Intermittent preexcitation: marked enhancement of anterograde conduction in the atrioventricular accessory pathway with isoproterenol.

A 29-year-old man presented with intermittent preexcitation and exertional syncope. Electrophysiological evaluation in the control state demonstrated a single, left posterior free-wall atrioventricular accessory pathway. The anterograde and retrograde effective refractory periods and block cycle lengths were long in the control state. No tachycardias were induced during programmed electrical stimulation. After intravenous administration of isoproterenol, anterograde conduction of the accessory pathway was markedly enhanced (block cycle length shortened 45% to less than 240 ms) and rapid antidromic reciprocating tachycardia (CL = 250 ms) associated with syncope was observed. Following successful surgical dissection of the accessory pathway the patient has been without tachycardia or exertional symptoms. We conclude that, under the influence of exercise or emotion, patients with intermittent preexcitation may be at risk for serious arrhythmias.