A defibrillator (AED) and method for using a defibrillator using two different ECG analysis algorithms which work sequentially to improve the accuracy of AED shock decisions. A first algorithm, such as (ART), is particularly suited for analysis in the presence of CPR periods. A second algorithm, such as (PAS), is particularly suited for analysis during hands-off periods. The AED switches algorithms depending on the period and on the current analysis of the cardiac rhythm. The inventions thus provide an optimized ECG analysis scheme in a manner that improves the effectiveness of the rescue, resulting in more CPR “hands-on” time, better treatment of refibrillation, and reduced transition times between CPR and electrotherapy.