Abstract:
A shunted coil telemetry transponder in an implant is employed as a magnetic pulse transducer for receiving externally transmitted data. Additional circuitry reproduces the pulse waveform from inductive spikes and interfaces a programming data input with an auxiliary reed switch used for the diagnostic mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following U.S. Patent applications, assigned to the assignee of the present application, each of which is incorporated in its entirety herein: 
     &#34;Implant Telemetry System,&#34; Slocum et al., U.S. Pat. application Ser. No. 153,093 filed May 27, 1980, now U.S. Pat. No. 4,361,153 issued Nov. 30, 1982; 
     &#34;Implantable Externally Programmable Microprocessor-Controlled Tissue Stimulator,&#34; Lesnick, U.S. Patent application Ser. No. 196,665 filed Oct. 9, 1980; 
     &#34;Interactive Programmer For Biomedical Implantable Devices,&#34; Mumford et al., U.S. Patent application Ser. No. 281,011, filed July 6, 1981. 
     BACKGROUND OF THE INVENTION 
     The invention relates generally to electromagnetic signalling and telemetry for biomedical implantable devices. 
     The increasing versatility of implanted stimulators such as cardiac pacers demands more complex programming capabilities. Programming in this context means noninvasively transferring parameter value data from an external device called the programmer to an internal device implanted in the patient&#39;s body. A number systems have been successfully employed in commercially available cardiac pacers, including magnetic programming and radio frequency (RF) programming. RF programming suffers from the unavoidable inherent disadvantage of electromagnetic interference in the environment which an active cardiac pacer patient may encounter. Magnetic programming, on the other hand, relies upon the generation of a series of strong magnetic impulses which actuate a reed switch inside the pacer. Additional circuitry recreates the magnetic pulse waveform from the openings and closings of the reed switch in response to the pulsating magnetic field. The output of the reed switch circuit forms the programming input to programming data registers in the implant, as shown, for example, in U.S. Pat. No. 3,805,796 to Terry et al., assigned to the assignee of the present application. 
     Reed switch programming is not to be confused with so called &#34;magnet rate, &#34; long a standard feature of demand cardiac pacers. Cardiac pacers which stimulate only when necessary to fill in &#34;missing beats&#34; are called standby or demand pacers. To check the inherent fixed rate of this pacer, i.e., the rate to which the pacer will revert in the absence of spontaneous cardiac activity, the physician can place a permanent magnet over the pacer site to actuate a reed switch. The pacer circuitry is designed to respond by removing the demand feature and switching to asynchronous or fixed rate operation so that the physician can not only check the fixed rate but can determine whether the pacer still is operating above the &#34;capture threshold&#34; and, in certain designs, check the pacer&#39;s batteries. In pacers such as Cordis &#34;Omnicor®&#34;, where the same reed switch is used for this diagnostic mode or magnet rate, and for programming (i.e., selecting new parameter values), the pacer is equipped with timing circuitry to distinguish between magnetic impulses and a constant magnetic field. 
     Reed switches have a number of desirable attributes. Besides having little or no associated current drain in the quiescent mode, the insensitivity of reed switches protects against spurious programming. In addition, of course, it is possible to use the same sensor for the conventional diagnostic mode as well as for programming. 
     On the other hand, with the trend to smaller pacer enclosures, the size reduction of the tiny mechanical reed switch element further reduces the sensitivity of the reed switch and its reliability and range of operation. The reed switch is essentially a threshold device dependent upon the proximity of the source of the magnetic field. Thus, with a given magnetic pulse train, the duty cycle of the pulse reproduced by the reed switch circuit depends on the distance between the programmer and the implant. In addition, a debounce circuit is necessary. 
     Some types of pacers only count magnetic impulses, such as the Cordis Omnicor, Telectronics and the Microlith-P® manufactured by Cardiac Pacemakers, Inc. of Minnesota. However, substantial duty cycle distortion is intolerable in pacer designs using pulse width modulation of magnetic impulses. 
