Since the 1960s, electrical heart defibrillators have been routinely used as a treatment for life-threatening cardiac dysrhythmias, especially ventricular fibrillation. The exact mechanism of action is yet to be determined, but is presumably linked to large-scale electrical depolarization of the heart muscle.
“The exact mechanism of defibrillation is not well understood. One theory is that successful defibrillation affects a critical mass of the heart, resulting in insufficient remaining heart muscle to continue the arrhythmia. Recent mathematical models of defibrillation are providing new insight into how cardiac tissue responds to a strong electrical shock.”