Many sudden cardiac arrests (SCAs) are caused by ventricular fibrillation (VF) that has been present for several minutes before defibrillation is attempted (long duration VF, LDVF). Yet, most studies of the basic mechanisms of VF have been performed during the first tens of seconds of VF (short duration VF, SDVF). However, LDVF and SDVF differ. Most studies of the mechanism of defibrillation have investigated shocks following SDVF, not LDVF. Spontaneous refibrillation after defibrillation is common following LDVF and survival progressively decreases as the number of refibrillation episodes increase. Therefore, a vitally important goal is the development of improved therapies for the treatment of LDVF, including better defibrillation techniques and ways to prevent refibrillation. During the last grant period, we made many new findings about the importance of focal activity arising in Purkinje fibers in the mechanisms of LDVF maintenance, defibrillation of LDVF, and spontaneous refibrillation following defibrillation of LDVF. The goals of this application are (1) to verify these new findings and investigate the mechanisms responsible for them, and (2) to obtain knowledge of how these basic mechanisms are altered by acute ischemia, an important cause of SCA, with the expectation that this knowledge will lead to the rational, directed development of improved therapy for SCA. The 3 specific aims listed below will be accomplished using intramural and endocardial electrical recordings, optical recordings, and microelectrode recordings. An advantage of the proposed studies is that the 3 specific aims will all be accomplished during the same experiments. Specific Aim 1. Investigate the mechanisms of LDVF maintenance. We will test the hypotheses that (1) focal activation arising in Purkinje fibers and (2) subendocardial mother rotors are the primary mechanism for LDVF maintenance. If hypothesis 1 is true, we will perform studies to help differentiate if the foci occur because of EADs, DADs, or abnormal automaticity. If hypothesis 2 is true, we will determine if the rotors are located in a particular ventricular region because of the anatomic structure there. Specific Aim 2. Investigate the mechanisms of failed defibrillation following LDVF. We will test the hypothesis that earliest post-shock activation following failed shocks arises focally in Purkinje fibers. If so, we will perform studies to help differentiate if the foci occur because of EADs, DADs, or abnormal automaticity. Specific Aim 3. Investigate the mechanisms of spontaneous refibrillation occurring ~1 min following defibrillation for LDVF. We will test the hypothesis that refibrillation following LDVF and defibrillation in HF is caused by focal activation arising in Purkinje fibers. If so, we will perform studies to help differentiate if the foci occur because of EADs, DADs, or abnormal automaticity. ) For all 3 aims, we will determine how these mechanisms are altered by acute regional ischemia. )