CARDIAC PACEMAKERS AND DEFIBRILLATORS

During the last ten years, two electronic instruments, the pacemaker and the defibrillator, have come into common use in the therapy of cardiac diseases. The task of a pacemaker for artificial stimulation of the heart is to deliver short electric pulses of appropriate voltage and length, rhythmically repeated at the desired heart rate. These pulses are fed to electrodes attached to the ventricular wall through flexible cables. Intracardiac electrodes in the right ventricle have also been tested during the last few years with promising results, and they may, in future, replace epicardial electrodes even for permanent use. Two main types of pacemakers are in use:1. The external type, in which the pulse generator is carried outside the body and connected to the electrodes by cables perforating the skin. 2. The implantable type has been developed mainly to avoid the risk of an ascending infection entering at the site in which the cables perforate the skin. Figure 1 shows a typical diagram which applies to both types of pacemakers. The pulse generator is a so-called blocking oscillator. I t consists of a transistor V,, a transformer, a condenser C , and two resistors R, + R,. The primary of the transformer is inserted between the collector and the minus pole of the battery. A resistor R, connects the emitter to plus. The path between base and plus goes across the condenser and the transformer secondary. If the condenser is charged as indicated in the figure, the voltage on the base is positive with respect to the emitter, and no current flows through the transistor. However, through the large resistors R, and R, a small current flows which eventually discharges the condenser and finally reverses its polarity. Now the transistor starts to conduct, and an increasing current flows in the primary. The increase of primary current causes a sudden voltage in the secondary, making the base still more negative. Thus, the transistor acts as a switch which is “on” as long as the current in the primary increases. When, because of the resistances in the collector circuit, the collector current cannot increase any further (or when the iron core of the transformer is saturated), the secondary voltage disappears. During the whole “on” period the base cur-