High speed telemetry system using transmission medium as a component of a telemetry link

The telemetry system allows a high speed transfer of digital data at for example 81,920 KHz, by utilizing the titanium can as a component of a telemetry link. The telemetry system includes a transmitter and a receiver that are interconnected by means of the telemetry link. Input data is fed to the transmitter where it is encoded, modulated, and transmitted through the can to the receiver. The can introduces a desired low pass filtering function that complements the signal encoding and modulation process implemented by the transmitter. The transmitter processes a binary data signal provided in Non-Return-to-Zero (NRZ) format. By a series of transformations, the transmitter produces a signal whose spectral properties closely match the telemetry link, including the titanium can. The titanium can becomes a source of controlled inter-symbol-interference (ISI) to be compensated for in the receiver.

FIELD OF THE INVENTION
 The present invention relates generally to cardiac pacemakers, and other
 types of implantable medical devices that can be programmed and/or
 analyzed following implantation using an external diagnostic/programmer
 system. Particularly, the invention relates to a high speed digital
 telemetry system for use in implantable devices. More specifically, the
 present invention relates to an implantable high bit rate telemetry
 transmitter and corresponding external receiver that utilize the
 transmission medium as a component of the telemetry link
 BACKGROUND OF THE INVENTION
 Implantable devices are implanted in a human or animal for the purpose of
 performing a desired function. This function may be purely observational
 or experimental in nature, such as monitoring certain body functions; or
 it may be therapeutic or regulatory in nature, such as providing critical
 electrical stimulation pulses to certain body tissue, nerves or organs for
 the purpose of causing a desired response. Implantable medical devices
 such as pacemakers, perform both observational and regulatory functions,
 i.e., they monitor the heart to ensure it beats at appropriate intervals;
 and if not, they cause an electrical stimulation pulse to be delivered to
 the heart in an attempt to force the heart to beat at an appropriate rate.
 In order for an implantable device to perform its functions at minimum
 inconvenience and risk to the person or animal within whom it is used,
 some sort of noninvasive telemetry means must be provided that allows data
 and commands to be easily passed back and forth between the implantable
 device and an external device. Such an external device, known by a variety
 of names, such as a controller, programmer, or monitor, provides a
 convenient mechanism through which the operation of the implantable device
 can be controlled and monitored, and through which data sensed or detected
 by the implantable device can be transferred out of the implantable device
 to an external (non-implanted) location where it can be read, interpreted,
 or otherwise used in a constructive manner.
 As the sophistication and complexity of implantable devices has increased
 in recent years, the amount of data that must be transferred between an
 implantable device and its accompanying external device or programmer, has
 dramatically increased. This, in turn, has resulted in a search for more
 efficient ways to effectuate such a data transfer at high speed. The
 telemetry must not only transfer the desired data without significant
 error, but it must do so at a high speed while preserving the limited
 power resources of the implanted device.
 Currently, three basic techniques have been used for communicating with an
 implantable device: (1) static magnetic field coupling; (2) reflected
 impedance coupling; and (3) RF coupling. In static magnetic field
 coupling, a static magnetic field is generated externally to the implanted
 device by using a permanent magnet, having sufficient strength to close
 (or open) a magnetic reed switch within the implanted device. While such a
 technique provides a fairly reliable mechanism for turning various
 functions within the implanted device ON or OFF, such as turning the
 telemetry circuits within an implanted device ON only when an external
 telemetry head is positioned a few inches from the implanted device, the
 technique is much too slow for efficiently transferring any significant
 amount of data. Further, for all practical purposes, the static magnetic
 system is mainly useful for transferring commands or data to the implanted
 device, not for transferring data or commands from the implanted device.
 This is because the weight and/or power requirements associated with the
 types of permanent magnets or electromagnets needed to operate a magnetic
 reed switch several inches distant therefrom is incompatible with the
 requirements of most implantable devices.
 In a reflected impedance coupling system, information is transferred using
 the reflected impedance of an internal (implanted) L-R or L-C circuit
 energized by an inductively coupled, external L-R or L-C circuit. Such a
 system is shown, for example, in U.S. Pat. No. 4,223,679. While such a
 system uses little or no current to transmit information, the speed at
 which the information is transferred is quite limited. The external
 circuit uses an RF (radio frequency) magnetic field carrier. In the cited
 patent, a voltage controlled oscillator (VCO), in the implanted device, is
 controlled by the signal to be telemetered. The VCO, in turn, varies the
 impedance that is reflected. If the signal controlling the VCO is a binary
 digital signal (having two possible values, e.g., a binary "1" and a
 binary "0"), this signal encodes the VCO so that the VCO varies from one
 frequency (representing a binary "1") to another frequency (representing a
 binary "0"). This technique is known as frequency shift keying (FSK). Each
 bit duration, i.e., the time in which the binary digit (bit) is expressed,
 requires a number of carrier cycles. Hence, the bit rate cannot generally
 be much higher than 10% to 30% of the VCO center frequency. On the other
 hand, the RF carrier frequency cannot be too high because of the metal
 enclosure of the implanted device acts as a low pass, single pole filter
 having an upper cut-off frequency of between 10-30 kHz. Further, the
 external oscillator L-C circuit typically has a Q (quality factor) of 20
 to 50, meaning that the useful modulation bandwidth is limited to around 2
 to 5 percent of the RF carrier frequency. This means that a 36 kHz carrier
 is typically only able to transmit data at a data rate of from 72 to 540
 bits per second (bps). Such a rate is generally considered inadequate for
 modern implantable devices, which devices may have thousands of bits of
 data to be transmitted.
