MIXED ANTAGONISTS LABETALOL
Labetalol blocks α1-, β1-, and β2-receptors. The ratio of α-blockade to β-blockade has been estimated to be approximately 1:7 following intravenous admin-istration. This mixed blockade reduces peripheral vascular resistance and arterial blood pressure. Heart rate and cardiac output are usually slightly depressed or unchanged. Thus, labetalol lowers blood pressure without reflex tachycardiabecause of its combination of α- and β-effects, which is beneficial to patients with coronary artery disease. Peak effect usually occurs within 5 min after an intravenous dose. Left ventricular failure, para-doxical hypertension, and bronchospasm have been reported.
The initial recommended dose of labetalol is 2.5– 10 mg administered intravenously over 2 min. Twice this amount may be given at 10-min intervals until the desired blood pressure response is obtained. Labetalol can also be administered as a slow contin-uous infusion at a rate of 0.5–2 mg/min. However, due to its long elimination half-life (>5 h), prolonged infusions are not recommended.