Evidence of three clinical subgroups in patients with dual atrioventricular nodal pathways.

We attempted to test the hypothesis that dual atrioventricular (A-V) nodal pathways with second-degree atrioventricular block (2nd A-V block) present as a different clinical entity from those with A-V nodal reentranttachycardia (AVNRT). By evaluation with Holter monitoring (2.9 +/- 2.5 recordings/patient) and 12-lead electrocardiogram (11.9 +/- 11.6), 177 patients with dual A-V nodal pathways could be divided into three subgroups. Thirty-two patients had 2nd A-V block only (2nd A-V block group), 57 had AVNRT only (AVNRT group), 88 had neither 2nd A-V block nor AVNRT (silent group), and none had 2nd A-V block and AVNRT both. Electrophysiologic studies showed that the atrio-His interval was significantly greater (P < 0.0001) and the maximal 1:1 atrioventricular conduction rate was lower (P < 0.0001) in the 2nd A-V block group than in the other two groups. These differences were nullified after the administration of atropine. These results suggest that patients with dual A-V nodal pathways can be classified into three clinical subgroups based on the presence of either 2nd A-V block or AVNRT. We suggest also that patients of the 2nd A-V block group may have a more augmented vagal tone on the A-V node than the other two groups.