The Essential Role of Atrioventricular Conduction Delay in the Initiation of Paroxysmal Supraventricular Tachycardia

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Abstract
Studies have shown that in patients with paroxysmal supraventricular tachycardia (SVT), spontaneous or stimulated atrial premature depolarizations (APD) falling within a specific portion of the relative refractory period of the atrioventricular (A-V) conduction system initiate SVT. The present study was designed to determine whether the A-V nodal conduction delay these APDs exhibit, or their coupling interval, is essential for the initiation of SVT. Multiple episodes of SVT were initiated and terminated by single APDs in four patients with a history of paroxysmal SVT. The atria were then paced at numerous fixed rates in excess of the spontaneous sinus rate. In each patient, at atrial rates where progressive A-V nodal conduction delay occurred from beat to beat (Wenckebach cycles), a specific degree of A-V nodal conduction delay always resulted in atrial reentry or SVT. Although the atrial coupling interval was considerably longer during Wenckebach cycles initiating SVT than for single APDs resulting in the arrhythmia, the prolongation of A-V nodal conduction was identical. Independent of atrial coupling interval, a requisite degree of A-V nodal conduction delay always resulted in atrial echoes or SVT. This supports the conclusion that SVT results from atrial reentry via the A-V node.