A defibrillator (112) is designed for implantation in a patient and for programming certain of its parameters after implantation, including energy content of a shock waveform and timing of delivery of the shock waveform. A shock waveform generator (59) of the device is responsive to a trigger signal for timed production of a shock waveform having a programmable shape and energy content designed for terminating atrial or ventricular fibrillation (AF or VF) of the patient. A detection circuit (50, 55, 56) processes a sensed cardiac signal of the patient to determine the relative timing of various portions of the cardiac signal, including the P-wave and the T-wave. In a test mode of the device, the energy content of a shock waveform is programmed to a magnitude exceeding the upper limit of vulnerability (ULV) of the patient, and the trigger signal times the delivery of a shock waveform of proper magnitude relative to the occurrence of a selected event in the cardiac signal, such as a cardiac stimulus, the P-wave or QRS complex, for application to the patient's heart coincident with the vulnerable period of a P-wave or a T-wave. The probable ULV of the patient's heart is determined or estimated, the defibrillator is set to deliver a shock of sufficient energy to exceed the probable ULV, and delivery of the shock is initiated into the vulnerable period of the P-wave or T-wave. The defibrillator (12) is selected with the capacity to deliver a shock of maximum energy exceeding the probable ULV by a margin deemed to provide an adequate safety margin for the patient. The occurrence of the vulnerable period of the P-wave or T-wave is precisely timed from a predetermined event in the patient's cardiac signal.