Reentry within the atrioventricular node in man: a reassessment

6 patients with reentrant supraventricular tachycardia were studied using the atrial extra-stimulus (S1-S2) technique. 3 patients had smooth curves of H1-H2 intervals plotted against A1-A2 intervals, and echoes occurred only in the relative refractory period of the AV node. These patients met traditional criteria for reentry within the AV node, but all 3 were found to have accessory pathways which functioned only in the retrograde direction. 3 other patients had discontinuous curves of H1-H2 intervals plotted against A1-A2 intervals, and the echo zone was confined to coupling intervals producing the longer of the two observed AV node responses. Accessory pathways could not be demonstrated in these latter 3 patients. In patients with accessory pathways the effect of changing atrial pacing site on the echo zone was studied. Pacing at a site remote from the accessory pathway shifted the echo zone to shorter coupling intervals. Some patients previously thought to have reentry within the AV node may have had accessory pathways which conducted only in the retrograde direction. The prevalence of this syndrome is unknown but is important because ablation of the accessory pathway is curative.