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Electrophysiologic Identification of Dual Atrioventricular Nodal Pathway Conduction in Patients with Reciprocating Tachycardia Using Anomalous Bypass Tracts

SUMMARYIn 67 consecutive patients with reciprocating tachycardia using an anomalous atrioventricular (AV) or nodoventricular (NV) bypass tract, electrophysiologic findings suggested the coexistence of dual AV nodal pathway conduction in eight patients. The evidence of coexistent dual AV nodal pathways and anomalous bypass tracts took three forms. In four patients, alternating short and long AV nodal conduction time (AH intervals), presumably caused by rate-dependent 2: 1 conduction in the fast AV nodal pathway, were recorded during AV reciprocating tachycardia. Intravenous administration of atropine invariably resulted in 1:1 fast AV nodal pathway conduction during tachycardia in all patients. In three other patients who had anomalous AV bypass tracts capable of only retrograde conduction, discontinuous AV nodal conduction curves (A,A2, H,H2) were generated during atrial extrastimulation. The remaining patient had an anomalous NV bypass tract bridging the slow AV nodal pathway and the right ventricle. During atrial extrastimulation, antegrade block in the fast AV nodal pathway caused antegrade conduction across a pathway composed of the slow AV nodal pathway and the NV bypass tract. This suddenly produced right ventricular preexcitation with inscription of the His bundle deflection within the QRS complex, disrupting the AV nodal conduction curves (A,A, H,H2). A sustained reciprocating tachycardia with a complete left bundle branch block pattern was subsequently initiated using the slow AV node-NV bypass tract pathway for antegrade conduction and the fast AV node-His-Purkinje systenm pathway for retrograde conduction. These observations suggest that dual AV nodal pathway conduction can be identified electrophysiologically in patients with reciprbcating tachycardia involving anomalous bypass tracts, but its manifestations may take several forms.