Biphasic defibrillation in a model of the rabbit heart

The mechanism by which biphasic defibrillation shocks lower defibrillation thresholds compared to monophasic shocks of the same energy remains not well understood. We have recently developed an active bidomain finite element model of anatomically accurate rabbit ventricles complete with fiber orientation. We utilize this model to simulate an episode of ventricular fibrillation initiated by rapid pacing. Biphasic defibrillation shocks are applied at various times after initiation of sustained fibrillation-like reentrant activity. In all cases, the biphasic defibrillation thresholds are lower than the defibrillation thresholds for the equivalent monophasic shock. Our simulations reveal that the difference is due to the decreased dispersion of refractoriness, absence of deexcitation, and the ability of biphasic shocks to excite all regions of membrane polarization, regardless of polarity.