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Aims: To elucidate the mechanism of spontaneous termination of ventricular fibrillation (VF) and to define an indicator of its occurrence, the phase angle, a novel measure to assess synchrony of activation, was evaluated., Methods and Results: In 7 isolated rabbit hearts, 7 monophasic action potentials were recorded simultaneously. Ventricular fibrillation was induced by T wave shocks. Cycle lengths (CL) and phase angles between all 7 recordings were analyzed until spontaneous termination or shock-induced defibrillation. Average phase angle was calculated as activation time difference to a reference channel and expressed as a fraction of the reference channel's CL with 1 equaling a complete CL. Initial CLs and phase angles were similar in sustained and terminating episodes (CL: 141 +/- 16 ms vs 142 +/- 24 ms, phase angle: 0.244 +/- 0.11 vs 0.263 +/- 0.1, p = NS). During spontaneous termination, CL increased slightly by 7%. Average phase angle converged gradually over the last three activations before termination of ventricular fibrillation by 22-48% (p < 0.0005), eventually resulting in phase angles similar to paced rhythms directly prior to spontaneous termination of ventricular fibrillation., Conclusions: Gradual synchronization of activation is part of the electrophysiological mechanism resulting in spontaneous ventricular fibrillation termination and can be detected three activations before termination. Phase angle convergence may be useful to detect spontaneous termination of ventricular fibrillation.