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9780521704632c13 CUFX213A/Peck 9780521618168 December 28, 2007 12:27
13 Adrenoceptor antagonists(RR)-stereoisomer probably confers theβ-blockade. The ratio ofα 1 :β-blocking
effects is dependent on the route of administration: 1:3 for oral, 1:7 for intravenous.Presentation and uses
Labetalol is available as 50–400 mg tablets and as a colourless solution containing
5 mg.ml−^1 .Itisused to treat hypertensive crises and to facilitate hypotension dur-
ing anaesthesia. The intravenous dose is 5–20 mg titrated up to a maximum of
200 mg. The oral form is used to treat hypertension associated with angina and dur-
ing pregnancy where the dose is 100–800 mg bd but may be increased to a maximum
of 2.4 g daily.Mechanism of action
Selectiveα 1 -blockade produces peripheral vasodilatation whileβ-blockade prevents
reflex tachycardia. Myocardial afterload and oxygen demand are decreased providing
favourable conditions for those with angina.Kinetics
Labetalol is well absorbed from the gut but due to an extensive hepatic first-pass
metabolism its oral bioavailability is only 25%. However, this may increase markedly
with increasing age and when administered with food. It is 50% protein bound.
Metabolism occurs in the liver and produces several inactive conjugates.