Patent GB-1382556-A (GB)
Title: Heart pacers
IPC: A61N1/365
Filed: 1973-03-29
Abstract: 1382556 Demand pacemakers AMERICAN OPTICAL CORP 29 March 1973 15192/73 Heading A5R [Also in Division H3] A demand pacemaker comprises electrodes E1, E2 for connection to a patient's heart, a pulse generator for supplying stimulating pulses to the electrodes, means for deriving a reference potential, means for deriving a timing potential which changes continuously in the  same direction from an initial value starting with the occurrence of a heart-beat, means responsive to a predetermined difference between the timing and reference potentials for operating the pulsing means, and means operative when a stimulating pulse is generated for first controlling a change in the reference potential in one direction and then controlling a gradual and continuous change in it in the opposite direction. As shown, a heart-beat detector 80 is of the kind described in Specification 1328926. Five batteries 3-7 connected in series provide power to the pacemaker and to heart-beat detector 80, and bias voltages to the latter. When a heart-beat occurs, a signal generated at electrodes E1, E2 is applied to the beat detector 80 which, produces a signal applied to a transistor T6, to discharge a timing capacitor 57 which is charged via variable resistors 35, 37 and resistors 61, 63. The junction of resistors 61, 63 is connected to the base of transistor T9, but the charging current of capacitor 57 is so small that transistor T9 is not turned on. When a spontaneous heart beat occurs, transistor T6 conducts and discharges capacitor 57, which then re-charges at a rate dependent upon the reference potential determined by a potential divider 82, 83 with its junction connected to a transistor trigger circuit T7, T8. The circuit is such that when the voltage across capacitor 57 exceeds the base voltage of transistor T7 sufficiently to turn transistor T7, on the trigger circuit operates to discharge capacitors 57, 65 through transistor T9, so providing a stimulating pulse. When the capacitors are fully discharged the transistors turn off. The reference voltage at the junction of potential divider 82, 83 is modified by a capacitor 84. If normal heart-beats have been occurring for some time, with the transistors non-conducting, capacitor 84 will be charged to the maximum voltage. Therefore, if normal heart-beats cease the trigger circuit is activated after a maximum charging time and capacitor 57 charges to a value related to the voltage across capacitor 84. The charging time of capacitor 84 is arranged to be greater than one second, so that, in the absence of further natural heart-beats, the voltage across capacitor 57 rises more rapidly than the voltage across capacitor 84; so that trigger action, with the consequent stimulating pulse, occurs after a shorter period. Ifno natural heart beats occur, subsequent pulses are generated at successively shorter periods until a minimum period is reached, subsequent pulses occurring at the same intervals. If a natural heartbeat occurs within the waiting period, i.e. the period after which a pulse would be generated, then capacitor 57 is discharged and no pulse is produced. If a series of natural heart-beats occurs, inversely the waiting period is gradually increased until capacitor 84 is charged to its maximum. The escape period may be adjusted by means of the variable resistor 35, while adjustment of variable resistor 37 controls the width of the stimulating pulse. In modifications, the capacitor 84 may be connected across resistor 82, instead of across resistor 83, and diodes may be connected in series between battery 6 and base of transistor T7.