Electrophysiologic effects of intravenous metoprolol

We evaluated the electrophysiologic effects of intravenous metoprolol, a selective beta-1-blocking agent, in 12 patients. Electrophysiologic parameters were measured during the control period, immediately following, and 4 to 6 hours after infusion of 0.15 mg/kg. Metoprolol serum concentration was serially measured in 6 of the 12 patients. Immediately after metoprolol infusion, the sinus cycle length increased by 20% from 738 +/- 166 msec to 885 +/- 181 msec (p less than 0.01), AH interval during sinus rhythm increased by 34% from 83 +/- 27 msec to 112 +/- 37 msec (p less than 0.01), and antegrade atrioventricular (AV) nodal Wenckebach cycle length increased by 20% from 360 +/- 188 msec to 432 +/- 199 msec (p less than 0.01). Sinus node recovery time and sinoatrial conduction time did not become abnormally prolonged in any patient. His-Purkinje conduction was unaltered. High atrial and ventricular refractory periods were modestly increased (less than 15 msec); the increase in refractoriness reached statistical significance when repeat measurements were made at 4 to 6 hours. No adverse effects were noted. Metoprolol serum concentration at the time of peak measured electrophysiologic effect was 54.6 +/- 15.2 ng/ml. We conclude that at this dose, intravenous metoprolol significantly prolongs sinus cycle length and AV nodal conduction, may modestly increase atrial and ventricular refractoriness, and appears to have electrophysiologic potency similar to propranolol. It is well tolerated following intravenous administration and may be of particular value in the acute treatment of supraventricular tachycardia when beta-receptor selectivity is desired.