     SUMMARY OF THE INVENTION 
     Accordingly, the general purpose of the present invention is to eliminate the dependency of implant programming on the reed swith used as a data transducer. 
     It has been discovered that the shunted coil telemetry transponder developed for a different purpose and disclosed in the above referenced copending application entitled &#34;Implant Telemetry Stystem,&#34; can be pressed into service not only as a transmitter but also as a receiver. The shunted coil telemetry transponder was invented in response to the need to develop a compatible telemetry system for signalling out of the implant within the energy budget constraints imposed by battery operated cardiac pacers. This passive transmitter system is implemented by a switching circuit, controlled by an implant data input, connected across a capacitor and a thin flat coil connected electrically in parallel. An externally generated myriametric (preferably 16 kHz) frequency magnetic carrier signal is resonantly reflected by the tuned coil in the implant. The lagging phase angle of the reflected signal is modulated at extremely low power by intermittently shunting the tuned coil with the data switch to accomplish transmission of data from the implant to the programmer or other external device. 
     In the present invention the tuned coil in the implant is employed as a magnetic pulse transducer for receiving externally transmitted data. Additional circuitry reproduces the magnetic pulse waveform from inductive spikes which appear across the tuned coil on the edges of the magnetic pulse. The spikes are rectified and used to trigger a flip-flop arrangement which exactly duplicates the magnetic pulse waveform without duty cycle distortion. Additional circuitry interfaces the programming data input of the implant with an auxiliary reed switch used solely for the diagnostic mode. During programming the reed switch is automatically isolated from the data programming input. In the diagnostic mode, the positive control of the reed switch is retained as in conventional pacers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 2 is an electrical schematic of the circuit according to the invention. 
     FIG. 2 is a composite waveform timing diagram of typical signals appearing in the circuit of FIG. 1 at the indicated test points. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The circuit of FIG. 1 operates as both data transmitter and data receiver for a biomedical implantable device designed to be used for two-way communication in conjunction with the programming head illustrated and described in connection with FIGS. 1-4 of the above referenced copending application entitled &#34;Implant Telemetry System&#34; (hereinafter the &#34;Implant Telemetry application&#34;). As described in that application, the programming head includes a relatively substantial magnetic programming coil for transmitting data to the implant as well as a coaxial triple coil assembly for activating the implanted transponder comprising the tuned coil combination L1 and C1 of FIG. 1 of the present application. When activated, the transponder in the implant can transmit data out to the external programmer. 
     A portion of the circuit of FIG. 1 designated 10 corresponds to the V-MOS circuit of FIG. 18 of the Implant Telemetry application. D-MOS FETs can also be used. When the myriametric transmitter system is in operation to allow data transmission from the implant, the binary data telemetry input to complementary transistors Q1 and Q2 shunts the tuned coil, correspondingly changing the phase of the reflected signal detected by the triple coil assembly in the external programmer (not shown). 
     The remainder of the implanted circuitry of FIG. 1 adapts the tuned coil and diode network associated with circuit 10 as a receiver for magnetic impulses. Magnetic programming pulses are produced by intermittently passing current through the magnetic programming coil in the external programming head (not shown). The resultant flux change, graphed in ideal form in the first line of FIG. 2, is sensed by coil L1 of FIG. 1 which acts like a secondary winding. Each edge 12 of the magnetic pulses produces a corresponding spike at test points 1 and 2 as shown in corresponding lines of FIG. 2. Schottky diodes CR1 and CR2 clip the positive excursion. Resistors R1 and R2 sum the complementary spikes to produce the resulting waveform at test point 3. Transistor Q3 is turned on abruptly by each spike corresponding to each edge of the magnetic impulses. Resistor R3 biases transistor Q3 off during the quiescent state. Roll-off capacitor C2 in parallel with resistor R3 prevents external interference from triggering the circuit. R4 is the collector load resistance for transistor Q3. 
