Determination of safety factor for defibrillator waveforms in cultured heart cells

We studied excitation thresholds and arrhythmias produced in cultured chick embryo myocardial cells subjected to electric shocks using rectangular, untruncated resistor-capacitor (RC), and critically damped resistor-inductor-capacitor (RLC) waveforms with variable intensities while photocell mechanograms were recorded. Strength-duration curves for excitation and production of a specific postshock arrhythmia (4-s arrest) were constructed. Excitation curves closely resembled those for in situ defibrillation threshold (or specific % success). The ratio between the shock intensity producing a 4-s arrest and that producing excitation at each duration (termed the "safety factor") was determined. Waveforms with a large safety factor in vitro defibrillated most effectively in situ with little postshock dysfunction. Waveforms with low safety factors had a low rate of success in situ and produced much postshock dysfunction. Safety factor of monophasic clinical waveforms were lower than that of the 5-ms rectangular wave. The close correspondence between in vitro safety factor and in situ defibrillating effectiveness, as reported in the literature, suggests that the cell culture system is an effective screening system for determining waveforms that will improve the efficacy and safety of defibrillation procedures.