Background--The aim of this study was to test the electrophysiological effects of continuous enhanced vagal tone on dual atrioventricular (AV) nodal and accessory pathways.

Methods and Results--This study included 10 patients with typical, slow-fast AV nodal reentrant tachycardia (AVNRT) and 10 patients with AV reciprocating tachycardia. Electrophysiological data were measured before and during continuous vagal enhancement by using phenylephrine infusion (0.6 to 1.5 µg/kg per min). For patients with AVNRT, during phenylephrine infusion, 1:1 conduction times over the anterograde fast and slow and retrograde fast pathways were prolonged (453±64 to 662±120 ms, P<0.001; 379±53 to 443±95 ms, P<0.05; 405±112 to 442±118 ms, P<0.05). The effective refractory period and functional refractory period of the anterograde fast pathway were prolonged with phenylephrine (394±73 to 544±128 ms, P<0.001; 454±60 to 596±118 ms, P<0.001). In contrast, the effective refractory period and functional refractory period of the anterograde slow and retrograde fast were not significantly changed. No significant change was observed in the conduction or refractoriness of the accessory pathways in patients with AV reciprocating tachycardia nor in atrial or ventricular refractoriness.

Conclusions--Enhanced vagal tone produces disparate effects on the refractoriness of the slow and fast AV nodal conduction pathways, with the anterograde fast pathway being the most sensitive. These changes are conducive to induction of AVNRT with a premature atrial complex and may explain in part the relatively common occurrence of AVNRT during sleep or other periods of presumed increased parasympathetic tone.