Differential effects of sotalol and metoprolol on induction of paroxysmal supraventricular tachycardia

Seventeen patients with recurrent paroxysmal supraventricular tachycardia (SVT) underwent serial electrophysiologic studies to compare the effects of i.v. sotalol (1.5 mg/kg) and i.v. metoprolol (0.15 mg/kg). The plasma concentrations of sotalol (2.1 ±1.1 μg/ml) and metoprolol (67 ± 15 ng/ml) were within the therapeutic range. Before drug administration, sustained SVT could be reproducibly induced in all patients. Sotalol prevented induction of sustained SVT in 10 of 17 patients (59%) and metoprolol in 4 (28%) (p < 0.05). In 6 of 8 patients with atrioventricular (AV) nodal reentrance, the site of action of sotalol was the anterograde or the retrograde limb, reflecting an increase in refractoriness in both pathways of the circus movement. In 4 of 9 patients with AV reentrance, the site of action of sotalol was exclusively the AV nodal pathway; conduction through the extranodal accessory tract appeared to be unchanged, but lts anterograde effective refractory period was prolonged (from 285 ± 25 to 322 ± 28 ms, p <0.001; mean ± Standarddeviation). In the 7 patients in whom sotalol did not prevent sustained SVT, the tachycardia cycle length increased from 347 ± 42 to 392 ± 45 ms (p <0.01). Compared with sotalol, metoprolol had qualitatively similar but quantitatively less potent effects on the AV nodal pathways; however, different from sotalol, metoprolol had no effect on extranodal accessory tracts.The study suggests that at therapeutic plasma concentrations, sotalol would be effective in preventing clinical SVT in a significant proportion of patients refractory to metoprolol; because sotalol not only has β-blocking properties but also results in acute prolongation of the action potential duration, this combination of class II and III activity may contribute to its superior prophylactic efficacy compared with pure β blockade.