 In an RF coupled system, information is transferred from a transmitting
 coil to a receiving coil by way of a carrier signal. The carrier signal is
 modulated with the data that is to be transmitted using an appropriate
 modulation scheme, such as FSK or PSK (phase-shift keying for reversing
 the phase of the carrier by 180 degrees). The modulated carrier induces a
 voltage at the receiving coil that tracks the modulated carrier signal.
 This received signal is then demodulated in order to recover the
 transmitted data. Because of the metal enclosure of the implanted device,
 which acts as a low pass filter (attenuating high frequencies), the
 carrier frequency cannot be increased above approximately 10-20 kHz
 without an unacceptable increase in transmitting coil power. Further,
 depending upon the type of modulation/demodulation scheme employed, the
 data or bit rate cannot exceed a prescribed fraction of the carrier
 frequency, without exceeding a specified amount of mutual interference,
 i.e., without being able to reliably distinguish between a modulation that
 represents a binary "1" and modulation that represents a binary "0".
 The maximum data transfer rate (bit rate) at which independent signal
 values can be transmitted over a specified channel without exceeding a
 specified amount of mutual interference is referred to as the "Nyquist
 rate." The maximum allowable Nyquist rate is directly related to the
 bandwidth of the channel through which the data is transferred.
 Conversely, the "Nyquist bandwidth" is that bandwidth required to allow
 independent signal values to be transmitted at a given rate without
 exceeding the specified levels of mutual interference. For example, if the
 bandwidth of the channel through which the data is transferred is W, the
 Nyquist rate (assuming an ideal channel) may be as high as 2W. Stated
 differently, if the data rate is 2W, the Nyquist bandwidth must be at
 least W. Because of these and other limitations, conventional implantable
 devices using RF coupling have generally not been able to transfer data at
 rates in excess of 2-4 kbps. It should be noted that a one-sided bandwidth
 definition is used, namely that a bandwidth W refers to a range of
 frequencies from 0 to W, or from -W to 0. Where a carrier signal having a
 frequency f.sub.c is used, the one-sided bandwidth W refers to a range of
 frequencies from f.sub.c to (f.sub.c +W), or from (f.sub.c -W) to f.sub.c.
 A further problem affecting the rate at which data can be transferred from
 an implantable device is electrical noise and/or EMI (electromagnetic
 interference). In particular, there are at least two primary sources of
 EMI associated with commonly used types of external devices that
 significantly affect the range of carrier frequencies and data rates that
 can be reliably and efficiently (at low power consumption levels) used to
 transfer data in an RF-type system. First, the input power line frequency
 (50-60 Hz) of the external device, and the associated switching magnetic
 fields (e.g., 30 Hz) used with a cathode ray tube (CRT) display,
 frequently used with external devices, create sufficiently large EMI
 harmonics to be troublesome as high as 2-6 kHz. Similarly, the 16 kHz line
 frequency of the horizontal scan of the cathode ray tube (CRT) commonly
 used with many electronic terminals, makes it extremely difficult to
 efficiently use a carrier frequency of 16 kHz or higher. In order to
 minimize the effect of such EMI on the transmission of data from an
 implanted device used in an environment where such interference is
 prevalent, and in order to maximize the speed at which the large amounts
 of data used with modern implantable devices may be transferred, it would
 be preferable to employ a narrow band telemetry channel to filter out as
 much EMI and noise as possible using a carrier signal in the 6-12 kHz
 range, and using a modulation scheme that permits a data bit rate as high
 as possible through such channel.
 A telemetry system that addresses this problems and that presents a
 solution to allow data to be transferred at an acceptably fast rate, e.g.,
 8 kHz, and to also allow the data at this fast rate to be transferred
 through a narrow bandwidth, thereby decreasing the susceptibility of the
 system to EMI and other noise sources is described in U.S. Pat. No.
 4,944,299 to Silvian.
 An additional problem present facing conventional telemetry systems is the
 presence of the titanium can along the telemetry link. Heretofore, this
 problem remains unsolved. The reason for considering the titanium can to
 be highly undesirable is that the titanium limits the bandwidth of the
 channel by attenuating the high frequencies in a manner similar to that of
 a low pass filter. In particular, the higher frequencies are attenuated as
 by a low pass filter with a -3 dB frequency of 10-15 KHz. In the current
 state of the art, this attenuation of higher frequencies causes increasing
 inter-symbol-interference (ISI) as the data rate approaches the cutoff
 frequency. The ISI, in turn, causes distortion of the received signal
 which degrades performance, limits the maximum data rate, or renders
 reliable reception impossible.
