An implantable cardioverter/defibrillator capable of detecting fibrillation and other tachyarrhythmias and of generating high energy cardioversion and defibrillation pulses. The device delivers cardioversion pulses and defibrillation pulses synchronized to heart rhythm, wherever possible. Delivery of cardioversion pulses requires verification of the continuing presence of a tachyarrhythmia, following charge up of the output capacitor in the pulse generator. In the event that cardioversion therapy is aborted, the charge on the capacitor is retained until subsequent detection of termination of the tachyarrthythmia. Defibrillation pulses are delivered synchronized to heart rhythm, wherever possible, but are delivered asynchronously if synchronization is not possible. Following delivery of a cardioversion or defibrillation pulse, the charge remaining on the capacitor is retained until detection of tachyarrhythmia or fibrillation termination, respectively.