Atrioventricular (AV) nodal reentry associated with 2:1 infra-His conduction block during tachycardia in a patient with AV nodal triple pathways.

We report a patient with atrioventricular (AV) nodal reentry in which a 2:1 infra-His conduction block was demonstrated during tachycardia. The electrocardiogram (ECG) at the time of attack showed two types of supraventricular tachycardias. The first type was a narrow QRS tachycardia associated with 1:1 AV conduction at a rate of 170 beats/minute. The second type was a narrow QRS associated with 2:1 AV block at a rate of 85 beats/minute. Electrophysiological study revealed AV nodal reentry based on AV nodal triple pathways. The AV conduction curve obtained by atrial premature stimulation showed two discontinuous points at two different basic cycle lengths (500 msec, 400 msec) and from two different pacing sites (high right atrium, distal coronary sinus). These two types of tachycardias were induced by both atrial premature and overdrive stimulation. In the first type, the impulse conducted in the slow pathway antegradely with 1:1 AV conduction and in the fast pathway retrogradely. In the second type, the impulse was conducted beat-to-beat by either a slow pathway or a very slow pathway antegradely with the retrograde limb being the fast pathway and 2:1 infra-His conduction block. Only when the impulse was conducted in the slow pathway antegradely was the infra-His conduction block observed during the tachycardia. The tachycardia in this patient was drug refractory and controlled by an anti-tachycardia pacemaker.