Retrograde gap in fast pathway conduction accentuated by the class I antiarrhythmic agent, flecainide.

A case is reported of a patient with functional duality of AV nodal conduction in whom, during ventricular extrastimulus testing, there was a gap in retrograde fast pathway conduction which allowed the temporary expression of retrograde slow pathway conduction. The administration of the antiarrhythmic agent flecainide, which has disparate effects on retrograde fast and slow pathway conduction characteristics, accentuated this phenomenon. The electrophysiological basis of gap phenomena is discussed.