Orthodromic tachycardia initiation dependent on 1:2 atrioventricular conduction in the presence of a left lateral bypass tract.

This report describes a patient who presented with atrial fibrillation associated with a rapid ventricular response (270 bpm) and wide QRS complexes. At the time of electrophysiology study, a left lateral bypass tract with both anterograde and retrograde conduction was demonstrated. Orthodromic tachycardia with atrial premature impulses was initiated and was dependent on two ventricular responses to one atrial impulse. Intravenous procainamide administration had little effect on anterograde or retrograde conduction in the accessory pathway; however, it prevented orthodromic tachycardia initiation with atrial premature impulses by prolonging atrial muscle refractoriness.