Cardiac Pacemakers

Artificial cardiac pacemakers have two components: a power source (battery with pulse generator) and an electrode that delivers current to the heart (transvenous, epicardial, transthoracic, and transcutaneous). In permanent pacemaker placement, the power source is implanted subcutaneously, and the electrodes are run through the veins to inside the heart or through the subcutaneous tissue to the epicardial surface. In temporary pacemaker placement, the power source is external to the body, and electrodes are placed in one of two ways: (1) transvenously to an intracardiac location or (2) transcutaneously with electrodes placed on the thoracic skin. The pulse generator can be designated to operate in either a fixed-rate mode (asynchronous or competitive) or a demand mode (synchronous or noncompetitive). In the fixed-rate mode, the pulse generator produces an electrical signal at the preset rate regardless of the patient's own intrinsic cardiac rhythm. Serious dysrhythmias or ventricular fibrillation may occur if the pacemaker discharges during the vulnerable period (T wave); for this reason, fixed-rate pacing is rarely done.

In the demand mode, the pulse generator has a sensing circuit that detects spontaneous cardiac activity and will discharge only if no cardiac depolarization is detected for a preset interval. Demand pacemakers may have two response modes, either inhibited or triggered. Pacemakers set to be inhibited have a pulse generator that is inhibited by the sensed cardiac activity and do not generate an impulse. In the triggered response mode, the pacemaker detects the patient's intrinsic cardiac activity and then discharges during the absolute refractory period. On ECG, this appears as pacing spikes following each intrinsic QRS complex.

The latest five-letter code system is shown in Iable...24:5.. This coding system has added antitachyarrhythmic and shocking capabilities of the latest pacemakers. Many patients carry cards indicating the type of pacemaker they have. The simplest type of pacemaker used—the ventricular-demand inhibited-response pacemaker—would be designated as VVI.

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