Patent GB-1335679-A (GB)
Title: Pacemakers
IPC: A61N1/368
Filed: 1970-12-15
Abstract: 1335679 Demand pacemakers AMERICAN OPTICAL CORP 15 Dec 1970 [15 Dec 1969] 59435/70 Heading A5R A demand pacemaker comprises a first timing circuit producing stimulating pulses at terminals E3, E4 for atrial stimulation, a second timing circuit producing stimulating pulses at terminals El, E2 for ventricular stimulation, means coupled to terminals El, E2 for detecting the beating of a patient's heart, means responsive to the detector for resetting the timing circuit, and means for preventing such resetting when the detector detects a signal on terminals El, E2 as a result of the generation of a stimulating pulse on terminals E3, E4. In the absence of inhibiting signals at the base of transistor T6 of the second timer, or if a switch S is open, the second timer is free running and a capacitor 57 charges through resistors 35, 37 until it reaches a voltage at which transistors T7, T8 and then T9 conduct to discharge a capacitor 65 via terminals E1, E2 for ventricular stimulation. Resistor 37 controls the ventricular stimulating pulse width and resistor 35 the repetition frequency. A similar first timing circuit comprising members T6' to T9' 35', 37', 57' produces atrial stimulating pulses coupled via transformer 44 to terminals E3, E4. In the absence of natural ventricular contractions a ventricular stimulating pulse is produced every 800 m.sec. The first timer produces an atrial stimulating pulse 600 m.sec after every detected ventricular contraction, whether natural or stimulated. Zener diodes 67, 52 short out any excessively large signals on terminals El, E2 and E3, E4 respectively. Signals picked up on terminals El, E2 are passed via differentiator 15, 17 to transistor T1, such signals being amplified by transistors T1, T2 and passed to capacitor 41, this amplifier including R-C circuits 19, 21; 29, 23; 27, 25; and 31, 43 arranged so that the amplifier gain decreases markedly below 20H2 arid at a lesser rate above about 30H2 so that ECG R-wave components are preferentially amplified. Signals from capacitor 41 pass to a rate discriminator circuit. Low-pass R-C filters 32, 91 and 36, 34 decrease the likelyhood that atrial stimulating pulses will effect resetting of the ventricular pulse timer. The Rate Discriminator comprises transistors T3-T5 which provide a unipplar pulse of constant current magnitude for charging, via resistors 45, 38, a capacitor 49 for each input signal above a threshold. Transistor T4 conducts on receiving negative pulses from capacitor 41, and transistor T3 and phase inverter transistor T5 conduct when transistor T3 receives positive pulses. When the charging pulses for capacitor 49 are spaced apart sufficiently the voltage swing across capacitor 49 between charging and discharging through diode 42 (effectively shorting resistor 38) and resistor 47 is sufficient to cause transistor T6' to conduct and also transistor T6 to conduct if switch S is closed, thereby discharging capacitors 57', 57 and resetting the timers. The circuit components are chosen so that transistors T6, T6' conduct in response only to signals at terminals El, E2 having a repetition rate about that of natural heart beats and frequency components above about 20 H2 but not much above 30H2. Resistors 45 and 38 are chosen so that the charging time constant for capacitor 49 is long enough to avoid causing transistors T6, T6' to conduct if a single narrow R.F. signal is picked up, and to avoid causing transistor T6 to conduct in response to a signal on terminals E1, E2 caused by an atrial stimulating pulse at terminals E3, E4. The atrial and ventricular pulse timers are free running in the presence of most noise signals, even if the heart is simultaneously beating normally.