An unusual induction of tachycardia: what is the tachycardia mechanism?--V.

A 48-year-old woman underwent an electrophysiologic procedure because of a 20-year history of paroxysmal supraventricular tachycardia. In the baseline stale, the sinus cycle length was 720 msec, the atrial-His (AH) and His-ventricular (HV) intervals were 110 and 50 msec, respectively, and tachycardia was not inducible by atrial or ventricular pacing. During infusion of 1 /zg/min of isoproterenol, the sinus cycle length was 520 msec, and tbe AH and HV intervals were 90 and 50 msec, re.spectively. There was decremental VA conduction, with a VA block cycle length of 3(X) msec. Programmed atrial stimulation with a single extrastimulus at coupling intervals of 220 to 250 msec reproducibly induced a narrow-QRS tachycardia, which had a cycle length of 330 to 340