Background: In this study, we examined the effect of changing capacitor values between phases of a biphasic waveform with the goal of lowering leading edge voltage (LEV), total delivered energy (TDE), and total stored energy (TSE). Methods: Defibrillation thresholds were determined in 18 open- chest swine using epicardial patch electrodes. In part I, three combinations of capacitors were tested: 150:150 μF; 150:300 μF; and 300:150 μF. Waveform durations were 6/0, 6/2, 6/4, 6/6, and 6/8 ms. In part II, phase 1 capacitance was 150 μF. Three phase 2 capacitance values were used: 150 μF; 75 μF; and 37.5 μF. Phase 2 LEV was a multiple of phase 1 training edge voltage: x 0.5; x 0.75; x 1; x 2; x 3; and x 4. A 3.5/2.0 ms biphasic waveform was used. In part III, thresholds were determined for two sets of capacitor values, which can be created by switching a pair of capacitors from in parallel to in series, 150:37.5 μF and 300:75 μF, and nine waveform durations, 4/0, 4/2, 4/4, 6/0, 6/3, 6/6, 8/0, 8/4, and 8/8 ms. Results: In part I, the 300:150 μF system defibrillated with the lowest LEV. TDE and TSE were not different for any of the biphasic waveforms tested except for the 6/8 ms, which was higher. In part II, there was no difference in LEV unsung any of the three phase 2 capacitor values. LEV was lowest for the x 2, x 3, x 4 multipliers. Peak voltage was lowest for the x 1 and x 2 multipliers. TDE was lowest for the x 0.5, x 0.75, x 1, and x 2 multipliers. In part III, the 300:75 μF system defibrillated at a lower LEV than did the 150:37.5 μF system. The 150:37.5 μF system defibrillated at a lower total delivered energy than did the 300:75 μF. Conclusion: These results suggest that defibrillation can be accomplished with lower LEV, TDE, and TSE if two capacitors are switched from a parallel configuration to a series configuration between phases of the biphasic waveform.