The effect of shock separation time on multiple-shock defibrillation

Reports in the literature of studies of the efficacy of multiple, staggered shocks used for cardiac defibrillation have shown widely varying results. Twin half sinusoids, separated by an integral number of 60-Hz half-cycles, were used for transthoracic defibrillation of dogs. The separation of the pulses was varied from 0 to 13 half-cycles (0 to 108 msec). An online computer was used to determine the energy content of each shock and to obtain curves of the percentage of success vs energy, current, and voltage. The results show that 2 pulses, separated by 9 half-cycles (75 msec), require 57 percent of the peak current and 87 percent of the peak voltage required by a single pulse. The energy requirement, however, was 9.9 percent higher with the staggered shock. Other pulse separations required higher voltages and currents. Possible mechanisms for the effect are discussed.