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To improve technics for electrical defibrillation of the heart, an apparatus was constructed for the application of a condenser discharge at any selected phase of the cardiac cycle. With this apparatus 2160 discharges were applied through the intact chests of 240 dogs under thiopental anesthesia. The voltage ranged from 0.5 to 6 kv, the condenser capacity from 0.5 to 100 μF, the total energy from 0.06 to 1800 watts. The associated changes in cardiac rhythm were graded from one (no abnormalities) through two (mild, brief extrasystoles), three (more protracted but fully reversible periods of bigeminy, extrasystoles, block, etc.), four (severe but reversible changes lasting several minutes, usually with fall in blood pressure), to five (severe arrhythmias changing into ventricular fibrillation). The severity of the arrhythmias increased progressively as the voltage was increased, and to a much smaller extent with increases in the total amount of electrical energy. Arrhythmias also were significantly more severe when the shock was delivered during the refractory period (S-T interval) than during the excitable (P interval) stage of the cardiac cycle. Two types of ventricular fibrillation of very different prognosis were revealed. One came on suddenly after low voltage shocks and could easily be terminated by a single adequate defibrillation impulse. The other came on more gradually after high voltages and energies and tended to be irreversible. Observations thus far made indicate that the severity of the arrhythmia following condenser shocks gives an approximate indication of the severity of pathological changes in the heart. Excessive currents could damage the heart to such an extent that myocardial contractions were weak or absent although electrical activity persisted. Therefore the parameters of condenser discharges used for clinical defibrillation should be such that they will produce only the mildest and briefest arrhythmias.

Cardiac Arrhythmias Following Condenser Discharges and Their Dependence Upon Strength of Current and Phases of Cardiac Cycle