Introduction

High intensity, pulsed d.c. electric fields are applied deliberately to the heart in the clinical setting for electrical counter shock of cardiac arrhythmia, using electrodes placed on the body surface, heart surface, or heart cavity. This chapter discusses those conditions of defibrillation under which cardiac tissue may be injured directly by the electric shock. Countershock is also used for cardioversion of cardiac arrhythmia, other than fibrillation, but is less likely to cause injury because of the lower energies used. The injury effect of high-energy d.c. shocks has been utilized for tissue ablation of some types of tachyarrhythmia, using endocardial catheters.

When the level of shock delivered to the heart is just at the threshold for defibrillation, electrical injury of the myocardium is absent or transient. However, defibrillation success rates are probabilistic in nature. Therefore, to assure a high probability of success with a minimal delivery of shock pulses in life-threatening situations, high suprathreshold levels of shock often are used. In animal studies, depending to some extent on waveform shape, if the shock level reaches three to five times the level of current, or 20–30 times the level of energy at the threshold of defibrillation, cardiac function is depressed significantly. In clinical human studies, myocardial injury has been documented following defibrillatory shocks, although not in every case, even with multiple shocks.

The experimental observations cited in this chapter are drawn primarily from animal studies on internal defibrillation in which electrodes are applied directly on, or within, the heart, since the myocardial injury is more clearly related to the shock site and shock level.