QRS and Cycle Length Alternans During Paroxysmal Supraventricular Tachycardia: What is the Mechanism?

A 36-year-old man was referred for management of paroxysmal supraventricular tachycardia. Clinical examination revealed no structural cardiac disease. An electrophysiologic procedure was performed in the absence of antiarrhythmic drug therapy. A sustained and irregular supraventricular tachycardia without visible P waves on surface ECG was repetitively induced by rapid atrial pacing. The tachycardia had a mean rate of 160 beats/min, could be interrupted by vagal maneuvers, and displayed cycle length alternans. The tachycardia terminated spontaneously with AV block. A narrow QRS complex without axis deviation terminated each long RR interval. Left-axis deviation was present after each short RR interval. Moreover, there was a right bundle branch block pattern after every other short cycle (i.e., each fourth complex) (Fig. 1). On intracardiac tracings, each ventriculogram was preceded by a His potential and a normal HV interval. Alternans was present in AH intervals but not in HA intervals (Fig. 2). There was no evidence of a manifest or concealed accessory pathway. What is the mechanism of the tachycardia, and what is the explanation for the varying QRS complex morphologies?