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 includes a confidence analyzer circuit which determines the confidence level of an electrotherapy shock decision based on the detection. If the confidence level is low, the apparatus adjusts its shock decision criteria.