 Therefore, there is a great, and still unsatisfied, need for a telemetry
 system that overcomes the problem associated with the presence of the
 titanium can, and that allows for a high data transfer of information
 particularly from the implantable device to the external programmer.
 SUMMARY OF THE INVENTION
 The present invention addresses these and other concerns by providing an
 improved telemetry system. According to a preferred embodiment, the
 telemetry system allows a high speed transfer of digital data at for
 example 81,920 KHz, and further utilizes the transmission medium, such as
 the titanium can as a component of the telemetry link.
 The telemetry system accomplishes this goal without including added new
 components, and without significantly increasing the overall cost of the
 implanted device.
 The foregoing and other features of the present invention are achieved by a
 telemetry system that includes a transmitter and a receiver that are
 interconnected by means of a telemetry link. The transmitter is generally
 comprised of a data encoder, a modulator, and a transmit coil. The
 receiver is generally comprised of a receive coil, an amplifier, a
 band-pass filter, and a demodulator. The telemetry link maintains data
 communication between the transmitter and the receiver 14, and includes
 the transmit coil, the receive coil, and a part of a titanium can that
 houses the transmitter.
 Input data is fed to the transmitter where it is encoded by the encoder,
 modulated by the modulator, and transmitted by the transmit coil, through
 the can, to the receiver. The can introduces a desired low pass filtering
 function, which complements the signal encoding and modulation process
 implemented by the transmitter. The signal transmitted over the telemetry
 link is received by the receive coil, amplified by the amplifier, filtered
 by the band-pass filter, and demodulated by the demodulator.
 The telemetry system can transmit data at a high rate, for example 81,920
 Hz. The transmitter processes a binary data signal provided in
 Non-Return-to-Zero (NRZ) format. By a series of transformations, the
 transmitter produces a signal whose spectral properties closely match the
 telemetry link, including the titanium can. Data rates in excess of those
 possible with the current state of the art are supported by including the
 spectral properties of the titanium can in the transfer function for the
 whole telemetry system. In effect, the titanium can becomes a source of
 controlled inter-symbol-interference (ISI), to be compensated for in the
 receiver.
 The presence of the titanium can in the telemetry link is desirable in that
 it becomes part of the encoding process, and overcomes the bandwidth
 limitations. The telemetry system employs a partial response signaling
 which is combined with the low pass filter characteristic of the titanium
 can.
 The particular channel response employed in the telemetry system 10 is
 termed a (1-D.sup.2) channel, where `D` is the delay operator and
 represents one bit time. The overall (1-D.sup.2) characteristic can be
 obtained by multiplying an input signal by (1-D) and (1+D) in succession.
 This channel response is implemented by using the low pass filter
 characteristic (1+D) of the titanium can, which is preceded by a (1-D)
 function in the modulator. The combined behavior of (1+D)*(1-D) produces
 the desired channel spectrum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 illustrates a telemetry system 10 according to the present
 invention. The telemetry system 10 includes a transmitter 12 and a
 receiver 14 that are interconnected by means of a telemetry link 16. The
 transmitter 12 is generally comprised of a data encoder 18, a modulator
 20, and a transmit coil 22. The receiver 14 is generally comprised of a
 receive coil 30, an amplifier 32, a band-pass filter 34, and a demodulator
 36. The telemetry link 16 maintains data communication between the
 transmitter 12 and the receiver 14. The telemetry link 16 includes the
 transmit coil 22, the receive coil 30, and a part of a titanium can 40
 that houses the transmitter 12. The titanium can 40 will also be referred
 to herein as housing 40.
 Input data (DATA IN) is fed to the transmitter 12 where it is encoded by
 the encoder 18, modulated by the modulator 20, and transmitted by the
 transmit coil 22, through the can 40, to the receiver 14. The can 40
 introduces a desired low pass filtering function, which complements the
 signal encoding and modulation process implemented by the transmitter 12.
 The signal transmitted over the telemetry link 16 is received by the
 receive coil 30, amplified by the amplifier 32, filtered by the band-pass
 filter 34, and demodulated by the demodulator 36.
 The telemetry system 10 can transmit data at a high rate, for example
 81,920 Hz. The transmitter 12 processes a binary data signal provided in
 Non-Return-to-Zero (NRZ) format, although any other suitable binary format
 may be used. NRZ signals consist of two levels, with a first level
 corresponding to a binary "1" and a second level corresponding to a binary
 "0". By a series of transformations explained more fully below, the
 transmitter 12 produces a signal whose spectral properties closely match
 the telemetry link 16, including the titanium can 40. Data rates in excess
 of those possible with the current state of the art are supported by
 including the spectral properties of the titanium can 40 in the transfer
 function for the whole telemetry system 10. In effect, the titanium can 40
 becomes a source of controlled inter-symbol-interference (ISI) to be
 compensated for in the receiver 14, as it will be explained later in
 greater detail.
 The presence of the titanium can 40 in the telemetry link 16 is desirable
 in that it becomes part of the encoding process, and overcomes the
 bandwidth limitations. The telemetry system employs a partial response
 signaling which is combined with the low pass filter characteristic of the
 titanium can 40.
