Susceptibility of Heart to Electric Shock in Different Phases or the Cardiac Cycle

In connection with an extended series of experiments on the effects of electric shock, the fact has been brought to light that shocks whose duration is a small fraction of the cardiac cycle have a marked difference in their tendency to cause ventricular fibrillation,. depending upon the phase of the cycle in which they occur. The shocks were tripped off by amplified cardiac currents after a controlled delay from a reference point in the electrocardiac cycle. The electrocardiogram together with the shock current and voltage were photographically recorded in time relation on a continuously moving film, which enabled checks to be made on the placement of the shocks in the cardiac cycle. Nearly 800 shocks with 60-cycle current of .03 and .1 second durations were administered to 92 anesthetized sheep in different parts of the cardiac cycle, with currents ranging from 1 to 17 amperes, through the pathway between the right fore-leg and the left hind-leg. A large proportion of the shocks which were given near the end of ventricular systole, as determined by their coincidence with the T-wave of the electrocardiogram, caused ventricular fibrillation and death, while but one shock (0.1 second duration) distinctly outside the T-wave resulted in fibrillation. It seems probable that the difference in the susceptibility of the heart to fibrillation from shocks in different phases of its cycle is related to the known variation of its excitability. Further investigation is being carried forward.