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Transthoracic impedance and current flow are determinants of defibrillation success with monophasic shocks. Whether transthoracic impedance, either independently or via its association with body weight, is a determinant of biphasic waveform shock success has not been determined.We studied 22 swine, weighing 18-41 kg. After 15 s of ventricular fibrillation, each pig received transthoracic truncated exponential biphasic shocks (5/5 ms), 70-360 J. Shock success was strongly associated individually with body weight, leading-edge transthoracic impedance and current at low energy levels (70 and 100 J, all P0.001). Multiple logistic regression analysis showed a significant association of body weight with shock success after adjusting for the effect of leading-edge impedance (odds ratio of success for 1 kg decrease in weight at 70 J was 1.29, 95% CI: 1.05-1.59, P=0.02; and at 100 J was 1.30, 95% CI: 1.14-1.49, P0.0001). The same result was observed after adjusting for the effect of leading-edge current. At 150 J or higher energy levels, no significant association was observed.Body weight is a determinant of shock success with biphasic waveforms at low energy levels in this swine model.

Our purpose was to compare biphasic versus monophasic shock success for VF termination in a porcine model of acute left ventricular (LV) dysfunction.For the termination of ventricular fibrillation (VF), transthoracic biphasic waveform shocks achieve higher success rates than monophasic shocks. However, the effectiveness of biphasic versus monophasic defibrillation in a setting of left ventricular dysfunction has not been reported.In 23 open-chest adult swine (15-25 kg), LV dysfunction [or =25% decline in cardiac output (CO)] was induced by continuous inhalation of halothane (1-1.75%). Each pig randomly received transthoracic biphasic and monophasic shocks at three energy levels (30, 50 and 100 J) in two conditions: baseline and LV dysfunction. Halothane effect on left ventricular size and contraction was measured by echocardiography in three additional swine.With halothane, pigs demonstrated a decline in CO (baseline 4.16+/-0.19, halothane 2.72+/-0.19 l/min, P0.01), mean arterial pressure (baseline 107.2+/-3.5, halothane 80.1+/-3.4 mmHg, P0.01) and increased left ventricular end-diastolic pressure (baseline 6.4+/-0.9, halothane 12.7+/-0.8 mmHg, P0.01). LV diameters increased and fractional shortening fell. During baseline, biphasic shocks achieved significantly greater success (termination of VF) compared to monophasic waveforms (100 J: biphasic 83.3+/-9.5 versus monophasic 38.9+/-9.5%, P0.01; 50 J: biphasic 67.1+/-8.8 versus monophasic 11.8+/-5.7%, P0.01; 30 J: biphasic: 31.9+/-6.4 versus monophasic 0+/-0%, P0.01). The superiority of the biphasic waveform to terminate VF was retained during LV dysfunction at all energy levels (100 J: biphasic 78.3+/-7.3 versus monophasic 37.5+/-8.1%, P0.01; 50 J: biphasic 65.5+/-11.5 versus monophasic 11.7+/-5.9%, P0.01; 30 J: biphasic: 40.6+/-8.0 versus monophasic 3.1+/-3.1%, P0.01). Within both waveforms, there were no significant differences in percent shock success at any energy level comparing baseline with LV dysfunction.In this porcine model of acute LV dysfunction, biphasic waveform shocks were not only superior to monophasic waveform shocks for termination of VF during baseline, but retained superiority to monophasic waveform shocks when LV dysfunction was present.