The calcium channel blocking agents, unlike the beta-adrenergic blocking agents, shorten the duration of the ventricular action potential, and by so doing, shorten the duration of the ventricular refractory period and the QT interval on the electrocardiogram. For this reason, calcium channel blocking agents are contraindicated in patients with WPW who develop atrial fibrillation because their administration may facilitate conduction through the bypass pathway while blocking conduction in the AV node and by so doing, lead to a faster ventricular rate and promote the developement of ventricular fibrillation.