A decision algorithm was developed for a semiautomatic defibrillator. The function of the algorithm is to evaluate the ECG of a patient and determine whether a defibrillation shock should be delivered. The development process included establishment of defibrillation criteria, creation of ECG databases, algorithm design, development of test protocols, and clinical testing. The result was an algorithm with sensitivity and specificity sufficiently accurate to allow a defibrillation shock to be delivered safely outside the hospital.