Patent GB-1162353-A (GB)
Title: Stimulator Analysis
IPC: A61N1/362
Filed: 1966-11-18
Abstract: 1,162,353. Cardiac pacer. GENERAL ELECTRIC CO. Nov.18, 1966 [Nov. 23, 1965], No.51731/66. Heading G1N. [Also in Divisions H3 and H4] An implanted cardiac pacer unit or the like supplying electrical stimulating pulses directly to the muscle of the heart or other body organ, may have the control of the amplitude and frequency of its pulses, which are normally controlled by a built-in oscillator, taken over by an external analyzer device which emits spaced trains of pulses, the number of pulses in each train being variable to adjust the reduction in amplitude of the stimulating pulses. The stimulating pulses are reduced in amplitude until unreliable stimulation of the heart is detected by an electrocardiograph, thus giving a measure of the threshold voltage required to stimulate the heart. The analyzer has a first coil 4, Fig. 4, used to locate the oscillator unit of the implanted pacer (11, Fig.1, not shown) and a second coil L2 to transmit the pulse train from the threshold measuring circuit, the two coils (L1, L2, Fig. 5, not shown) being mounted together in a portable unit (18, Fig. 1 not shown) movable over the body surface. The pacer Fig. 3A (not shown), comprises two transistors (Q101 and Q102) arranged in a circuit so as to charge a capacitor (C100) with a sharp rise of voltage which cuts off the second transistor (Q102) so that a further voltage transient cannot occur until the capacitor (C100) has discharged through a resistor (R101). The sharp rise in voltage on the capacitor (C100) causes a current pulse to pass through the equivalent impedance (C h , R h ) of the heart. An inductor (L100) serves both to transmit weak signals due to the stimulating pulses to facilitate location of the pacer by the search coil (L1), and thereafter to receive the pulse trains from the analyzer threshold circuit. Each pulse of such a train received from the external analyzer, when received by the inductor (L100) switches on the transistor (Q102) briefly, to maintain the charge on the capacitor (C100), and thereby the pulse train inhibits the freerunning of the oscillator. During the period after a train of pulses ceases (V c100 , Fig. 3c, not shown) the capacitor (C100) starts to discharge and is recharged again when the next train starts. Thus the voltage to which it is permitted to discharge and consequently the corresponding stimulating pulse generated when it is rapidly recharged, are reduced, and are controllable by adjusting the time duration of each pulse train, the repetition frequency of each train also being controlled at the same time. The locator circuit, Fig. 4 (lower half) basically contains a transistor Q10 amplifying the output of search coil L1, a rectifier CR8, and a meter 22 (also Fig. 6, not shown) for observing variations of signal. When a satisfactory degree of coupling is reached, a lamp LM1 lights, and the pulse generating circuit may be switched on to measure threshold. The pulse generating circuit (upper half of Fig. 4) includes a bi-stable Q5, Q6, which is switched by respective oscillating unijunction transistors Q1, Q4, the timing circuits of which are respectively C1, R3, and C2, R48, R6, P6A. The voltage of V1 is adjustable by potentiometer P2 to adjust the period of the bi-stable output, and potentiometer P6A is adjustable to vary the mark-space ratio of the bi-stable output. The bi-stable output controls the conduction of a unijunction transistor Q7 which produces pulses only during one state of the bistable, and drives the coil L2. Safety features are incorporated, such as; an interlock circuit CR13 which prevents production of pulses if the lamp LM1 extinguishes indicating loss of satisfactory coupling; a switch SW3 which must be held manually depressed for production of pulses; and a switch SW2 which prevents lighting of lamp LM1 unless the pulse train length is set to minimum at P6A at commencement of test. The setting of P6A which represents percentage reduction of stimulating pulse energy (i.e. as pulse train length is increased) is measured by indicating on another scale of the meter Fig. 6 (not shown) the voltage on a second potentiometer P6B ganged to P6A and having a stable voltage across it.