Effect of tiapamil in the Wolff-Parkinson-White syndrome

The effect of tiapamil was studied in 9 patients with the Wolff-Parkinson-White syndrome using programmed stimulation of the heart. Before the drug, sustained orthodromic tachycardias could be initiated in 7 patients and antidromic tachycardia in 2 by premature atrial and/or ventricular stimulation. An intravenous bolus of tiapamil, 2 mg/kg, terminated the tachycardia in 7 out of 8 cases by a block in the atrioventricular (AV) node. Tiapamil lengthened the effective refractory period of the AV node in the only patient in whom it could be measured and the atrial effective refractory period in 1 out of 9 cases, but the drug had no influence on antegrade or retrograde refractory periods of the accessory pathway or on that of the ventricle. The AV nodal conduction time (A-H interval) was prolonged. Following tiapamil, it was not possible to initiate the tachycardia in 4 cases, in 2 patients the tachycardia zone widened, and in 3 it was unaltered. In the latter cases, the cycle length of the tachycardia was increased. Tiapamil appears to be useful for the termination of tachycardia and also for its prevention in some cases. In others, it may facilitate the inhibition of tachycardia. The delayed AV nodal conduction during sinus rhythm augments the area of ventricular preexcitation, which may facilitate the electrocardiographic localization of the accessory pathway.