 Partial response channels, which employ partial response signaling, allow
 controlled ISI by incorporating the effects of adjacent symbol
 interactions into the encoding and decoding process. For example, if it is
 known that a portion of an adjacent symbol will spread over into the
 current symbol space, it is possible to subtract this effect at the
 receiver end. The telemetry system 10 takes advantage of the partial
 response signaling feature which is offered by the titanium can 40 and
 which is incorporated into the overall transfer function of the telemetry
 system 10.
 The particular channel response employed in the telemetry system 10 is
 termed a (1-D.sup.2) channel, where `D` is the delay operator and
 represents one bit time. The overall (1-D.sup.2) characteristic can be
 obtained by multiplying an input signal by (1-D) and (1+D) in succession.
 This channel response is implemented by using the low pass filter
 characteristic (1+D) of the titanium can 40, which is preceded by a (1-D)
 function in the modulator 20. The combined behavior of (1+D)*(1-D)
 produces the desired channel spectrum.
 With reference to FIG. 1, the data encoder 18 divides the binary input
 signal (DATA IN) by (1-D.sup.2) to simplify the decoding, by the receiver
 14, of the signal that has been processed by the transmitter 12 and the
 can 40. Simplification occurs because the combined effect of the encoder
 (i.e., division by 1-D.sup.2), and the rest of the channel (multiplication
 by 1-D.sup.2), results in a signal at the receiver 14, which after
 suitable equalization, closely resembles the original input signal. That
 is, the transfer function of the telemetry system 10 between the original
 data source and the receiver 14 is unity or close to unity.
 The modulator 20 modifies the signal encoded by the encoder 18 by
 multiplying it with a (1-D) factor in preparation for transmission through
 the telemetry link 16. The transmit coil 22 has a ferrite core with low
 impedance, to support higher transmission rates in accordance with the
 present invention. For illustration purpose only, the coil 22 is used for
 a 81,920 bps transmission rate.
 As used herein, a transmission medium includes any material in the
 telemetry link 16, which conducts the signal between the transmit coil 22
 and the receive coil 30. This includes portions of the body in which the
 device is implanted, air, and the titanium can 40. Optionally, the
 transmission medium can include any material or component used by the
 receiver 14 to change the reception characteristic of the received signal.
 According to the present invention, the treatment of the titanium can 40 as
 a component of the telemetry link 16 allows the telemetry link 16 to
 operate at considerably higher data rates by combining the filtering
 characteristic (or spectral response) of the can 40 with the particular
 partial response function chosen. While in a conventional telemetry system
 the presence of the titanium can imposes a bandwidth limitation on higher
 data rates because it produces distortion associated with ISI, in the
 present invention, however, the combination of the spectral response of
 the titanium can 40 with the modulator function satisfies the partial
 response requirements for the overall channel which is designed to operate
 properly with controlled ISI.
 FIG. 2 illustrates an exemplary implementation of the transmitter 12
 according the present invention. In this embodiment, the encoder 18 is
 comprised of a logic gate 50, such as an exclusive OR, which is connected
 at one of its inputs to the input data. The output of the logic gate 50 is
 fed to a 2T-delay circuit 55, for introducing a delay of 2T, and therefrom
 to the other input of the logic gate 50. As used herein, T refers to the
 period of the data rate, for example 1/81,920 Hz.
 The modulator 20 is comprised of 1T-delay circuit 60 that introduces a
 delay of T to the signal at the output of the encoder 18, and that feeds
 the delayed signal to the negative terminal of a summer (or a summation
 circuit) 65 configured as a subtractor. The output of the encoder 18 is
 fed to the positive terminal of the summer 65. The overall effect of the
 modulator 20 is to provide the desired (1-D) response function. The signal
 at the output of the summer 65 is amplified by an amplifier 70, and is
 transmitted over the telemetry link 16 via the transmit coil 22.
 As explained above, the inclusion of the titanium can 40 as a component of
 the telemetry link 16 adds a desired low pass filter characteristic whose
 spatial spectrum is illustrated in FIG. 4, and which is closely
 approximated by a (1+D) response function. This response function can also
 be represented mathematically by the following equation (1):
EQU h(t)=.delta.(t)+.delta.(t-T), (1)
 where h(t) is the response function in FIG. 4, .delta.(t) is the data bit
 at time t, and .delta.(t-T) is the data bit at time (t-T).
 Equation (1) can be expressed in the frequency domain by the following
 equations (2) and (3):
EQU H(.function.))=1+e.sup.-j2.pi..function.T (2)
EQU .vertline.H(f).vertline.=2 cos.pi.fT (3)
 It can be seen that the cosine function of equation 3 can be represented by
 the graph of FIG. 4.
 FIG. 4A is a diagram of a circuit 82 that provides an equivalent (1+D)
 function to that provided by the can 40, and approximated by the cosine
 function of FIG. 4. The circuit 82 includes a 1T-delay circuit 83 and a
 summer 84. The summer 84 adds the data bit .delta.(t) at time t and the
 data bit .delta.(t-T) at time (t-T), to generate the response function
 h(t) expressed by equation (1) above.
