Fast-slow type of atrioventricular nodal reentrant tachycardia: horizontal dissociation of the AV node during tachycardia.

Two patients with recurrent supraventricular tachycardia are presented. The tachycardia was initiated and terminated by atrial extrastimulation beyond the atrial relative refractory period and the atrial activation sequence during the tachycardia was low to high. The induction of tachycardia was dependent on a critical AH interval. In patient 1 who had ventriculoatrial conduction, the tachycardia was initiated by the premature ventricular stimulation followed by double atrial response. In patient 2 the ventriculoatrial conduction was not observed. In both patients, the unchanged atrial cycle length during the tachycardia with antegrade Wenckebach AH block was observed. When AH block occurred during tachycardia the first AH interval was shorter than the subsequent HA interval. In patient 2 verapamil (5 mg) prolonged the atrial cycle length during tachycardia and rapid intravenous injection of adenosine triphosphate (10 mg) terminated the tachycardia. Oral diltiazem (180 mg/day) suppressed the tachycardia in patient 1. These findings suggest that the mechanism of tachycardia may be fast-slow type of AV nodal reentry in the upper portion of the AV node and this type of arrhythmia has tendency to show incessant form.