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

Abstract

To illustrate the complexity of the electrophysiological behaviour of the human alrioventricular (A–V) node, two patients suffering from A–V nodal tachycardia are described. During tachycardia an A–V nodal slow pathway was used for anterograde conduction, and an A–V nodal fast pathway for retrograde conduction. Patient 1 showed smooth A–V nodal conduction curves in both the anterograde and the retrograde direction. Tachycardia could only be initiated by ventricular premature beats. No critical delay in ventriculo-atrial conduction time was required for initiation of tachycardia. Patient 2 showed smooth A–V nodal conduction curves at the lowest rate of pacing during atrial and ventricular stimulation. The curves became discontinuous in both directions when the basic drivencycle length was decreased. Tachycardia could only be initiated by atrial premature beats. Ventricular premature beats induced non-sustained A–V nodal reentry that used an A–V nodal fast pathway for anterograde conduction, and an A–V nodal slow pathway for retrograde conduction. Accepting dual pathways in the anterograde and retrograde directions in the A–V node, means that depending upon their electrophysiological properties a large number of combinations of anterograde and retrograde conduction are possible. When more than two A–V nodal pathways are present, the number of possible combinations will increase markedly. These considerations are of help in understanding electrophysiological findings in patients with A–V nodal tachycardia. They are also useful to explain the importance of autonomic tone and the results of drug administration in these patients.