His-Bundle Activation During VT. The use of multiple His bundle-right hundle branch recordings in electrophysiologic studies has facilitated definition of the mechanism and elucidation of the direction of impulse propagation in bundle branch reentrant tachycardia, “Mahaim” fiber reciprocating tacbycardia, and retrograde His depolarization in fascieular or ventricular tacbycardias. This report details the electrophysiologic evaluation of pleomorphic ventricular tachycardia in a patient with advanced coronary heart disease. The ventricular tachycardia at baseline revealed variation in the QRS duration without alteration of the elec-trocardiographic (ECG) morphology. Following flecainide administration, a ventricular tachycardia with close resemblance of tbe ECG morpbology to sinus rbytbm was induced. Proximal and distal His-bundle recordings revealed early antegrade His-bundle activation during tbis tacbycardia. Programmed stimulation converted this tachycardia back to tbe clinical ventricular tacbycardia witb intermittent narrowing of the QRS complexes. Early His activation was evident only during the narrower complexes but not in tbe tacbycardia beats witb wide complex. Penetration of the His bundle by ventricular tacbycardia, witb resultant fusion from intramyocardial ventricular activation and His-Purkinje activation, could have accounted for tbe near normalization of tbe QRS morphology during the ventricular tachycardia.