An implantable system for the defibrillation or cardioversion of the heart of a patient in need of such treatment comprises a plurality of primary electrodes, a power supply, and a control circuit. Preferably, at least one auxiliary electrode is also included. The plurality of primary electrodes are configured for delivering a defibrillation pulse along a predetermined current pathway in a first portion of the heart, the current pathway defining a weak field area in a second portion of the heart. The at least one auxiliary electrode is configured for delivering an auxiliary pulse to the a portion of the heart where the primary shock field intensity is at or near a minimum. The control circuit is operatively associated with the primary electrodes, the auxiliary electrode, and the power supply, with the control circuit configured for delivering a cardioversion sequence comprising an auxiliary pulse sufficient to induce a cessation of propagation in the weak field area through the auxiliary electrode, followed by a defibrillation pulse through the primary electrodes delivered during the cessation of propagation in the weak field area.