Safety factor for precordial pacing: minimum current thresholds for pacing and for ventricular fibrillation by...

Temporary cardiac pacing by means of rapidly applied, precordial electrodes can be of great value in selected patients suffering cardiac arrest or profound bradycardia, post-defibrillation atrioventricular block, or digitalis intoxication. This study provides data pertaining to the safety of precordial pacing measured as the ratio of the minimum current required to induce ventricular fibrillation (VF) to the minimum current required to pace the ventricular myocardium. Single rectangular pulse stimuli having durations of 1-50 ms were evaluated. In 6 anesthetized dogs, a cutaneous electrode was centered over the shaved apex beat area of the left chest and paired with a larger electrode sutured to the right chest wall. Using a specially fabricated, synchronized, high-energy pulse generator, the "most vulnerable" time in the cardiac cycle at which the least current was required to induce VF with a single shock was identified. This current was compared to the much lower current required to pace the heart in the diastolic interval. While the current required to produce either fibrillation or pacing decreased as pulse duration increased, the safety factor (i.e., the ratio of fibrillation current to pacing current) remained nearly constant, averaging 12.6 for all pulse durations examined. That is, on the average, a stimulus of any given duration between 1 and 50 ms required at least 12 times the current required for pacing to produce ventricular fibrillation. We conclude that in normal canine hearts, the risk of inducing VF during precordial pacing is small.