CA1187140A - Telemetry system for a medical device - Google Patents

Abstract

ABSTRACT
Conventional digital modems have not been applicable to pacemaker telemetry systems since their use would require the periodic conversion of analog data to a numerical value prior to transmission. In contrast, the pulse interval telemetry system of the present invention is capable of transmitting analog data without conversion to a numerical value, and is capable of sequen-tially transmitting both digital and analog data. This data is individually and serially transmitted in either an analog or digital format to a remote receiver. The apparatus of this invention includes a resonant tank circuit for producing a radio frequency carrier signal. This tank circuit is operative at periodic intervals determined by the output frequency or period of a variable frequency oscillator (VFO). Thus, the VFO output modulates the time interval between consecutive pulses of the carrier signal. Sources of digital and analog information are interfaced to the VFO to vary the VFO output period. In the digital mode the VFO output is varied to a first preselected period to encode a logic one, and to a second preselected period to encode a logic zero.
In the analog mode the VFO output is continuously variable over a limited range about a nominal time period to encode the analog data. An additional feature of the invention is the use of the VFO output to synchronize the translation of digital data from a parallel format in memory to a serial format suitable for telemetry transmission.

Description

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This application is a division of our Canadian patent application Seri.al No. 387,385 fi.led October 6, 1981.
This invention relates to implantable medical devices such as pacemakers, and more particularly, to a telemetry system for transmitting in-formation from the pacemaker to a remote receiver for diagnostic purposes.
Pacemakers for providing stimulating pulses to the heart in the absence of natural cardiac activity are well-known. Originally, such pacemakers were fabricated from discrete analog components. More recently designed pace-makers employ digital circuitry realized in monolithic form. The additional com-plexity resulting from monolithic digital implementation has been used to provide desirable pacemaker features, including programmability. One example of such prior art is United States Patent No. 4,276,883 to McDonald et al. issued July 7, 1981. This patent discloses a pacemaker having a number of programmable features including -the pacing rate and pulse width. Information concerning these operating parameters is stored in digital form in the pacemaker's memory.
After implantation it is desirable to read out these memory locations for dia-gnostics purposes. Additional information which is useful for diagnostic pur-poses, such as lead impedance, battery voltage, and the patient's intracardiac electrogram are inherently analog in nature and not directly compa~ible with -the other digital inforlDation within the pacemaker. Consequently, conventional digital modems have not been applicable to pacemaker telemetry systems since their use would require the periodic conversion of the aforementioned analog data to a numerical value prior to transmission.
In contrast, the pulse interval telemetry system of the present invention is capable of transmitting analog data without conversion to a numeri-cal value, and is capable of sequentially transmitting both digital and analog data. This data is individually and serially transmitted in either an analog , ~

