A Preexcited Left Bundle Branch Block Tachycardia

A 52-year-old woman underwent an electrophysiologic prtKedure because of a 30-year history of recurrent episodes of paroxysmal tachycardia. The baseline sinus cycle length wa.s 600 msec and the QRS complexes had a left bundle branch block configuration, with a QRS duration of 120 msec. The atrial-His (AH) and His-ventricular (HV) intervals were 90 and 20 msec, respectively. Programmed atrial stimulation with a sitigle extrastimulus demonstrated progressively greater degrees of ventricular preexcitation. with the HV interval shortening to -15 msec as the AV interval increased from 110 to 200 msec. Programmed atrial stimulation with two extrastimuH resulted in the induction of a tachycartha that had a cycle length of 310 msec and a left bundle branch block conflguratit^n (Fig. I), identical to the QRS complexes generated in response to