AV nodal reentrant tachycardia with a bystander Mahaim fiber.

A 13 year old boy had a wide QRS complex tachycardia. A discontinuity in the AV nodal functional curve was observed in the electrophysiologic study. The AV internal was prolonged in association with progressive ventricular preexcitation. At maximal preexcitation, the HV interval was -20 msec and the QRS complex was identical to that seen during clinical tachycardia. No VA conduction was found and atrial premature beats did not affect the tachycardia. The His deflection was found at variable timing when tachycardia was induced. These findings confirmed that tachycardia originated within the AV node and was conducted to the ventricle over the Mahaim fiber. The short effective period of the Mahaim fiber had clinical significance since when atrial fibrillation developed, a rapid ventricular response was observed.