 FIG. 5 illustrates the operation of the telemetry system 10 by considering
 an exemplary data string (DATA IN), and tracking its transformation along
 various points. The input data string (DATA IN) is a string of binary or
 digital data that switches between two levels +1 and -1 representing a
 binary "1" or binary "0", respectively.
 The encoder 18 (FIG. 2) divides the DATA IN by (1-D.sup.2). The encoder
 output at point A is the mod 2 addition of the DATA IN, and the signal at
 point B is the encoder output delayed by two clocks cycles. The operation
 of the encoder 18 on the DATA IN assures that the signal output at point A
 is the original DATA IN divided by (1-D.sup.2).
 The modulator 20 receives its input from the encoder 18 and generates a
 (1-D) function which is applied to the input signal. The signal at point
 C, is a one clock cycle-delayed version of the signal at point A. The
 signal at point D is the result of subtracting the signal at point C from
 the signal at point A, and is a three-level signal where "short steps",
 e.g. "0" to "2", "0" to "-2", "2" to "0", or "-2" to "0" are to be
 interpreted as binary "1s" and "long steps", e.g. "-2" to "2" and "2" to
 "-2" represent binary "0s". The absence of any step also represents a
 binary "0".
 The signal at point D is amplified by the amplifier 70 without changing its
 characteristics, and is thereafter transmitted at point E, via the
 transmit coil 22 and the can 40 to the receiver 14. The can 40 generates a
 (1+D) function which is applied to the signal at point D as amplified.
 At the receiver 14, the signal at point E is amplified by the amplifier 32
 without changing its characteristics, and is passed through the band-pass
 filter 34 that corrects for the (1+D) factor introduced by the can 40. The
 signal at the output of the band-pass filter 34, at point F, is
 demodulated by the demodulator 36 by multiplying it with a (1-D.sup.2)
 factor. The signal at the output of the demodulator 36, at point F,
 becomes a substantial replicate of the input signals DATA IN.
 The receive coil 30 is designed to pick up the signals produced by the
 transmit coil 22 (or coil 120 in FIG. 8) after those signals have passed
 through the can 40. The received signals are attenuated by the loosely
 coupled coils 22, 20 and by the can 40, and are amplified by the amplifier
 32 to a level suitable for introduction into the band-pass filter 34.
 In a partial response channel, the filter 34 normally serves two functions.
 The first function is the attenuation of high frequency noise which can
 otherwise produce errors in the receiver 14, and the second function is to
 equalize or "shape" the signal so that any distortion introduced by the
 telemetry link 16 are suppressed.
 The properly equalized signal which is available at the output of the
 filter 34 is applied to the demodulator 36, whose role is to convert the
 input signal into a series of digital values. The receiver 14 generates a
 sequence of binary digital signals which represent the original,
 transmitted data (DATA IN).
 The overall function of the telemetry system 10, in accordance with the
 present invention, is to transmit the binary signals originating in an
 implanted device to a suitable configured receiver 14 which restores the
 original binary signals. Having successfully recreated the original data
 at the remote receiver location, the data may then be further processed or
 interpreted, as desired.
 FIG. 3 illustrates a transmitter 80 according to an alternative embodiment
 of the present invention. The transmitter 80 is generally similar in
 function and design to the transmitter 12 of FIG. 2. In the transmitter
 80, the summation circuit 65, the amplifier 70 and the transmit coil 22
 are replaced by a simpler design comprised of the transmit coil 22. It
 should be clear to a person of ordinary skill in the field that other
 transmitter designs are also operable with the present invention.
 FIG. 6 illustrates an exemplary clock generation circuit 90 that provides
 the clock signals to the transmitter 12 of FIG. 1. The clock generation
 circuit 90 provides a stable reference clock for the operation of the
 encoder 18, the modulator 20, and the transmit coil 22, to enable
 operation at 81,920 Hz, in accordance with the present invention. Timing
 reference signals at other frequencies are also provided. The clock
 generation circuit 90 takes into account the fact that most implantable
 devices already have a 32,768 Hz crystal-controlled oscillator.
 The clock generation circuit 90 includes a crystal 92 connected across a
 32,768 Hz oscillator 93. The output of the oscillator 93 is connected to a
 phase detector 94, which, in turn, is connected to a low pass filter 95.
 The output of the low pass filter 95 is connected to a voltage controlled
 oscillator (VCO) 96. The output of the VCO 86connected to the phase
 detector 94 via a divider 97, and to the input of the telemetry circuit 10
 of FIG. 1, via a divider 98, in order to provide the desired 81,920 Hz
 clock signal to the telemetry circuit 10.
 The telemetry system 10 of FIG. 1 requires a bandwidth of 40,960 Hz (1/2
 81,920 Hz), and has a peak distribution at 20,480 Hz, with no DC response.
 The current consumption of the telemetry system 10 may exceed the design
 expectations for a particular application, in which event a backup
 telemetry mode might be useful.
 FIGS. 7 and 8 illustrate such a telemetry system 100 which is capable of
 transmitting data in one of two distinct modes: a high data rate mode
 operating at 81,920 Hz, and a lower data rate mode operating at 8,192 Hz.
