Comparison of monophasic with single and dual capacitor biphasic waveforms for nonthoracotomy canine internal defibrillation

Monophasic and single capacitor and dual capacitor biphasic truncated exponential shocks were tested in pentobarbital-anesthetized dogs with use of a nonthoracotomy internal defibrillation pathway consisting of a right ventricular catheter electrode and a subcutaneous chest wall patch electrode. Seven dogs weighing 20.2 ± 0.5 kg were utilized. Monophasic pulses of 10 ms duration were compared with three biphasic pulses. All biphasic waveforms had an initial positive phase (PI) followed by a terminal negative phase (P2) and the total duration of Pt Plus P2 was 10 ms. The dual capacitor biphasic waveform (PI 9 ms, P2 1 ms) had equal initial voltages of Pt and P2. Two simulated single capacitor biphasic waveforms were also tested, the first designed to minimize the magnitude of P2 (P19 ms, P21 ms with initial voltage of P2 equal to 0.3 of the initial voltage of P1) and the second to maximize P2 (PI 5 ms, P2 5 ms with initial voltage of P2 = 0.5 PI). Alternating current was used to induce ventricular fibrillation and four trials of eight initial voltages from 100 to 800 V were performed for each of the four waveforms. Stepwise logistic regression was utilized to construct curves relating probability of successful defibrillation and energy. In the logistic model, the dual capacitor biphasic and single capacitor biphasic waveforms that maximized P2 were associated with significantly (p < 0.001) lower energy requirements for defibrillation than those of the monophasic waveform. The single capacitor biphasic waveform that minimized P2 was not significantly better than the monophasic waveform. The biphasic waveforms associated with the lowest energy requirements for defibrillation were characterized by a higher P2/PI energy ratio. These results demonstrate that single capacitor biphasic waveforms can be constructed that are superior to similar duration monophasic waveforms and comparable with some dual capacitor biphasic waveforms. The relative magnitude of PI and P2 appears to be an important determinant of defibrillation efficacy.