Unusual properties of accessory pathways.

accessory fibers may be quite variable. The unusual responses to electrical stimulation in these patients included: 1) failure to depolarize the His bundle from the site of ventricular pre-excitation; 2) supraventricular tachycardia with two types of re-entry; 3) anterograde block in the accessory pathway with intact retrograde conduction; and 4) retrograde delay or block in the accessory pathway and atrioventricular (A-V) node with intact anterograde conduction. Understanding these variations may provide important therapeutic implications regarding drug and surgical management. In addition to these anomalies in conduction over accessory pathways we demonstrated in these patients the occurrence of retrograde functional right bundle branch block and retrograde "gap' phenomenon, and ventricular reciprocation without recording intervening atrial discharge.