An external defibrillator system is disclosed that generates and applies a diagnostic signal to the patient in conjunction with defibrillation therapy. The diagnostic signal is designed to elicit a physiologic response from the patient's heart, namely, mechanical cardiac response and electrical cardiac response, electrical cardiac response only, or no cardiac response. Depending upon the type of cardiac response detected, the system selects an appropriate resuscitation protocol that considers the likely responsiveness of the patient to defibrillation therapy. In one practical embodiment, a stimulus signal is applied to patients that show mechanical and electrical capture in response to the diagnostic signal. The stimulus signal maintains the mechanical capture (and, therefore, perfusion) for a period of time prior to the delivery of a defibrillation pulse.