Available clinical data indicate that an initial shock energy of 200 J will defibrillate the majority of patients. There is no advantage in starting at a higher energy, and lower energy shocks may be safer. Measurements of transthoracic impedance may permit the use of even lower energy levels for initial shocks. If the initial shock fails to defibrillate it should be repeated immediately, at the same energy level, and then increased if defibrillation is still not achieved. If the initial shock defibrillates but refibrillation occurs later there is no reason to increase the energy; it should be repeated at 200 J. We suggest the following energy selection algorithm for defibrillation (VF = ventricular fibrillation): (Formula: see text).