One to one atrioventricular conduction during atrial pacing at rates of 300/minute in absence of Wolff-Parkinson-White Syndrome.

One to one atrioventricular (A-V) or atrio-His bundle (A-H) conduction occurred during right atrial pacing at rates of 300/min in two patients with short P-R (and A-H) intervals, narrow QRS complexes and recurrent supraventricular tachyarrhythmias. Patient 1 had episodes of reciprocating A-V tachycardia and of atrial fibrillation with very fast rates (270 to 290/min) that were slowed to 100 to 135/min after administration of intravenous verapamil. Enhanced A-V (A-H) conduction was exposed only during stimulation from the high right atrium, but not from the low lateral right atrium or coronary sinus. Patient 2 had episodes of atrial flutter with 1:1 A-V conduction and rates of 290/min. The H-V interval was short (25 ms) during sinus rhythm and atrial pacing presumably because conduction occurred through an atrio-"distal" His bundle (atriofascicular) tract. In contrast, the H-V interval was normal (40 ms) in echo beats or when the "proximal" His bundle was stimulated. In these two patients, having as "common denominators" short P-R (and A-H) intervals, narrow QRS complexes and recurrent supraventricular tachyarrhythmias, enhanced A-V (A-H) conduction was (1) possible due to different electrogenetic mechanisms; (2) pacing-site dependent; (3) manifested, during atrial fibrillation and atrial flutter, by extremely fast ventricular rates; and (4) unrelated to the rate of reciprocating A-V tachycardias because the latter was predominantly a function of anterograde conduction through the "slow" nodal pathway.