Duality of conduction in an atriofascicular pathway during antidromic tachycardia

Cycle length alternation in atrioventricular reentrant tachycardia due to alternating conduction time over the dual atrioventricular (AV) nodal pathways has been well described. Atriofascicular pathways with decremental conduction characteristics (Mahaim fibers) are known to contain accessory AV nodal tissue. We describe a case of cycle-length alternans in antidromic tachycardia through an atriofascicular pathway because of alternation in conduction time in the antegrade limb. The possible mechanisms of this phenomenon, rarely described in atriofascicular pathways, are discussed. (PACE 2009; e7-e8).