Patent GB-1518741-A (GB)
Title: Cardiac pacers
IPC: A61H31/00
Filed: 1976-05-14
Abstract: 1518741 Stimulating and condition monitoring heart pacer MEDALERT CORP 14 May 1976 [19 May 1975] 19990/76 Heading A5R [Also in Division G1] A cardiac pacer has sensing means 42, 46, 48, 54 sensing the response of the heart to the pacer stimulating pulses ST, such response normally including a biphasic cardiac signal (D), (R), line B Fig. 2 (not shown), for each such pulse ST, and the sensing means responds to the positive (D) and negative (R) parts of the cardiac signal falling below respective positive and negative thresholds by causing the pacer pulse generator 10, 12 to generate increased amplitude stimulation pulses and to apply to the. stimulating electrodes 20, 22, a respective alarm pulse (A 3 ), line A Fig. 2, a predetermined time after each stimulation pulse ST, the alarm pulses (A 3 ) being detectable on an EKG trace. In response to pulses from a generator 10, a constant current driver 12 produces 1m. sec current pulses at terminals 20, 22 to stimulate the heart, generator 10 and driver 12 being inhibited, with switch 40 in its DEMAND position, if amplifier 26 senses spontaneous heart activity at above a given rate. Amplifier 26 has an intrinsic refractory delay 32 following each stimulating pulse and accordingly cannot sense the response of the heart to the stimulating pulses. In order to determine any failure of such pulse to properly stimulate the heart (loss of capture) or failure of amplifier 26 &c to sense spontaneous activity and reset generator 10 (sensing failure), fast recovery switches 44 eliminate any charges on capacitors in driver 12 and in a differential amplifier 42 so that the amplifier 42 can sense the heart response to the stimulating pulses from driver 12. These stimulating pulses ST pass to a circuit comprising delay circuits (209), (200), (202), (204) (206) and monostables (210), (201), (203), (205), (207), Fig. 5 (not shown), which give various pulses in timed relation to the stimulating pulses ST and including recovery pulses PR to cause switches 44 to unblock amplifier 42 and driver 12. Sampling pulses SPA, SPB from the Fig. 5 circuit enable gates 46, 48 to respectively pass the (positive) depolarising response pulse (D), Fig. 2 and the (negative) repolarising response pulse (R) from amp. 42 via amp. 52 to + and - comparators 56, 58 of a threshold detector 54 giving, via amp. 60, a unipolar pulse output ABV each time the depolarising pulse (D) or repolarising pulse (R) is respectively more +ve or -ve than a threshold, amp. 60 output being zero below the threshold. A loss of captive monitor, Fig. 3 (not shown), comprising an N stage counter (62) and 2<SP>N</SP> line decoder (64) and a court display (305), receives the ABV pulses from amp. 60. Whenever this monitor senses a predetermined number, which may be adjusted by a switch (70) in an external pacer, of successive loss of captive events (absence of ABV pulses), a flip-flop (66) therein is set to give a high signal on line 78, Fig. 1, to energise a pulse doubler 80 which thereby causes driver 12 to increase (e.g. double) the stimulating pulse ST magnitude, the signal on line 78 together with one of the pulse outputs ST 2c  of the Fig. 5 circuit causing driver 12 to respond to a pulsing circuit 84 so that at terminals 20, 22 each stimulating pulse ST is accompanied shortly thereafter by an alarm pulse (A 3 ), Fig. 2A, to indicate loss of captive. The higher level ST pulses and accompanying alarm pulses (A,) continue until the counter (62) and flipflop (66) are manually reset, e.g. by a magnetic reed switch (86) in an implanted pacer. A sensing failure of amp. 26 &c., giving rise to a stimulating pulse ST at a time when spontaneous activity should have suppressed it, is detected by a circuit comprising a flip-flop (96), Fig. 4 (not shown), set directly by a spontaneous R wave and indirectly by such R-wave via a 0.6 sec delay (94), so that a gate (327) is enabled for 0.6 sec following each spontaneous R-wave. In response to generation of a stimulating pulse ST in this 0.6 sec, gate (327) causes a flip-flop (328), Fig. 3, to be set giving an input FF s5  to gate 98, Fig. 1, coincidence of this input and an input SF to gate 98 from Fig. 5 causing driver 12 to respond to pulsing circuit 100 so that at terminals 20, 22 each stimulating pulse ST is accompanied by an alarm pulse (A 4 ), Fig. 2A delayed relative thereto, and to the alarm pulse (A 3 ) if present, and which can also be sensed by EKG, pulses (A 4 ) continuing until manual reset of the circuit. A second counter/ decoder and count display circuit can be used so that the alarm pulses (A 4 ) are only produced after a predetermined number of successive sensing failures.