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The current concept of defibrillation correlates defibrillation threshold to cardiac cell excitation threshold (ET). Previous work has shown a 10-15 ms biphasic waveform preferential window (BWPW), identified in the early diastolic phase of an action potential, where ET of the biphasic waveform (BW) is much lower than that of the monophasic waveform (MW). In this paper we investigated the ionic mechanism behind this phenomenon using computer simulations and found that waveforms of the electrical field which can recruit more h and j gates and can provide enough force to drive the membrane potential over the sodium current threshold lowered the ET. This finding may explain some questions arising from basic research and clinical defibrillation in exciting cardiac tissue using different waveforms.