Extent of Atrial Participation

Twenty-one patients with atrioventricular (AV) bypass tracts underwent electrophysiologic studies. The bypass tract was left-sided in 15 patients, septal in five and right-sided in one patient. Orthodromic AV-reciprocating tachycardia was induced in all 21 patients, with a mean tachycardia cycle length of 342 59 msec. The introduction of single stimuli in the high right atrium during tachycardia resulted in simultaneous dissociation of the high right atrial and low septal atrial electrograms in nine patients. In six patients, high right atrial overdrive pacing during tachycardia resulted in simultaneous dissociation of the high right atrial and low septal atrial electrograms for two to five consecutive beats. All patients in whom the low septal atrial electrogram was dissociated from the tachycardia had a left-sided bypass tract. In no patient was the coronary sinus atrial electrogram dissociated from the tachycardia by high right atrial pacing. Dissociation of the low septal atrial electrogram (as recorded in the His bundle electrogram) from AVreciprocating tachycardia suggests that the portion of the right atrium adjacent to the AV node may not be a necessary link in the tachycardia circuit. This observation suggests that the site of entry of left-sided impulses into the AV node may be different from that of right-sided impulses.