An external defibrillator with a microprocessor for testing the integrity of an output circuit prior to and during the delivery of a multiphasic defibrillation pulse from an energy storage capacitor coupled to the output circuit is disclosed. The output circuit contains a plurality of switches. The integrity of the output circuit and the output switches is determined by selectively monitoring the changes in the voltage level on the energy storage capacitor while certain tests or functions are being performed. A scaling circuit steps down the voltage level of the high-energy storage device so that it can be measured and monitored by the microprocessor. The microprocessor may compensate for the failure of an output switch by locating a usable set of output switches and using them to provide a discharge path through the output circuit for application of a monophasic pulse.