or digital -format to a remote receiver.
The apparatus of this invention includes a resonant tank circuit for producing a radio frequency carrier signal. This tank circuit is operative at periodic intervals determined by the output Erequency or period of a vari-able frequency oscillator (VF0). Thus, the VF0 output modulates the time interval between consecutive pulses of the carrier signal.
Sources of digital and analog information are interfaced to the VF0 to vary the VF0 output period. In the digital mode the VF0 output is varied to a first preselected period to encode a logic one, and -to a second preselected period to encode a logic zero. In the analog mode the VF0 output is continuously variable over a limited range about a nominal time period to encode the analog data.
An additional feature of the invention is the use o:E the VF0 out-put to synchronize the translation of digital data from a parallel format in memory to a serial format suitable for telemetry transmission.
Thus, in accordance with one broad aspect of the invention, there is provided a pulse interval modulation telemetry system for sequentially transmitting analog and digital i.nformation from an implantable medical device to a remote receiver comprising: a source of digital data, a source of analog data, fixed frequency oscillator means for providing a radio frequency tele-metry carrier signal~ variable frequency oscillator means responsive to a charging rate for periodically energizing said fixed frequency oscillator for establishing the telemetry pulse intervals, and a plurality of current sources cooperating with said sources of analog and digital data for establishing a charging rate proportional to the instantaneous value of said information.
In accordance with another broad aspect of the invention, there is provided a pulse interval modulation telemetry system for sequentially transmitting analog and digital information from an implantable pacemaker of the type having memory means for storing digital information and of the type having analog measuring means providing a source of real time analog informa-tion; said telemetry system comprising: a fixed frequency oscillator for generating a telemetry carrier signal, a current level responsive variable frequency oscillator for providing a modulation signal to periodically activate said fixed frequency oscillator at time intervals proportional to the magnitude of said current level, current means for providing a current level proportional to the instantaneous value of said analog information and for providing a current level proportional to the instantaneous value of said digital informa-tion.
The invention will now be further discussed in conj~mction with the accompanying drawings, in which:
FIGURE 1 is a block diagram of the function elements of the system for encoding and transmitting informatlon from the implanted medical device.
FIGURE 2 is a truth table showing the relationship between the encoding scheme and the corresponding states of the various current sources of the system;
FIGURE 3 is a waveform diagram showing the analog and digital data format; cmd FIGURE 4 is a schematic diagram showing the VF0, and current sources in a form suitable for implementation in a bipolar integrated circuit.
As previously described, the pulse interval modulation telemetry system is used to transmit analog and digital information from the implanted medical device to a remote receiver. In the context o-f a pacemaker application the analog information may include battery voltage, lead impedance, or the patient's intracardiac electrogram. Similarly, typical digital data may in-~8~

clude programmed pulse width and rate settings as well as identification infor-mation. An example o:f a pacemaker suitable for use as a source of digital information is taught by the previously mentioned United States Patent No.
~,276,883. This application discloses a digitally implemented pacemaker having memory for storing digitally programmed information shown in FIGURE 611 o:E the reference patent. This information is stored in a parallel format as a sequence of binary digits.
A suitable source for analog information such as the patient's intracardiac electrogram may be found in United States Patent No. ~,266,551 to Stein et al., issued May 12, 1981. The circultry disclosed in ~.his patent may be used to provide a source of intracardiac analog information to the telemetry system of the present invention.
As shown schematically in FIGlJRE 1 the heart 10 has an indwelling catheter 11 for sensing cardiac depolarizations and for stimulating cardiac tissue. Pacer logic receives signals via sense amplifier 3~ and delivers stimu-lating pulses by way of output amplifier 33. The pacer logic 12 shown operates under the control of parameter data stored in memory 15. The memory 15 contains the parameter data in parallel form which is seriali~ed for data transmission by shift register 16 which forms a portion of the telemetry system.
In operation, the transmissiotl of data is remotely initiated by the closure of a magnetically actuated reed switch within the pacemaker in the well-known manner. Digital data is then transmitted twice to a remote receiver where it is decoded and checked for errors. The digital data transmission is followed by the transmission of analog data in an analog format. The telemetry system is disabled by removing the magnet from the pacemaker site which opens the reed switch and disables the telemetry circuitry.
Additionally, the telemetry circuitry of the present invention in-cludes a receiver blanking circuit which permits the transmission of analog or digital data to be interrupted by the remote programmer thus truncating the transmission of telemetry information so that the pacemaker may receive higher priority programming information from the remote programmer. This function is achieved by digital circuitry which detects the presence of a long duration burst of RF energy from the remote programmer which is received by the pacemaker and which is decoded to turn off the telemetry transmission systems and to pre-pare the digital circuitry for the reception of programming information from the remote programmer.
Oscillators Referring to FIGURE 1 the radio frequency carrier signal is deve-loped by a radio frequency oscillator tank in FIGURE 1. The tank circuit 1~ is energized at periodic intervals determined by a variable frequency oscillator (VFO) 12. Radio frequency energy from the resonant tank circuit 1~ is coupled to antenna 16 which radiates this energy to a remote receiver (not shown).
The repetition rate of the variable frequency oscillator is set by a number of cooperating current sources which establish a net charging rate at the input node 18 of the VFO 12. When operating in the digital mode for the transmission of digital information the current sources establish a first char-acteristic charging rate for encoding a logic one and a second characteristic charging rate for encoding a logic zero.
As shown in FIGURE 1, three cooperating current sources 26, 28 and 30, are energized by control logic, operating switches 20, 22 2~o When each of these current sources is turned on, a characteristic current I, .5I or .25I is supplied to the capaci.tor 32 which establishes a voltage at node 18.
When the voltage on capacitor 32 reaches a trip level, the VFO output will change state i.nitiating a burst of RF energy from the tank circuit 1~. Consequently, 37~