     The positive going edge of the collector of transistor Q3 clocks the dual &#34;D&#34; flip-flop U1. The lower flip-flop U1-A divides the input clock string by two in order to exactly reproduce the original magnetic impulse waveform with the same pulse widths. The other flip-flop U1-B performs two functions. First, if U1 is set by powering up the circuit or if an odd number of pulses has been received, U1-B provides a reset via resistor R5 and capacitor C3. Resistor R5 can be an active trim external resistor. The combination R5 and C3 is picked so that the time constant will not interfere with programming or telemetry. Diode CR3 provides a rapid discharge of capacitor C3 once flip-flop U1-B has been reset. Thus, the circuit is ready for a new transmission almost immediately. 
     The second function of the upper flip-flop U1-B is to remove the effect of the diagnostic mode reed switch S1 from the programming input. Since the reed switch S1 closes during the magnetic input from the electromagnet, its effect would be to stretch the pulses at the output (PROG OUT). Accordingly, when flip-flop U1-B is clocked, one side of the reed switch S1 is pulled to V B  by the Q bar output of flip-flop U1-B. Diode CR4 is then back biased and reed switch S1 will not load down the PROG OUT pulse. This effectively disconnects the reed switch S1 from the circuit. Diode CR5 eliminates loading by the Q output of flip-flop U1-A which would be at V B  when the flip-flop is reset. The PROG OUT output of FIG. 1 corresponds for example, to the data input of FIG. 3 of the above referenced copending Ser. No. 195,665 by analogy. 
     In the quiescent state, transistors Q1,Q2 and Q3 are off, dual flip-flop U1 is reset, reed switch S1 is open and PROG OUT is at V B  by virtue of resistor R6. In this condition, the circuit of FIG. 1 is ready to receive incoming programming data via the tuned coil L1 or constant magnetic flux to actuate the reed switch S1 for the diagnostic mode. The diode logic arrangement of FIG. 1 ensures that programming and diagnostic modes have mutually exclusive sensors. 
     The following table lists typical component values for the corresondingly designated elements of FIG. 1 by way of illustration only. Actual values for other versions of the same circuit will vary depending on the application. For example, the resonant frequency of the tuned coil in the preferred embodiment is 16 KHz and the incoming magnetic pulse widths are on the order of 1 millisecond. 
     
                       TABLE______________________________________R1             100 kilohmsR2             100 kilohmsR3             1.5 megohmsR4             1.2 megohmsR5             8 megohms (active trim)R6             1 megohmC1             .015 microfaradC2             390 picofaradsC3             .01 microfaradL1             3.89 millihenriesQ3             2N2605U1             CD4013______________________________________ 
    
     The advantage of using an electronic circuit in place of the reed switch for programming data is that it greatly expands the operating range without distortion of the duty cycle. Because the function of the reed switch itself is restricted to the diagnostic mode, the performance specifications for the reed switch are minimized so that a smaller reed switch can be used. In addition, because of the OR gate configuration of the programming output to the programming circuitry of the implant, the reed switch may still be used for programming if the tuned coil transponder circuit becomes nonfunctional for any reason. This redundancy in programming data input gives the pacer an extra margin of safety. 
     The above described circuitry can be varied and modified in many respects without departing from the underlying principle of the invention. For example, the reed switch S1 and associated circuitry may be omitted without affecting the reception of programming data. In fact, the diagnostic mode can be accessed by programming if desired. The circuit of FIG. 1 is also usable in connection with non-telemetry pacers, in which case the transistors Q1 and Q2 would not be present. In this form, the circuit could be operated using standard Cordis Omnicor programmer such as the Model 222. For example, in cardiac pacers which are programmed by counting the number of pulses received, pulse width is less critical and an analog pulse forming circuit, for example, a low current operational amplifier can be used in place of the flip-flop arrangement of FIG. 1 if desired. 
     The foregoing description is intended to be illustrative rather than restrictive, the scope of the invention being indicated by the appended claims.