Beta-adrenoreceptor antagonists

Which one of the following statements regarding β-blockers is TRUE?

a) When using esmolol, a loading dose of 0.1 mg/kg intravenously over one minute is

reasonable

b) Propranolol has high oral bioavailability

c) Atenolol has high β1 receptor selectivity

d) Metoprolol has low lipid solubility thus its absorption and bioavailability are

limited

e) Labetalol antagonises α1:β adrenoreceptorswith a ratio of 1:7 when administered orally

Answer: c

Explanation

β-blockers are all competitive antagonists and demonstrate varying degrees of selectivity

for the β1 adrenoreceptor. This ‘cardioselectivity’ is advantageous as β2 antagonism

could potentially result in bronchoconstriction. Cardioselectivity can be expected

from atenolol, esmolol and metoprolol at moderate therapeutic doses but is lost at

higher doses. Propranolol and metoprolol are both highly lipophilic and are well

absorbed from the gut; however, for both drugs, significant first-pass metabolism

limits their bioavailability. Labetalol antagonises α1:β adrenoreceptors with a ratio of

1:7 when administered intravenously; when given orally the ratio is 1:3. With esmolol,

a loading dose of 0.1 mg/kg would be unlikely to exert a clinically detectable effect.

A bolus of 0.5 mg/kg is the recommended starting point and often needs repeating

five minutes later before an infusion is started at 50 to 200 mcg/kg per min. Beware of

the widely different presentation concentrations of esmolol where 10mL may contain

100mg (10 mg/mL) or 2.5 g (250 mg/mL).

Reference

Peck T, Hill S, Williams M. Pharmacology for Anaesthesia and Intensive Care, 2nd edn.

London: Greenwich Medical Media, 2004; pp.213–20.