Patent GB-1266907-A (GB)
IPC: A61N1/08
Filed: 1969-03-26
Abstract: 1,266,907. Demand pacemakers. AMERICAN OPTICAL CORP. 26 March, 1969 [11 April, 1968], No. 15855/69, Heading A5R. [Also in Division H3] A demand pacemaker comprises terminals El, E2 for connection to the patient, means for generating stimulating impulses for application to the terminals, means for deriving a control pulse in response to each signal pulse on said terminals produced by heart beats or by extraneous noise signals, and controlling means responsive to the control pulses so that if the time separation of successive control pulses is greater than a predetermined value said controlling means causes or inhibits the generation of said electrical impulses in dependence on the beating action of the heart but if the time separation is less than said value the controlling means causes generation of the electrical impulses at a fixed rate independent of the timing of the natural heart beating action. In the absence of inhibiting signals at the base of transistor T6, or if switch S is closed, the pacemaker is free running, a capacitor 57 charging through resistors 35, 37 until it reaches a voltage at which transistors T7, T8, forming part of a relaxation oscillator, conduct causing transistor T9 to conduct to discharge a capacitor 65 charged through resistor 59, through the heart. Resistor 37 controls the stimulating pulse width and resistor 35 the repetition frequency. Signals picked up at terminals El, E2 are passed via differentiator circuit 15, 17 to transistor T1, such signals being amplified by transistors T1, T2 and passed through capacitor 41. A zener diode 67 shorts out excessively large signals. This amplifier includes R-C circuits 19, 21; 29, 23; 27, 25; and 31, 43 arranged so that the amplifier gain decreases markedly below 20Hz and at a lesser rate above about 30 Hz so that the amplifier preferentially amplifies the ECG R-wave which has components in the range 20-30 Hz. Signals from the amplifier pass via capacitor 41 to a rate discrimination circuit, comprising transistors T3-T5, which provide a unipolar pulse of constant current magnitude for charging capacitor 49 for each input signal pulse above a threshold level. Transistor T4 conducts on receiving positive pulses from capacitor 41, and transistor T3 and phase inverter transistor T5 conduct when transistor T3 receives negative pulses. When the charging pulses for capacitor 49 are spaced apart sufficiently the voltage swing across capacitor 49 between charging and discharging through resistor 47 is sufficient to cause transistor T6 to conduct, if switch S is open, thereby discharging capacitor 57 and preventing conduction of transistor T9 and discharge of capacitor 65. The circuit component values are selected so that transistor T6 conducts 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 but not much above 30 Hz. Resistor 45 is provided so that the time constant of the charging circuit for capacitor 49 is long enough to avoid causing transistor T6 to conduct if a single narrow R.F signal is picked up. The pacemaker is free running in the presence of most noise signals even if the heart is simultaneously beating normally.