 To accomplish this modal duality, the telemetry system 100 is provided
 with a transmitter 105 and a receiver 14. The receiver 14 is generally
 similar to that of the telemetry system 10.
 The transmitter 105 is substantially similar to the transmitter 12 of the
 telemetry system 10 of FIGS. 1 and 2, but additionally includes a transmit
 coil drive circuit 111 and another coil 120. The transmit coil drive
 circuit 111 selects and drives one of the two coils 22, 120 to produce the
 signal which is coupled into the telemetry link 16. The selection of the
 coil 22, 120 is based on the requirement of the telemetry system 100 to
 transmit at the lower 8,192 bps data rate or at the higher 81,920 bps data
 rate.
 As described above, the coil 22 has a ferrite core with low impedance, for
 supporting the higher transmission rates, such as 81,920 bps in accordance
 with the present invention. The coil 120 is used for transmission and
 reception of signals at lower transmission rates, such as 8,192 bps, and
 has a mumetal core and high impedance, to provide a low power transmission
 path for data at lower rates.
 FIG. 9 shows a simplified functional block diagram of an ICD device 125,
 and FIG. 10 shows a simplified functional block diagram of a dual-chamber
 pacemaker 127, which incorporate the telemetry system 10 of the present
 invention. It should also be noted that in some instances the functions of
 an ICD and a pacemaker may be combined within the same stimulation device.
 However, for teaching purposes, the devices will be described as separate
 stimulation devices.
 It is the primary function of an ICD device 125 to sense the occurrence of
 an arrhythmia, and to automatically apply an appropriate electrical shock
 therapy to the patient's heart 126 aimed at terminating the arrhythmia. To
 this end, the ICD device 125, as shown in the functional block diagram of
 FIG. 9, includes a control and timing circuit 128, such as a
 microprocessor, state-machine or other such control circuitry, that
 controls a high output charge generator (or pulse generator) 129. The high
 output charge generator 129 generates electrical stimulation pulses of
 moderate or high energy (corresponding to cardioversion or defibrillation
 pulses, respectively), e.g., electrical pulses having energies of from 1
 to 10 joules (moderate) or 11 to 40 joules (high), as controlled by the
 control/timing circuit 128.
 Such moderate or high energy pulses are applied to the patient's heart 126
 through at least one lead 130 having at least two defibrillation
 electrodes, such as coil electrodes 138 and 140. The lead 130 preferably
 also includes at least one electrode for pacing and sensitivities, such as
 electrode 132. Typically, the lead 130 is transvenously inserted into the
 heart 126 so as to place the coil electrodes 138 and 140 in the apex of
 the heart 126 and in the superior vena cava, respectively. While only one
 lead 130 is shown in FIG. 9, it is to be understood that additional
 defibrillation leads and electrodes may be used as desired or needed in
 order to efficiently and effectively apply the shock treatment generated
 by the high voltage generator 129 to the patient's heart 126.
 The ICD device 125 also includes a sense amplifier (or detection circuit)
 142 that is coupled to at least one sensing electrode 132. It is the
 function of the sense amplifier 142 to sense the electrical activity of
 the heart 126, such as R-waves which occur upon the depolarization, and
 hence contraction, of ventricular tissue; and P-waves which occur upon the
 depolarization, and hence contraction, of atrial tissue. Thus, by sensing
 R-waves and/or P-waves through the sense amplifier 142, the control/timing
 circuit 128 is able to make a determination as to the rate and regularity
 of the patient's heart beat. Such information, in turn, allows the
 control/timing circuit 128 to determine whether the heart 126 of a patient
 is experiencing an arrhythmia, and to apply appropriate stimulation
 therapy.
 The control/timing circuit 128 further has a memory circuit 144 coupled
 thereto wherein the patient's historical data, and the operating
 parameters used by the control/timing circuit 128 are stored. Such
 operating parameters define, for example, the amplitude of each shock
 energy pulse to be delivered to the patient's heart 126 within each tier
 of therapy, as well as the duration of these shock pulses. The memory 144
 may take many forms, and may be subdivided into as many different memory
 blocks or sections (addresses) as needed to store desired data and control
 information. In some embodiments, the ICD device 125 has the ability to
 sense and store a relatively large amount of data as a data record, which
 data record may then be used to guide the operation of the device, i.e.,
 the present operating mode of the device may be dependant, at least in
 part, on past performance data.
 Advantageously, the operating parameters of the implantable device 125 may
 be non-invasively programmed into the memory 144 through telemetry
 transmitter 12, in telecommunicative contact with the external programmer
 or receiver 14 by way of the coupling coil 22. The coil 22 may serve as an
 antenna for establishing a radio frequency (RF) telemetry link 16 with the
 receiver 14. The coil 22 may serve as a means for inductively coupling
 data between the transmitter 12 and the receiver 14. Reference is made to
 U.S. Pat. No. 4,809,697 (Causey, III et al.) and U.S. Pat. No. 4,944,299
 (Silvian) that are incorporated herein by reference. Further, the
 transmitter 12 allows status information relating to the operation of the
 ICD device 125, as contained in the control/timing circuit 128 or memory
 144, to be sent to the receiver 14 through the telemetry link 16.
