, an optical link could be used to communicate in both directions.  
         [0031]    It is contemplated that implant transmitter  308  includes an LED that emits light in the infrared (&lt;4.3×10 14  Hz), visible (4.3×10 14 -7.3×10 14  Hz) or ultraviolet (&gt;7.3×10 14  Hz) frequency ranges, and that wand receiver  310  includes a light sensor sensitive to light emitted by implant transmitter  308 . The various frequencies (colors) of light experience differing amounts of attenuation by body tissues  306 . The light emitted by implant transmitter  308  preferably experiences relatively small losses while passing through body tissues  306 . Experiments have been done using a light frequency of 5.42×10 14  Hz (green light), but somewhat lower frequencies such as 4.3×10 14 -5.0×10 14  Hz may be preferred, and 4.5×10 14 -4.7×10 14  Hz may be more preferred.  
         [0032]    [0032]FIG. 4 is a block diagram of portions of one embodiment of a programmer  110 . Programmer  110  includes a microprocessor  402 , a modulator  404 , a transmit coil  406 , a light sensor  408 , and a demodulator  410 . Microprocessor  402  accepts and responds to user input (via controls not shown) and initiates communications with pacer  106 . For example, if a user requests a download of data from pacer  106  to programmer  110 , microprocessor  402  formulates a command signal, and sends the signal to modulator  404 . Modulator  404  converts the command signal into a modulated signal for driving transmit coil  406 . The signal driving the transmit coil produces a changing magnetic field which induces a current in a receive coil in pacer  106 . Pacer  106  processes the induced current in a manner described further below. Pacer  106  can transmit signals to programmer  110  by modulating a light signal. The modulated light signal may be greatly attenuated by body tissues. When enabled, light sensor  408  detects and amplifies the modulated light signal to produce a detection signal. Demodulator  410  demodulates the detection signal and converts it into the data transmitted by the pacer  106 . Demodulator  410  then provides the data to microprocessor  402  for eventual analysis and display.  
         [0033]    Because the optical signal may be greatly attenuated (i.e. reduced in intensity) by body tissue, light sensor  408  preferably is highly sensitive and must be protected from ambient light. This protection may be provided in the form of an enable signal which is asserted only when the ambient light is blocked, e.g. when the wand is pressed flat against the torso. In one implementation, the enable signal may be asserted when a mechanical switch is closed upon pressing the wand against the torso. In another implementation, the enable signal may be asserted when a photo-transistor adjacent to the light sensor  408  detects that the light intensity has fallen below a predetermined threshold.  
         [0034]    One light sensor which is contemplated for use in wand  108  is a PMT (photo-multiplier tube) such as R5600-01 PMT from Hamamatsu Corporation. PMT&#39;s are well known and widely available, and are able to detect single photons while maintaining a low noise level. This light sensor is advantageously sensitive to light in the frequency range from 4.3×10 14  to 20.0×10 14  Hz.  
         [0035]    In another embodiment, light sensor  408  comprises a photo-diode which may be robust enough to withstand ambient light and sensitive enough to detect attenuated light emissions from the pacer. This light sensor advantageously does not require an enable signal and the means for generating the enable signal.  
         [0036]    [0036]FIG. 5 shows a block diagram of an illustrative telemetry module  218  of pacer  106 . Telemetry module  218  comprises an implant receiver coil  502 , a current sensor  504 , a demodulator  506 , a power converter  508 , a modulator  510 , and a light source  512 . A communication signal from wand  108  induces a current in coil  502 . Current sensor  504  detects the induced currents and produces an amplified detection signal representative of the communication signal sent by wand  108 . Demodulator  506  demodulates the communication signal to obtain the commands, data and/or parameters being sent by wand  108 . Microprocessor  204  processes the demodulated signal and determines an appropriate response. For example, if the transmitted data represents a download request, microprocessor  204  will initiate a download of the requested data stored in memory  206 , i.e. the microprocessor will cause data from memory  206  to be supplied to modulator  510 .  
         [0037]    Referring still to FIG. 5, power converter  508  is coupled to implant receiver coil  502  to convert the induced currents into stored energy. As modulator  510  converts the data from microprocessor  204  into a modulated signal, it uses stored energy from power converter  508  to drive light source  512  in accordance with the modulated signal. Light source  512  may be an LED (light emitting diode) which emits light with a frequency suitable to pass through the body to the wand. Preferably the LED emits light with a frequency between 4.3×10 14  and 5.0×10 14  Hz, but other frequencies may be used as well. The light emitted is modulated in accordance with the modulated signal from modulator  510 . The modulated light may be detected and demodulated by wand  108  to recover the data stored in memory  206  as described above.  
