['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 P<0.001). 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, P<0.0001). 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.'] Is body weight a predictor of biphasic shock success for low energy transthoracic defibrillation?