Supraventricular origin of bidirectional tachycardia. Report of a case.

We have confirmed a supraventricular origin of bidirectional tachycardia with simultaneous standard electrocardiograms and a His bundle electrogram. The patient had sinus rhythm, variable trifascicular block and atrioventricular block below the His bundle. Ventricular excitation varied among three patterns which included right bundle branch block with right axis deviation (RBBB-RAD), right bundle branch block with left axis deviation (RBBB-LAD), and right axis deviation without right bundle branch block (RAD). Sinus beats with alternating patterns of RBBB-RAD and RBBB-LAD resulted in bidirectional tachycardia as did sinus beats with alternating patterns of RBBB-LAD and RAD. Our observations prov-e that bidirectional tachycardia may be initiated by a supraventricular pacemaker and that the distinguishing bidirectional quality of this arrhythmia need not always occur with constant right bundle branch block.