Electrophysiologic effects of tolamolol on atrioventricular conduction in man

The electrophysiologic effects of tolamolol (UK-6558-01), a beta-adrenergic blocking agent, were studied in 13 patients by means of intracardiac electrograms and the extrastimulus method. Tolamolol (4 to 30 mg. intravenously) resulted in : (1) prolongation of sinus cycle length (SCL) in all patients (p less than 0.01); (2) prolongation of sinus escape time (SET) in 11 of 13 patients (p less than 0.001); (3) prolongation of A-V nodal conduction time during sinus rhythm in 1i of 13 patients (p less than 0.001); (4) onset of A-V nodal Wenckebach block at longer paced cycle lengths in 10 of 11 patients (p less than 0.001); (5) prolongation of the functional refractory period (FRP) of the A-V node in 11 of 11 patients (p less than 0.001); and (6) prolongation of the effective refractory period (ERP) of the A-V node in 10 of 10 patients (P less than 0.001). Tolamolol had no effect on His-Purkinje system (HPS) conduction time in any patient, including 3 patients with abnormal H-V intervals. Because of the marked increase in A-V nodal conduction time encountered by premature atrial depolarizations, the relative and effective refractory periods of the HPS could not be determined in any patient after tolamolol. Atropine (0.5 or 1.0 mg. intravenously) significantly reversed the effects of tolamolol on: sinus cycle length (4 of 5 patients); sinus escape time (3 of 3 patients); A-V nodal conduction time (4 of 5 patients); and A-V nodal refractioriness (5 of 5 patients).