the time period between successive bursts of radio frequency energy will be determined by the number of current sources which are on. The truth table FIGURE 2 indicates the relationship between the encoding scheme of the present invention and the states of the various current sources. As indicated in the diagram, the logic "one" signal is encoded by energizing current source 30 by closing switch 2~, which provides a constant current charging rate to capacitor 32 of magnitude I. In the preferred embodiment this characteristic charging rate results in a pulse interval of 1,000 microseconds. Similarly, a logic "zero" is encoded by energizing the two current sources 28 and 30 resulting in a net charging current of 1.5I which results in a shorter, 667 microsecond pulse interval. This is accomplished by closure of switches 22 and 2~.
In the analog mode, an alternate pair of current sources 26 and 30 are energized to provide a nominal charging rate corresponding to an 800 microsecond pulse interval. A suitable analog signal such as the intracardiac electrogram derived from the pacemaker lead system is used to modulate one of the current sources 26 to vary the nominal charging rate in a positive or negative direction. This current modulation results in a varying pulse inter-val which corresponds to the amplitude variations of the intracardiac signal.
As shown in FIGURE 3, digital data corresponding to a serial stream of logic one and logic zeroes is cncoded by time periods between shorter and longer time period between bursts of radio :Erequency energy. It is import-ant to note that the longer interval of 1,000 microseconds is not an even multiple of the shorter time period of 667 microseconds used to encode a logic zero. This scheme results in a lower error rate than systems wherein the logic zero and logic one are related as integer multiples. As shown in the lower analog traces of FIGURE 3, a nominal time period of 800 microseconds corresponds to the zero level of the analog signal to be transmitted. Positive and negative , . .

excurslons indicated by the phantom wave traces are used to encode the minimum and maximum excursions about the nominal value.
Although the telmetry system has been described with reference to only a single analog channel, it should be clear that a time division multiplex-ing scheme could be employed to simultaneously transmit more than one channel of analog data 36 as shown in FIGURE 1. The sequential transfer of more than one analog channel is desirable for use with dual chamber pacemakers whose perform-ance depends upon intrinsic atrial and ventricular electrograms. One possible scheme for achieving this time division multiplexing is using a multiplexer 35, shown in FIGURE 2 wherein an additional analog chanllel, labelled "Analog B", is encoded by activating both current sources 28 and 26.
In a similar fashion, other analog signal sources 36 such as lead impedance or battery voltage could be suitably buffered and applied to variable current source 26 to establish a charging rate proportional to the analog signal.
Co rol Logic and Current Sources The block diagram of FIGURE 1 shows the two constant current sources 28 and 30 and one variable current source 26 energi~ed by suitable switching means interfaced to control logic 38. In practice, the switching and current sourcing function may be combined by the use of bipolar ~ransistors which have a characteristic collector-emitter current which corresponds to the magnitude of injected base current. One suitable bipolar implementation for these current sources is shown in FIGURE 4. Referring now to FIGURE 4 the operation of this circuit is initiated by a reed switch closure connecting node 100 to the positive supply voltage. This connection supplies bias current to transistors 102, 104, 106, 108 which, in turn, supply bias current to tran-sistors 110, 111, 112, 113, 114, 116, 118, 120 and to transistors 122, 12~l and 126. Input node 99 interfaces the current source system with the sources of digital and analog data. This node 99 is connected to the positive supply voltage through a tri-state buffer when a logic "zero" is to be transmitted.
The node 99 is connected to ground through the tri-state buffer for the trans-mission of "analog" information. The node 99 is disconnected and is floating when the tri-state buffer is in the high impedance configuration for the transmission of a logic "one".
For the transmission of a logic "one", transistor 118 is off and transistor 120 supplies approximately 225 nanoamps of current to the junction of the base of transistor 128 and the VF0 capacit.or 32. Assuming that capacitor 32 is near ground potential, then transistors 129, 130, 132, 13~ and 136 are off. The voltage on capacitor 32 increases because of the charging current supplied by transistor 120 until the bases of transistors 128 Qnd 129 are equal.
This allows current flow in transistors 129 and 138. When the collector-emitter current of transistor 129 exceeds the current flow through transistor 138, excess current flows into transistor 134, which turns it on. This, in turn, turns on transistor 13~ which sinks current through the tank circui~ 1~ and causes the emission of a pulse of radio frequency. The circuit formed by tran-sistors 130, and 13~ form a latch arrangement which will not change state until the capacitor 32 discharges to approximately 0.5 volt whereupon these transis-tors shut off. The discharge of capacitor 32 takes approximately 2 microseconds and determines the time transistor 136 is on, which de-termines the width of the pulse applied to the tank circuit. When capacitor 32 is discharged, transistor 129 is off and transistor 128 is on which permits the cycle to begin again.
When a logic "one" is applied to input node 99, transistor 118 is activated which adds additional current to the VF0 input node 18, shortening the time required to reach the trip level of the VF0 circuit, thus shortening 71~¢~