 The control/timing circuit 128 includes appropriate processing and logic
 circuits for analyzing the output of the sense amplifier 142 and for
 determining if such signals indicate the presence of an arrhythmia.
 Typically, the control/timing circuit 128 is based on a microprocessor, or
 similar processing circuit, which includes the ability to process or
 monitor input signals (data) in a prescribed manner, e.g., as controlled
 by program code stored in a designated area or block of the memory 144.
 FIG. 10 is a block diagram of the circuitry needed for the dual-chamber
 pacemaker 127. The pacemaker 127 is coupled to the patient's heart 126 by
 way of leads 274 and 276, the lead 274 having an electrode 275 that is in
 contact with one of the atria of the heart 126, and the lead 276 having an
 electrode 277 that is in contact with one of the ventricles of the heart
 126. The leads 274 and 276 are electrically and physically connected to
 the pacemaker 127 through a connector 273 that forms an integral part of
 the housing wherein the circuits of the pacemaker 127 are housed. The
 connector 273 is electrically connected to a protection network 279, which
 network 279 electrically protects the circuits within the pacemaker 127
 from excessive shocks or voltages that could appear on the electrodes 275
 and/or 277 in the event such electrodes were to come in contact with a
 high voltage signal, e.g., from a defibrillation shock.
 The leads 274 and 276 carry stimulating pulses to the electrodes 275 and
 277 from an atrial pulse generator (A-PG) 278 and a ventricular pulse
 generator (V-PG) 280, respectively. Further, electrical signals from the
 atria are carried from the electrode 275, through the lead 274, to the
 input terminal of an atrial channel sense amplifier (P-AMP) 282; and
 electrical signals from the ventricles are carried from the electrode 277,
 through the lead 276, to the input terminal of a ventricular channel sense
 amplifier (R-AMP) 284. Similarly, electrical signals from both the atria
 and ventricles are applied to the inputs of an intracardiac electrogram
 (IEGM) amplifier 285. The amplifier 285 is typically configured to detect
 an evoked response from the heart 126 in response to an applied stimulus,
 thereby aiding in the detection of "capture". Capture occurs when an
 electrical stimulus applied to the heart is of sufficient energy to
 depolarize the cardiac tissue, thereby causing the heart muscle to
 contract, or in other words, causing the heart to beat. Capture does not
 occur when an electrical stimulus applied to the heart is of insufficient
 energy to depolarize the cardiac tissue. The dual-chamber pacemaker 127 is
 controlled by a processor or control system 286, which is comprised of
 control and timing circuitries that carry out control and timing
 functions. The control system 286 receives the output signals from the
 atrial (P-AMP) amplifier 282 over signal line 288. Similarly, the control
 system 286 receives the output signals from the ventricular (R-AMP)
 amplifier 284 over signal line 290, and the output signals from the IEGM
 amplifier 285 over signal line 291. These output signals are generated
 each time that a P-wave or an R-wave or an evoked response is sensed
 within the heart 126. The control system 286 also generates trigger
 signals that are sent to the atrial pulse generator (A-PG) 278 and the
 ventricular pulse generator (V-PG) 280 over signal lines 292 and 294,
 respectively. These trigger signals are generated each time that a
 stimulation pulse is to be generated by the respective pulse generator 278
 or 280. The atrial trigger signal is referred to as the "A-trigger", and
 the ventricular trigger signal is referred to as the "V-trigger".
 During the time that either an A-pulse or V-pulse is being delivered to the
 heart 126, the corresponding amplifier, P-AMP 282 and/or R-AMP 284, is
 typically disabled by way of a blanking signal presented to these
 amplifiers from the control system over signal lines 296 and 298,
 respectively. This blanking action prevents the amplifiers 282 and 284
 from becoming saturated from the relatively large stimulation pulses that
 are present at their input terminals during this time. This blanking
 action also helps prevent residual electrical signals present in the
 muscle tissue as a result of the pacemaker stimulation from being
 interpreted as P-waves or R-waves.
 The pacemaker 127 further includes a memory circuit 300 that is coupled to
 the control system 286 over a suitable data/address bus 302. This memory
 circuit 300 allows certain control parameters, used by the control system
 286 in controlling the operation of the pacemaker, to be programmably
 stored and modified, as required, in order to customize the pacemaker's
 operation to suit the needs of a particular patient. Further, data sensed
 during the operation of the pacemaker may be stored in the memory 300 for
 later retrieval and analysis.
 As with the memory 144 of the ICD device 125 shown in FIG. 9, the memory
 300 of the pacemaker 127 (FIG. 10) may take many forms, and may be
 subdivided into as many different memory blocks or sections (addresses) as
 needed in order to allow desired data and control information to be
 stored.
 In some embodiments, the pacemaker 127 has the ability to sense and store a
 relatively large amount of sensed data as a data record, which data record
 may then be used to guide the operation of the device. That is, the
 operating mode of the pacemaker 127 may be dependent, at least in part, on
 past performance data. For example, an average atrial rate may be
 determined based on the sensed atrial rate over a prescribed period of
 time. This average rate may then be stored and updated at regular
 intervals. Such stored rate may then be compared to a present atrial rate
 and, depending upon the difference, used to control the operating mode of
 the pacemaker. Other parameters, of course, in addition to (or in lieu of)
 atrial rate, may be similarly sensed, stored, averaged (or otherwise
 processed), and then used for comparison purposes against one or more
 currently-sensed parameters. Modern memory devices allow for the storage
 of large amounts of data in this manner.
