AV nodal reentrant tachycardia with Mahaim fiber conduction.

Paroxysmal tachycardia with widened QRS complexes was found in a 46-year-old woman. In sinus rhythm, the patient had electrocardiographic evidence of type B preexcitation with a left bundle branch block pattern. The resting PR interval (160 msec) and A-H interval (100 msec) were within normal limits, but the H-V interval (10 msec) was abnormally short. Programmed atrial extrastimuli at progressively shorter coupling intervals resulted in sudden prolongation of the A-H interval from 120 msec to 250 msec, and the His bundle activities became incorporated just after initiation of the QRS complexes. The QRS morphology was changed but the change was minimal, and atrial echo beats or sustained tachycardia with wide and preexcited QRS complexes were elicited. It is postulated that the site of reentry is within the AV node with preexcitation occurring as the result of conduction in an anomalous nodoventricular pathway.