Determinants of fast- and slow-pathway conduction in patients with dual atrioventricular nodal pathways.

Electrophysiological studies were performed in two patients with documented paroxysmal supraventricular tachycardia and dual atrioventricular (AV) nodal pathways as defined by the atrial extra-stimulus technique. Both patients manifested two ranges of A-H intervals (AV nodal conduction times) at critical cycle lengths, reflecting fast- and slow-pathway conduction. The occurrence of fast- and slow-pathway conduction at the same cycle length depended on a long fast-pathway effective refractory period relative to the spontaneous or driven cycle length. At critical cycle lengths with fast-pathway conduction, a shift to slow-pathway conduction could be induced by a premature atrial impulse falling within the effective refractory period of the fast pathway. Repetitive retrograde concealed conduction to the fast pathway then maintained antegrade slow-pathway conduction. Resumption of fast-pathway conduction was induced with premature atrial impulses falling within the effective refractory periods of both the fast and the slow pathways, allowing recovery of the fast pathway for antegrade conduction. Atrial echoes and AV nodal reentrant paroxysmal supraventricular tachycardia occurred when sufficient slow-pathway delay was achieved to allow recovery of the fast pathway for retrograde conduction.