An apparatus and method for delivering electrical shock therapy in order to treat atrial tachyarrhythmias such as fibrillation in which the energy stored in a capacitor used to deliver a shock pulse is monitored and adjusted. A charging circuit is used to charge the capacitor from a supply voltage upon detection of an atrial arrhythmia, and a controller monitors sensed ventricular depolarizations until R-wave synchrony requirements are met so that an atrial shock pulse can be safely delivered. The controller also attempts to maintain the voltage of the capacitor at a specified voltage before delivery of the shock pulse by operation of the charging circuit.