An automated external defibrillator (AED) (10) having a treatment decision processor (28) is described which follows a “shock first” or a “CPR first” rescue protocol after identification of a treatable arrhythmia, depending upon an estimate of the probability of successful resuscitation made from an analysis of a patient parameter measured at the beginning of the rescue. The invention may also follow different CPR protocols depending on the estimate. The invention also may use the trend of the measured patient parameter to adjust the CPR protocol either during a CPR pause or after the initial CPR pause. The AED (10) thus enables an improved rescue protocol.