Patent GB-1419531-A (GB)
Title: Rechargeable cardiac pacemaker
IPC: A61N1/378
Filed: 1973-06-28
Abstract: 1419531 Heart pacemaker JOHNS HOPKINS UNIVERSITY 28 June 1973 [28 June 1972] 30787/73 Heading A5R [Also in Divisions H2 and H3] An implantable heart pacer includes a rechargeable battery 19, Figure 3, a stimulating pulse generator 10, 11, 12, and means 20 for coupling alternating magnetic input recharging energy to the battery 19 from a source external to the patient, an inner shield 13 encompassing the pulse generator and constructed of a material effective to shield the pulse generator from said alternating magnetic input energy and' sources of electromagnetic energy, and a second shield 18 encompassing the coupling means 20 battery 19 and the pulse generator to form a hermetic shield against body fluids and constructed of a material effective to permit the coupling of recharging energy by said coupling means to battery 19 from said source without substantial alternation. Shield 13 may be made of specified thickness pure nickel or pure gold and shield 18 may be made of specified thickness stainless steel, titanium, glass or ceramic. Shields 13, 18 are in the form of open sided cans, with their contents encapsulated therein and the open sides engaged on and sealed to shoulders 16, 17 of a cap 15, which may be of pure nickel and receives one end of a co-axial catheter electrode unit 21. A continuous outer coating 27 of one or more layer of plastics is provided. The Pacer Circuitry comprises a ferrite core transformer 20, Fig. 1a, 1b, receiving the charging energy to charge single-cell nickel-cadmium battery 19 via current monitoring resistor 32, current linking FET 31 and resistor 33. Resistor 32 is coupled to voltage controlled oscillator 35 which feeds winding 39 to give an externally detectable signal having a frequency dependent on the charging current. Charging current through resistor 33 increases the intrinsic pulse rate of the stimulating pulse generator giving a second means to monitor battery charging. In the absense of spontaneous heart beats the pulse generator is free running at about 72 ppm. A capacitor 45 charges via transformer 12 and resistors 44, 42 until transistor 40 and therefore transistor 41 turn on connecting capacitor 45 to the power supply in the opposite sense to its connection during said charging and about 1 m.sec later transformer 12 saturates current through capacitor 45 decreases and transistors 40, 41 turn off, each such cycle giving in the secondary of transformer 12 a stimulating pulse comprising a positive going part, coupled to electrode 55 by transistor 49, followed by a negative going part coupled to electrode 55 by transistor 50, transistor 49, 50 only conducting when an output pulse appears in transformer 12 secondary. Heart beats are amplified by transistors 56, 57, 58 in an input amplifier. When the output of the input amplifier exceeds a threshold transistor 61 and thereby transistors 63, 62 turn on, transistors 62 maintaining transistor 63 on for a fixed interval after the input to transistor 61 disappears determined by R-C circuit 67, 68 and typically equal to 100 m.secs, whereby a 100 m.sec wide pulse appears on the collector of transistor 63. This pulse turns transistor 64 on the discharge capacitor 70. Transistor 65 conducts and thus transistor 66 turns on until capacitor 70 charges to a level sufficient to turn off transistor 65 and thereby 66, this time delay typically being 200 m.sec whereby the output of one-shot circuit 65-66 is a negative going pulse whose leading edge corresponds to the start of an R- wave and which is (100 + 200) = 300 m.sec long. This 300 m.sec output pulse at the collector of transistor 66 will be extended beyond 300 m.sec for as long as pulses less than 300 m.sec apart are detected. Thus this circuit prevents the pacer being inhibited by normally timed T-waves and by spacious noise signals exceeding 200 p.p.m., the stimulating pulse generator only being inhibited by the leading edge of the output pulse on transistor 66 collector. This leading edge momentarily turns on transistor 73 and is coupled to a one-shot circuit comprising transistors 75, 76 whereby these transistors conduct and capacitor 77 charges until it reaches a level at which transistors 75, 76 turn off, the output at transistor 75 collector being a negative going square wave of 500 m.secs duration whereby transistors 46 conducts for 500 m.secs and discharges timing capacitor 45 in the stimulating pulse generator, a further 500 m.secs then being required following turn-off of transistor 46 before generation of the next stimulating pulse. Thus following a spontaneous heart beat the pacer waits for 500 + 500 m.sec = 1 sec. and only then generates a pulse if no further spontaneous beat occurs, (<SP>5</SP>/ 6  sec. being the normal waiting period between successive stimulating pulses). Thus in general if the heart is beating at between about 60 and 180 beats/min the power remains inhibited. The stimulating pulse is also coupled from transistor 41 collector via capacitor 78 and resistor 79 to turn on transistor 81 whereby the heart beat resulting from the stimulating pulse will not operate the pulse inhibitor 73, 75, 76, 46. Closure of a magnetic reed switch 82 produces a fixed rate mode of operation at 72 stimulating pulses/min.