Incessant supraventricular tachycardia with constant 1:2 atrioventricular ratio: a longitudinally dissociated atrioventricular node?

We report a patient with incessant, exercise-limiting supraventricular tachycardia on the ventricular level and a constant 1:2 atrioventricular relation. Careful mapping of the AV nodal region revealed His alternans in the inferior AV nodal area and nonalternating His morphologies in the superior His region. Radiofrequency catheter ablation in the inferior AV node cured the patient (11-month follow-up). Constant dual ventricular activation, His alternans, distinct His morphologies in the superior and inferior His, and long-term suppression of the tachycardia by ablation in the so-called slow pathway region of the AV node are suggestive of permanent dual anterograde AV nodal conduction in this patient.