A defibrillator and method for using a defibrillator which adopts an ECG analysis algorithm that can detect a cardiac arrhythmia in the presence of noise artifact induced by cardio pulmonary resuscitation (CPR) compressions. The apparatus and method provides both of a continuous and scheduled mode of operation for interleaving periods of CPR with electrotherapy, in a manner that improves the effectiveness of the rescue, resulting in more CPR “hands-on” time, better treatment of refibriUation, and reduced transition times between CPR and electrotherapy.