Comparisons of the Electrophysiological Effects of Intravenous Sotalol and Propranolol on the Immature Mammalian Heart

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Sotalol is a β blocker that has also been reported to exert class III antiarrhythmic effects. To evaluate the effects of sotalol on the immature heart, and specifically to assess the relative importance of its class III action, the electrophysiologic effects of incremental doses of intravenous dl-sotalol (cumulative dose of 8 mg/kg) were studied in 11 intact canines (ages 4-15 days) utilizing intracardiac programmed stimulation and electrogram recording techniques. These results were compared to the electrophysiologic effects obtained in an additional 14 neonatal canines given 0.6 mg/kg of the β blocker propranolol intravenously. Sotalol caused a greater increase than propranolol in the resting sinus cycle length (45 vs. 4%). Importantly, sotalol resulted in greater increases in atrial and ventricular muscle refractoriness than did propranolol (AERP-77 vs. 4%, AFRP-57 vs. 6%; VERP-53 vs 4%, VFRP-51 vs. 7%). Thus, the electrophysiologic effects of sotalol include large changes in myocardial refractoriness that are not observed with simple β blockade induced by propranolol. These results suggest that sotalol exerts a significant class III effect in the immature mammalian heart, and thus may be useful as an antiarrhythmic agent in the neonate.