New considerations on the threshold of ventricular fibrillation for a.c.shocks at 50-60 Hz

The most common cause of death in electric shock is thought to be ventricular fibrillation, a condition in which the circulation is arrested and death ensues very rapidly. A.C. shocks at 50-60 Hz are the most frequent cause of electrical accidents and therefore a careful analysis of available data must be the basis of protective measures. During the last few years valuable new results concerning the threshold of ventricular fibrillation have been obtained in experiments with animals, and progress in the field of physiology of the heart makes it possible to explain the influence of various parameters on the phenomena. For a given shock duration, the distribution of the threshold of current to produce ventricular fibrillation is log-normal. Probability limits for 05% and 50% for ventricular fibrillation are determined for shock durations from 8.3 ms to 5 s. When the 50% probabilities at different durations are examined a discontinuity is observed in the vicinity of the period of one heart cycle. This divides the threshold into two levels, a high level at durations shorter than one third of the period of the cardiac cycle and a level which is more than 20 times lower at shock durations longer than about six heart cycles. At the higher level, ventricular fibrillation only occurs if the shock falls within the vulnerable period of the cardiac cycle; at the lower level the initiation of ventricular fibrillation does not depend on the point of the cardiac cycle in which the shock starts.