The authors note that knowledge of the extracellular potential gradient field of the defibrillation shock and the cardiac-tissue response to the shock should lead to better understanding of the mechanism of defibrillation and stimulate improvement of defibrillation techniques. By measuring the potential distribution of the defibrillation shock throughout the heart and the location of the recording electrodes, the potential gradient field may be calculated. By recording local electrograms throughout the heart immediately before and after the shock, the tissue response to the shock can be evaluated.>