Background Certain biphasic waveforms with specific time ratios of positive and negative components require less energy for successful defibrillation of the fibrillating ventricles than monophasic waveforms.However, if more efficient waveforms were also to be associated with more injurious effects on myocardial function, they might not provide a true biological advantage. This study investigates the relation between defibrillation efficacy and potential toxicity of monophasic and asymmetric, single capacitor, biphasic waveforms with equal durations of positive and negative components.Methods and Results The myocardial lactate extraction rate (LER) was used to measure the injurious effects on myocardial oxidative metabolism of two synchronized 35-J shocks in sinus rhythm. LER, mean arterial pressure (MAP) and, in a subset of experiments, cardiac output (CO) and coronary blood flow (CBF) were measured at baseline, 30 seconds, 60 seconds, 90 seconds, 150 seconds, 300 seconds, and 600 seconds after the shocks. In 12 dogs, three different waveforms (M 10: monophasic 10 milliseconds; BI 10: biphasic 10 milliseconds; BI 20: biphasic 20 milliseconds) were tested as series of two consecutive shocks (60 seconds apart) resulting in a total of 36 sets of data. At baseline, LER was 25+-11%. After monophasic shocks, LER decreased significantly more than after biphasic shocks (LER at 150 seconds: M 10: -6+-31% versus BI 10: 21+-15% versus BI 20: 21+-16%; M 10 versus BI 10 and M 10 versus BI 20, P<.05) and showed also a slower recovery (LER at 300 seconds: M 10: 1+-24% versus BI 10: 20+-11% versus BI 20: 20+-15%; M 10 versus BI 10 and M 10 versus BI 20, P<.05). The maximal decrease in LER was 41+-27% for M 10 compared with 18+-15% for BI 10 and 15+-11% for BI 20 (both, M 10 versus BI 10 and M 10 versus BI 20, P<.05). There was a similar decrease in CO and MAP, with the lowest MAP after monophasic shocks. The maximal decrease in MAP was significantly greater after M 10 compared with BI 20 (-29+-15 mm Hg versus -13+-11 mm Hg, P<.05). The defibrillation threshold was 18.6+-8 J for M 10 compared with 11.5+-4.0 J for BI 10 (P<.05) and 15.0+-6.1 J for BI 20, respectively (P=NS).Conclusions Our results suggest that these specific biphasic waveforms are associated with less injurious effects on myocardial oxidative metabolism and hemodynamic performance.Given their higher defibrillation efficacy as well, biphasic waveforms may provide important long-term benefits in patients receiving frequent shocks from implantable cardioverter-defibrillators. (Circulation. 1994;90:2501-2509.)