a method is provided for controlling an automatic external defibrillator without stopping cpr . while chest compressions continue to be applied to the victim , the system differentiates between a perfusing rhythm that has the capability of leading to a beating heart without a shock and ventricular fibrillation which sometimes occurs in the presence of ventricular tachycardia , in which there is no capability for leading to a beating heart without a shock . defibrillation shocks should be applied only when needed and that is in the presence of vf and sometimes in the presence of vt . electrocardiographic signals obtained from electrodes applied to the patient &# 39 ; s chest are analyzed so that the presence of a qrs signal characteristic of a rhythm which has the potential of supporting a beating heart , or the absence of a qrs signal which indicates ventricular fibrillation , may be detected in the presence of artifacts resulting from chest compressions .