A method for cardiac defibrillation is described, for use with an implanted defibrillator. A patient's R-waves are sensed to detect the R-R intervals. If an arrhythmia is detected, the charging of a capacitor is commenced. Determination are made whether the arrhythmia is still in progress and whether the capacitor is charged a predetermined amount. If the arrhythmia is not still in progress, the charging is discontinued. If the arrhythmia is still in progress and the capacitors are charged a predetermined amount, another R-R interval is detected and if the R-R interval is shorter than a selected amount, a shock is delivered to the heart.