Postshock arrhythmias—a possible cause of unsuccessful defibrillation

Clinical and experimental information exists in the literature which suggests that defibrillation with higher energies than are required results in a decreased percentage of success. Previous work in this laboratory which showed the occurrence of postshock arrhythmias caused by a prolonged depolarization of the cell membrane in myocardial cells in vitro, led to the hypothesis that the decreased percent success at high energies in vivo might be due to the development of similar shock-induced arrhythmias which could immediately refibrillate the heart. The purpose of these experiments was to test this hypothesis. Myocardial cells grown in vitro were subjected to rectangular wave electric field stimulation of varying intensity and duration. Postshock arrhythmias were evaluated using a photovoltaic cell mounted on a closed-circuit television monitor. The photocell converted the change in light intensity produced as the cell contracted to an electrical signal which was read out on a strip chart. Strength-duration curves were formed both for excitation (production of a single extrasystole) and for specific degrees of arrhythmia. These were compared with strength-duration curves obtained for a specific percent success defibrillation in vivo by other investigators. These experiments showed a close similarity between the in vivo and in vitro data, thus, strengthening the hypothesis that decreasing percentage of success of defibrillation with increasing intensity at high energies is due to secondary arrhythmias produced by the shock. The experiments further suggest that in vitro myocardial cells are a valuable screening system for determining waveforms which maximize the ratio between the voltages producing postshock arrhythmias and those producing excitation (defibrillation). This ratio, defined as the "safety factor" of the waveform, varies with the duration of the rectangular wave. Durations having high safety factors can produce defibrillation with a high percentage of success; however, waveforms having low safety factors make it impossible to achieve a high percentage of success defibrillation with any applied voltage. This information suggests that the minimum voltage required for successful defibrillation always be used and that defibrillators be produced with waveforms which maximize the safety factor.