Atrioventricular nodal tachycardia with and without discontinuous anterograde atrioventricular nodal conduction curves: a reappraisal of the dual pathway concept

To illustrate the complexity of the electrophysiological behaviourof the human alrioventricular (A–V) node, two patientssuffering from A–V nodal tachycardia are described. Duringtachycardia an A–V nodal slow pathway was used for anterogradeconduction, and an A–V nodal fast pathway for retrogradeconduction. Patient 1 showed smooth A–V nodal conductioncurves in both the anterograde and the retrograde direction.Tachycardia could only be initiated by ventricular prematurebeats. No critical delay in ventriculo-atrial conduction timewas required for initiation of tachycardia. Patient 2 showedsmooth A–V nodal conduction curves at the lowest rateof pacing during atrial and ventricular stimulation. The curvesbecame discontinuous in both directions when the basic drivencyclelength was decreased. Tachycardia could only be initiated byatrial premature beats. Ventricular premature beats inducednon-sustained A–V nodal reentry that used an A–Vnodal fast pathway for anterograde conduction, and an A–Vnodal slow pathway for retrograde conduction. Accepting dualpathways in the anterograde and retrograde directions in theA–V node, means that depending upon their electrophysiologicalproperties a large number of combinations of anterograde andretrograde conduction are possible. When more than two A–Vnodal pathways are present, the number of possible combinationswill increase markedly. These considerations are of help inunderstanding electrophysiological findings in patients withA–V nodal tachycardia. They are also useful to explainthe importance of autonomic tone and the results of drug administrationin these patients.