the pulse interval time to approximately 667 microseconds.
When input node 99 is grounded through the operation control logic, the analog transmission mode is enabled and an analog voltage signal applied to the base of transistor 142 is converted to a proportional charging current by transistors 142, 144, 146, 148, 140. As the analog voltage varies, the current of transistor 148 is modulated and the result of pulse interval is shifted with respect to the nominal 800 microsecond pulse interval.
Although the current sources and VF0 have been shown implemented in bipolar technology, it should be appreciated that equivalent structures exist in other technologies including metal oxide semiconductor technologies, and that other modifications may be made without departing from the scope of the invention.

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pulse interval modulation telemetry system for sequentially transmitting analog and digital information from an implantable medical device to a remote receiver comprising:
a source of digital data, a source of analog data, fixed frequency oscillator means for providing a radio frequency telemetry carrier signal, variable frequency oscillator means responsive to a charging rate for periodically energizing said fixed frequency oscillator for establish-ing the telemetry pulse intervals, and a plurality of current sources cooperating with said sources of analog and digital data for establishing a charging rate proportional to the instantaneous value of said information.

2. A pulse interval modulation telemetry system for sequentially transmitting analog and digital information from an implantable pacemaker of the type having memory means for storing digital information and of the type having analog measuring means providing a source of real time analog informa-tion; said telemetry system comprising:
a fixed frequency oscillator for generating a telemetry carrier signal, a current level responsive variable frequency oscillator for pro-viding a modulation signal to periodically activate said fixed frequency oscil-lator at time intervals proportional to the magnitude of said current level, current means for providing a current level proportional to the instantaneous value of said analog information and for providing a current level proportional to the instantaneous value of said digital information.

3. The telemetry system of Claim 2 wherein said current means com-prise a plurality of scaled current sources, at least two of which are selectivc-ly operated to encode logic one and logic zero information, and at least two of which are selectively operated to encode analog information.

4. The telemetry system of Claim 4 further including means for converting digital data stored in said memory in a parallel format to a serial bit stream delivered to said current means at a bit rate equal to said modu-lation signal.

5. The telemetry system of Claim 2 or Claim 4 wherein said current means further comprise: a first current source for establishing a bias charging rate activated during the telemetry mode, a second current source selectively operated in response to said digital data for establishing a first charging rate proportional to a logic one condition and for establishing a second charging rate proportional to a logic zero condition, and a third current source selectively energized in response to said analog data for establishing a continuously variable charging rate proportional to the instantaneous magni-tude of said analog data.