Patent GB-1492990-A (GB)
Title: Rechargeable tissue stimulating system
IPC: A61N1/37
Filed: 1974-09-26
Abstract: 1492990 Heart pacemakers PACESETTER SYSTEMS INC 26 Sept 1974 [27 Sept 1973] 41884/74 Heading A5R [Also in Divisions E2, G1, H2, H3 and H4] A rechargeable stimulating system for living tissue, comprises an implantable stimulator 11 for applying electrical pulses, e.g. a pacemaker, powered by a rechargeable D.C. source connected to a charging circuit 10, an external power source 13 for supplying power to the charging circuit from outside the body a telemetry circuit 12 connected to charging circuit 10 for providing a signal indicating the magnitude of the current charging the D.C. source, an external receiver for said signal, and external control means connected to source 13 for controlling the charging current below a predetermined upper limit. In the embodiment described power is coupled into the body by induction, power source 13 employing a 21 KHz sine wave oscillator (104), Fig. 4 (not shown), the frequency of which is mainly determined by a tuned circuit (100, C28). The oscillator (104) supplies an inductor (20). incorporated in a charge head 42 adapted for temporary attachment to a patient's upper torso, so as to couple the inductor (20) with another inductor (17), Fig. 2 (not shown), in the patient's chest. The current induced into inductor (17) is full-wave rectified by diodes (CR1, CR2) and the output passes through the rechargeable D.C. source (15), Fig. 3 (not shown), via a current sampling resistor (R9), and a resistor (R8) which with a transistor (Q7) form a shunt current regulator to limit the current reaching the rechargeable source (15). A Zener diode (VR1) in parallel with the charging leads (51, 52) limits the voltage which can be applied to the stimulator 11, and a series diode (CR5) protects the source (15) should a short - circuit occur in charging circuit 10. Telemetry circuit 12 comprises an inductor (18) connected to a multivibrator driven by two transistors (Q2, Q3), the frequency of which is controlled almost linearly by the charging current flowing throueh the current sampling resistor (R9). The 21 KHz wave is amplitude modulated at the frequency of the multivibrator, Fig. 9 (not shown), and the modulated signal is received in an inductor (21) in the charging head 42. This signal is full - wave rectified by a bridge rectifier (25), Fig. 4A (not shown), and the modulation frequency component passes successively through a band pass filter (105), a tuned amplifier (106), and a low pass filter (107) and then into a frequencyto-voltage converter (108), Fig. 4B (not shown) the output of which is a voltage dependent on the strength of the charging current. Converter (108) drives a comparator circuit (109) including a potentiometer (R42), and the output of comparator (109) goes positive when the output of the frequency-to-voltage converter (108) goes below the voltage setting of potentiometer (R42). The output of comparator (109) passes to a current regulator (60), Fig. 4 (not shown), which controls the output level of the 21 KHz oscillator (104). Thus the setting of the potentiometer (R42) determines the charging current value which is maintained by the current regulator (60). A second output from converter (108) passes through a comparison circuit (110) Fig. 5 (not shown), to a lamp driver circuit (111) which actuates on L.E.D. (26) indicating when correct charging current is reached and another L.E.D. (27) and a buzzer (28) to provide an alarm signal if the charging current is too low. A timing circuit 61, Fig. I may also be provided with a register (31) indicating the charging time, and the time registered may also be decremented to correspond to periods of discharge. Conventional inhibiting circuitry may prevent the register from decrementing to a number less than zero or incrementing to a number greater than a given maximum. An alarm may be provided to guard against over-charging of the battery. In the embodiment described the stimulator 11 is a fixed rate pace-maker of conventional form, Fig. 3 (not shown), but the invention would be equally applicable to a demand pacemaker. The physical configuration of the implantable package, Fig. 10 (not shown), is substantially the same as that disclosed in Fig. 3 of Serial No. 1419531. The stimulator circuit includes an output transformer (54) which is specially screened from the charging magnetic field by an inner copper band and an outer soft iron box. The power source 13, transducer 14 and timing circuit 61 may be housed in a belt-supported unit (37). Figs. 6 and 7 (not shown), connected to the charge head 42. The latter may be attached to a harness (45), Fig. 8 (not shown), worn by the patient, by a fastening comprising a multiplicity of flexible hooks projecting from one contact surface which engage with a loop pile upon the other contact surface. Alternative telemetrv systems may use (a) an e.m. signal at a frequency different from the charging frequency. (b) current signal transmissions during intervals when the power source is switched off, (c) an acoustic signal generated by a piezoelectric crystal, (d) a single frequency modulation linearly related to charging current, (e) two different frequencies indicating adequate or inadequate charging, (f) various combinations of amplitude and frequency modulation. Reference has been directed by the comptroller to Specification 1419533