Demonstration of dual A-V nodal pathways in patients with paroxysmal supraventricular tachycardia.

Electrophysiological evidence suggestive of dual atrioventricular (A-V) nodal pathways is presented in two patients with normal P-R interval and reentrant paroxysmal supraventricular tachycardia (PSVT). His bundle recordings and atrial stimulation were used to obtain this electrophysiological evidence. Refractory periods were measured with the atrial extra-stimulus technique. Plotting of H,-H2 responses against A,-A2 coupling intervals revealed that as A,-A2 decreased, H,-H, decreased appropriately. At a critical A,-A2, a sudden marked increase in H,-H2 occurred, suggesting failure of fast pathway, (defining the fast pathwav effective refractory period ERP). Further shortening of A,-A2 defined a second H,-H2 curve. The longest A,-A2 with no Ho response was defined as the slow pathway ERP. Echo zones coincided with A,-A2 intervals equal to or less than the fast pathway ERP. These results provide the first electrophysiological demonstration of dual A-V nodal pathways in patients with normal P-R interval and PSVT, as manifest by dual A-V nodal conduction times and refractory periods. Antegrade failure of the fast pathway with subsequent availability for retrograde conduction could allow A-V nodal reentry. These findings provide a basis for reentrance in some patients with reentrant PSVT.