Effect of single doses of labetalol, metoprolol, and placebo on ventilatory function in patients with bronchial asthma: interaction with isoproterenol.

The effects of labetalol, metoprolol, and placebo on pulmonary function and their interaction with isoproterenol were evaluated in 18 patients with reversible bronchial asthma [isoproterenol-induced increase in forced expiratory volume in 1 second (FEV1) of greater than or equal to 15%]. Two dose levels of each medication were studied (on consecutive days), with the lower doses of these drugs given randomly. When the lowest values during the 2-hour postdrug evaluation period were considered, FEV1 significantly increased (10.9%; 15.4%) following labetalol (200 and 400 mg, respectively), significantly decreased (-11.2%; -5.4%) after metoprolol (100 and 200 mg, respectively), and was unchanged after placebo. The effects of the three treatments on forced vital capacity (FVC) and maximal midexpiratory flow (MMEF) were qualitatively similar to those observed in FEV1. When isoproterenol was administered following labetalol (200 and 400 mg), FEV1 further increased with a resulting combined increase of 18.9 and 19.7%, respectively, indicating an additive interaction. The effect of isoproterenol was blunted by metoprolol (100 and 200 mg) (p less than 0.01, difference from the effect observed after labetalol and isoproterenol), indicating antagonistic interaction between these two drugs. Labetalol may be a safer antihypertensive than metoprolol in patients with concomitant bronchial asthma.