Systematic evaluation of the determinants of defibrillation efficacy

OBJECTIVES We studied the effect of varying shock capacitance, shock impedance, and pulse duration on defibrillation efficacy in a randomized, crossover manner for biphasic shocks. BACKGROUND The relationship between the electrical determinants of defibrillation efficacy is incompletely understood. METHODS Biphasic shocks were delivered to 12 dogs through epicardial patches (to vary impedance) after 15 seconds of ventricular fibrillation using one of 100- or 155-muF capacitors at each of four pulse durations (2.5, 5, 10, 20 ms), in a balanced random order. There were two impedance groups: six with higher impedance (mean 97 +/- 15 Omega, range 80-120) and six with lower impedance (mean 39 +/- 3 Omega, range 34 - 44). Voltage requirements were estimated as the average of three defibrillation threshold (DFT) tests. RESULTS Shock capacitance, resistance, and pulse duration all had significant effects upon the minimum voltage DFT (P = .0065, P = .0066, and P = .0001, respectively). The tilt associated with the lowest voltage and current requirement for each of the four capacitance/resistance combinations varied widely, between 34 +/- 5% and 63 +/- 3%, depending on capacitance and impedance. The optimal pulse duration associated with minimum DFT lies between 5.11 and 5.34 ms. CONCLUSIONS Defibrillation voltage requirements for biphasic shocks are affected by pulse duration, capacitance and impedance, but not "tilt".