 A clock circuit 303 directs an appropriate clock signal(s) to the control
 system 286, as well as to any other needed circuits throughout the
 pacemaker 127 (e.g., to the memory 300) by way of clock bus 305.
 A telemetry transmitter 12 is further included in the pacemaker 127. The
 telemetry transmitter 12 is connected to the control system 286 by way of
 a suitable command/data bus 306. In turn, the telemetry transmitter 12,
 which is included within the implantable pacemaker 127, may be selectively
 coupled to an external programming device or programmer or receiver 14 by
 means of an appropriate telemetry link 16, which telemetry link 16 may be
 any suitable electromagnetic link, such as an RF (radio frequency)
 channel, a magnetic link, an inductive link, an optical link, and the
 like. Through the receiver 14 and the telemetry link 16, desired commands
 may be sent to the control system 286. Similarly, through this telemetry
 link 16 with the receiver 14, data commands (either held within the
 control system 286, as in a data latch, or stored within the memory 300)
 may be remotely received from the receiver 14. Similarly, data initially
 sensed through the leads 274 or 276, and processed by the microprocessor
 control circuits 286, or other data measured within or by the pacemaker
 127, may be stored and uploaded to the receiver 14. In this manner,
 non-invasive communications can be established with the implanted
 pacemaker 127 from a remote, non-implanted, location.
 The pacemaker 127 additionally includes a battery 293 which provides
 operating power to all of the circuits of the pacemaker 127 via a POWER
 signal line 295.
 It is noted that the pacemaker 127 is referred to as a dual-chamber
 pacemaker because it interfaces with both the atria and the ventricles of
 the heart 126. Those portions of the pacemaker 127 that interface with the
 atria, e.g., the lead 274, the P-wave sense amplifier (or detection
 circuit) 282, the A-PG 278, and corresponding portions of the control
 system 286, are commonly referred to as the "atrial channel". Similarly,
 those portions of the pacemaker 127 that interface with the ventricles,
 e.g., the lead 276, the R-wave sense amplifier (or detection circuit) 284,
 the V-pulse generator 280, and corresponding portions of the control
 system 286, are commonly referred to as the "ventricular channel".
 As needed for certain applications, the pacemaker 127 may further include
 at least one sensor 312 that is connected to the control system 286 of the
 pacemaker 127 over a suitable connection line 314. While this sensor 312
 is illustrated as being included within the pacemaker 127, it is to be
 understood that the sensor may also be external to the pacemaker 127, yet
 still be implanted within or carried by the patient. A common type of
 sensor is an activity sensor, such as a piezoelectric crystal, that is
 mounted to the case of the pacemaker. Other types of sensors are also
 known, such as sensors that sense the oxygen content of blood, respiration
 rate, pH of blood, body motion, and the like. The type of sensor used is
 not critical to the present invention. Any sensor or combination of
 sensors capable of sensing a physiological or physical parameter relatable
 to the rate at which the heart should be beating (i.e., relatable to the
 metabolic need of the patient), and/or relatable to whether a
 tachyarrhythmia is likely to soon occur, can be used. Such sensors are
 commonly used with "rate-responsive" pacemakers in order to adjust the
 rate (pacing cycle) of the pacemaker in a manner that tracks the
 physiological or metabolic needs of the patient.
 The pacemaker 127 further includes magnet detection circuitry 287, coupled
 to the control system 286 over signal line 289. It is the purpose of the
 magnet detection circuitry 287 to detect when a magnet is placed over the
 pacemaker 127, which magnet may be used by a physician or other medical
 personnel to perform various reset functions of the pacemaker 127, and/or
 to signal the control system 286 that an receiver 14 is in place to
 receive data from, or send data to, the pacemaker memory 300 or control
 system 286 through the transmitter 12.
 The control system 286 may be realized using a variety of different
 techniques and/or circuits. A preferred type of control system 2286 is a
 microprocessor-based control system. It is noted, however, that the
 control system 286 could also be realized using a state machine. Indeed,
 any type of control circuit or system could be employed for the control
 system 286.
 Representative of the types of control systems that may be used with the
 invention is the microprocessor-based control system described in U.S.
 Pat. No. 4,940,052, entitled "Microprocessor Controlled Rate-Responsive
 Pacemaker Having Automatic Rate Response Threshold Adjustment". Reference
 is also made to U.S. Pat. Nos. 4,712,555 and 4,944,298, wherein a
 state-machine type of operation for a pacemaker is described; and U.S.
 Pat. No. 4,788,980, wherein the various timing intervals used within the
 pacemaker and their inter-relationship are more thoroughly described.
 These patents are incorporated herein by reference.
 While certain preferred embodiments of the invention have been described,
 these embodiments have been presented by way of example only, and are not
 intended to limit the scope of the present invention.