THE PROBABILITY OF DEFIBRILLATION SUCCESS AND THE INCIDENCE OF POSTSHOCK ARRHYTHMIA AS A FUNCTION OF SHOCK STRENGTH

The effects of high voltage defibrillation shocks given to six swine w ere studied to determine if there is a limit to the advantage gained f rom increasing the shock strength. An endocardial electrode was placed in the right ventricle, and a 114-cm2 cutaneous patch was placed on t he left lateral thorax. Monophasic (10 msec) and single capacitor biph asic (5/5 msec) shocks with leading edge voltages of 200, 400, 600, 80 0, and 990 volts (approximately 2.3-59 J) were tested. For monophasic shocks, the probability of successful defibrillation ranged from 0% at 200 V to 90% at 990 V. The incidence of postshock arrhythmia increase d from 0% for successful shocks at 600 V to 67% for successful shocks at 990 V. For biphasic shocks, the probability of success peaked at 97 % for the 600-, 800-, and 990-V shocks. The incidence of postshock arr hythmia increased from 8 % at 400 V to 55 % at 990 V. Although more po stshock arrhythmias occurred at lower strengths for biphasic than for monophasic shocks, an efficacy criterion, quantifying the probability of defibrillation success and the probability that a postshock arrhyth mia will not occur, was always higher for biphasic shocks. The probabi lity of success never reached 100% for either waveform while the incid ence of postshock arrhythmia increased as the shock strength increased . In conclusion, for the catheter-patch electrode configuration, incre asing the shock strength does not always improve the probability of su ccess and may increase the incidence of postshock arrhythmia.