Atrioventricular node reentry: intravenous verapamil as a method of defining multiple electrophysiologic types.

Fourteen patients with recurrent supraventricular tachycardia (SVT) underwent electrophysiological evaluation. Each patient was shown to have reentry confined to the region of the atrioventricular (AV) node. Verapamil, 0.075 to 0.15 mg/kg was administered intravenously to each patient during a stable episode of SVT, resulting in termination in each instance. There was more than one mechanism for termination of SVT. Nine patients showed termination by anterograde AV node block preceded by an increase in conduction time in the anterograde limb of the tachycardia circuit (Ae-H intervals) with no change in the conduction time in the retrograde limb (H-Ae intervals). Three patients showed termination by block in the retrograde limb of the circuit preceded by increases in both Ae-H and H-Ae intervals. An additional example of termination by spontaneous ventricular premature complexes and usurpation by sinus rhythm were also seen. Common features were that verapamil had significant effects on anterograde and retrograde conduction and refractoriness in the AV node. It prolonged the refractory periods of both fast and slow pathways in patients with dual anterograde AV node pathways, and observable effects on retrograde conduction and refractoriness were seen even in patients with constant ventriculoatrial conduction times during incremental ventricular pacing in a control study. However, three distinct groups of patients were identified on the basis of their response to ventricular pacing in a control study and upon verapamil effects recorded during their SVT. An explanation for these latter findings may be that there is a normal variation in the retrograde response of parts of the AV node to ventricular pacing, and a variability in some of the patients' responses to verapamil.