Metoprolol in the treatment and prophylaxis of paroxysmal reentrant supraventricular tachycardia.

The effect of metoprolol on paroxysmal reentrant supraventricular tachycardia was studied in 30 patients. In 16 cases (12 accessory pathway, 4 pure AV nodal reentry) programmed stimulation was performed before and after 0.1 mg/kg intravenous metoprolol. When administered during sustained tachycardia, metoprolol terminated the arrhythmia in 4 out of 12 cases. Metoprolol prolonged the AV nodal functional and effective refractory period and the A-H interval but had no effect on refractory periods and conduction intervals of accessory pathways. Following metoprolol no tachycardia could be initiated in five patients. Premature atrial pacing revealed widened tachycardia zones in 10 cases; premature ventricular pacing showed varying responses to the drug. Twenty two patients (13 accessory pathway, 9 pure AV nodal reentry) received two times 50 to two times 100 mg metoprolol orally per day and were followed for 4 weeks. Five patients did not complete the entire term, four because of frequent tachycardia and one because of side effects. Ten patients were free of arrhythmia, most over several months, and seven continued to have attacks. Plasma levels were higher on oral metoprolol. Intravenous testing had correctly predicted the response to oral metoprolol in seven out of eight patients. Higher plasma concentrations, suppression of premature beats, or the different patient population may explain the more favorable prophylactic action of oral metoprolol.