Defibrillation of arrhythmias of increased complexity

Energy requirements for successful defibrillation shocks are arrhythmia specific. However, it remains unclear why probability of shock success decreases with increase in arrhythmia complexity. We hypothesize that this is due to differences in transmural postshock behavior. To test this hypothesis, we assess shock efficacy in a bidomain defibrillation model of a 4mm thick slice of canine ventricles. Shocks are applied between an RV cathode and a distant anode to terminate one of two types of arrhythmias: VT (single scroll wave) or VF (multiple scroll waves). The ED50 shock strength for VT is found to be 13% less than that for VF. The results demonstrate that an isoelectric window in the range of 34 to 43ms exists following both failed shocks and those that succeed in Type B defibrillation for both VT and VF. Further, shocks given in VT result in longer isoelectric window durations as compared to shocks in VF. Analysis of the results demonstrates that the shock induces strong virtual electrode polarization on the slice surfaces that rapidly diminishes in tissue depth. We find that transmural excitable pathways allowing wavefront propagation leading to breakthrough on the surface are formed earlier following shocks given in VF as compared to VT.