At least 450,000 cases of unexpected cardiac arrest occur annually in the United States (Figure 1). The majority occur in places other than hospitals in people with recognized heart disease. However, the accurate identification of potential victims is not possible, and sudden death is often the initial manifestation of cardiac disease. Most such events occur at home, but up to a quarter occur in public places and are often witnessed by trained personnel who attempt resuscitation. Prompt defibrillation is the most important determinant of survival. Years ago, observations from a Seattle cardiac-rehabilitation program showed that the survival rate was almost . . .