Paroxysmal atrioventricular tachycardia involving an anomalous pathway with antegrade unidirectional block

37 consecutive patients with frequent episodes of palpitation and/or dizziness underwent electrophysiological study. They had no signs of ventricular preexcitation during either sinus rhythm or atrial pacing. In 17 (46%), the supraventricular tachycardia was sustained by a reentry mechanism involving an anomalous AV pathway with unidirectional antegrade block (AV tachycardia). Some findings were accepted to demonstrate the existence of the anomalous pathway (AP) and others, its participation in the reentry circuit. The possible location of the AP was established in 15 cases at the following sites: left lateral in 5 cases, left posterior in 5 cases, left posteromedial in 1 case, right posterior in 1 case, right anterior in 1 case, posterior septal in 1 case, and anterior septal in 1 case. The rate of the AV tachycardia can be influenced not only by the ventriculoatrial interval but also by the other pathways included in the reentry circuit. It may change conspicuously in cases with dual AV pathway and anomalous AV pathway in relation to the modality of antegrade conduction by the AV node.