GB1408489A - Heart pacers - Google Patents

Abstract

1408489 Heart pacemakers AMERICAN OPTICAL CORP 26 Oct 1973 [27 Oct 1972] 50015/73 Heading A5R The invention relates to heart pacemakers of the type in which a stimulating pulse is produced if a timing capacitor charges to a given level and in which the capacitor is discharged through a transistor if detection means detect a beat. According to the invention the output circuit of the discharging transistor is connected to the input circuit of a transistor output stage of the detection means by a regenerative resistive feedback path so as to form a monostable circuit. This can ensure complete discharge by a marginal beat and thus avoid mistiming. In addition reflection signals occurring immediately after a true beat are not effective. In the free-running mode of the pacemaker shown a switch S is closed maintaining transistor TC off. Accordingly a capacitor 57 charges through resistor 35 and 37 until it reaches a threshold value whereupon T7 and T8 become conducting to discharge the capacitor and render conducting a transistor T9 which discharges a capacitor 65 through the ventricular electrodes E1, E2. In the demand mode switch S is open. The R wave of a natural beat picked up by electrode El is selected by a frequency selective amplifier T1, T2 and passed through a low pass filtering arrangement to the 'base of transistor T6 so that the capacitor is discharged and a new timing period commenced. A regenerative feedback path through resistor 91 forms T4, T6 into a monostable circuit so that complete discharge of capacitor 57 by marginal waves is ensured and false operation due to reflected pulses avoided. The low pass filter comprises a capacitor 49 and a constant current circuit T3. The amount of charge and discharge of capacitor 49 during each pulse applied to the base of T4 depends upon the frequency of the pusles and if this is above 30KH the amplitude of the pulses is insufficient to make transistor T6 conduct. A transistor T5 is provided to compensate for variations in the emitter-base threshold voltage of transistor T7. In addition a diode 40 ensures that in the free-running mode the discharge of capacitor 57 is less than the discharge which would occur through T6 so that the frequency is somewhat higher. The arrangement also comprises a circuit T10, T14 similar to T6 to T9 for providing atrial stimulation pulses, the output circuit however being somewhat different from that of the ventricular pulse circuit.