"Clockwise" and "counterclockwise" bundle branch reentry as a mechanism for sustained ventricular tachycardia masquerading as supraventricular tachycardia.

A 35-year-old man presented with a regular tachycardia having a typical right bundle branch block morphology. Electrophysiological testing revealed inducible tachycardia exhibiting counterclockwise as well as clockwise bundle branch reentry, leading to left and right bundle branch block morphologies. The disappearance of spontaneous episodes of these tachycardias following resolution of right bundle branch block in the resting electrocardiogram suggested that slow conduction in the right bundle branch played an essential role in the tachycardia circuit.