A controlled-power defibrillator and method of defibrillation for automatically compensating for changes in patient resistance during delivery of a defibrillation pulse. A switching power converter is connected between a storage capacitor and a pair of electrodes adapted for connection to a patient's body, either internally or externally. A closed-loop power control circuit is operatively connected to the power converter for control of the power delivered to the patient, the power control circuit generating a target voltage signal for the storage capacitor, the target voltage signal having a defined relationship with the waveform of the power desired to be delivered to the patient. The power control circuit monitors the actual voltage on the storage capacitor and adjusts the delivery of power to the patient as a function of the difference between desired and actual storage capacitor voltage. The power converter is adapted to capture energy from the discharge of an existing defibrillator and to convert captured energy from the existing defibrillator to a waveform different from that generated by the existing defibrillator.