Orthodromic tachycardia with atrioventricular dissociation: Evidence for anodoventricular (Mahaim) fiber

We describe a patient in whom two tachycardias with AV dissociation were in ducible by ventricular extrastimulation. The first tachycardia as character ized by a narrow QRS preceded bt a His deflection with an HV interval ident ical to that recorded in sinus rhythm (40 ms). Premature ventricular depola rization delivered rr hen the His bundle rr as refractory advanced the next His deflection. These findings suggest the presence of a nodoventricular b ypass tract involved in an orthodromic tachycardia. The second tachycardia was induced after propafenone infusion and exhibited a wide QRS complex wit h left bundle branch block morphology each ventricular complex as consisten tly associated with a His deflection with a HV-interval of -15 ms. The seco nd tachycardia may be considered to represent an antidromic tachycardia thr ough the nodoventricular tract. However, a ventricular tachycardia cannot b e excluded.