method and apparatus that uses dual - chamber , three - chamber , or four - chamber implantable cardiac devices including pacemakers , defibrillators and cardioverters , which stimulate cardiac tissue electrically to control the patient &# 39 ; s heart rhythm . the method and apparatus are further configured to create a far - field intra - atrial electrogram and a far - field intra - ventricular electrogram that are independently filtered , scaled , and then summed to form a composite far - field electrogram .