         [0038]    In one embodiment, power converter  508  employs a full-wave rectifier to convert the currents induced in coil  502  into a unidirectional charging current. The power converter also includes a bank of switching capacitors to be charged by the unidirectional charging current and thereafter step up the voltage to charge an energy storage capacitor. Current sensor  504  may be configured to detect the induced currents by sensing the voltage drop across one or more diodes in the full-wave rectifier.  
         [0039]    In another embodiment, the LED is powered by power supply  202  of pacer  106 . Power converter  508  may be included for the purpose of recharging power supply  202 .  
         [0040]    Various modulation schemes may be employed for the communication channels. The wand-to-implant communications channel may use pulse-width modulation (PWM), frequency-shift keying (FSK), or other suitable techniques. The implant-to-wand communications channel may also employ any suitable techniques such as pulse-code modulation (PCM) and simplex signaling. Both channels may employ channel coding for error detection, timing, and/or constraining power usage. Such channel coding techniques are known to those of ordinary skill in the art. It is noted that light sensor  408  may be configured to generate a detection signal which is proportional to the light intensity, and that consequently both digital and analog amplitude modulation signaling is also supported by the contemplated configuration.  
         [0041]    [0041]FIG. 6 shows an exemplary configuration of wand  108  and pacer  106  shown in cross-section. Wand  108  is pressed against body tissues  306  proximate to the location of pacer  106  and in active communication with pacer  106 . Pacer  106  comprises power supply  202 , electronics module  602 , implant receiver coil  502 , light source  512 , and header  604 . Electronics module  602  includes microprocessor  204 , memory  206 , timers  208 ,  210 , sensor/stimulators  212 ,  214 , current sensor  504 , demodulator  506 , power converter  508 , and modulator  510 . Electronics module  602  and the components it contains may be constructed as a circuit board. Header  604  is a transparent portion of pacer  106  which may include electrical connectors for the heart leads (FIG. 2) and light source  512 . Alternatively, light source  512  may be located in electronics module  602 . As electronics module  602  is normally placed in an opaque portion of pacer  106 , light source  512  is configured to emit light in the direction of the transparent header  604 . A mirror may be located within header  604  to redirect the modulated light toward wand  108 . This mirror may be concave to reduce dispersion of the modulated light signal. For either placement of light source  512 , header  604  may also have a portion of its exterior surface configured as a lens to reduce the dispersion of the modulated light signal. Some of these configurations are described in U.S. Pat. No. 5,556,421, which is hereby incorporated by reference in its entirety.  
         [0042]    Wand  108  illustratively comprises modulator  404 , transmit coil  406 , light sensor  408 , demodulator  410 , ambient light detector  606 , reflective surface  608 , interface module  610 , and user interface  612 . In one embodiment, light sensor  408  is placed near a convergence point of light rays that reflect from reflective surface  608 . Reflective surface  608  is designed to increase the light-gathering ability of wand  108 . Ambient light detector  606  is positioned within the concavity defined by reflective surface  608  and/or adjacent to light sensor  408 . Ambient light detector  606  provides the enable signal discussed in FIG. 4 when a sensitive light sensor  408  is employed. Ambient light detector  606  may be a photo-transistor or photo-diode or any other photo-sensitive device robust enough to withstand anticipated light levels when wand  108  is separated from torso  102 . Interface module  610  may be a line driver/buffer for communications with the rest of programmer  110 , and may further comprise a power supply or converter for powering the electronics of wand  108 . User interface  612  may comprise buttons for user input (e.g. begin download) and lights for user feedback regarding the status of the communications link with the implanted device. Directional lights may also be provided to aid the user in positioning the wand to achieve the highest communications signal-to-noise ratio and the maximum communications rate for downloading information from the memory of the pacer.  
         [0043]    [0043]FIG. 7 shows a second exemplary configuration of wand  108 , in which mechanical switches  702  rather than ambient light detector  606  are used to provide the enable signal of FIG. 4. Mechanical switches  702  are pressure sensitive and positioned on the face of the wand so that when the wand is correctly pressed against the torso, the normally open switches are all closed. Variations on this may be employed so long as the enable signal is only asserted when the light sensor  408  is shielded from excessive light levels. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.