Pharmacology for Anaesthesia and Intensive Care

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Adrenoceptor antagonists

α-Adrenoceptor antagonists


β-Adrenoceptor antagonists


Combinedα- andβ-adrenoceptor antagonists

α-Adrenoceptor antagonists

α-Adrenoceptor antagonists (α-blockers) prevent the actions of sympathomimetic

agents onα-adrenoceptors. Certainα-blockers (phentolamine, phenoxybenzamine)

are non-specific and inhibit bothα 1 - andα 2 -receptors, whereas others selectively

inhibitα 1 -receptors (prazosin) orα 2 -receptors (yohimbine). The actions of specific

α-adrenoceptor stimulation are shown in Table13.1.

Non-selectiveα-blockade

Phentolamine

Phentolamine (an imidazolone) is a competitive non-selectiveα-blocker. Its affinity

forα 1 -adrenoceptors is three times that forα 2 -adrenoceptors.

Presentation

Itis presented as 10 mg phentolamine mesylate in 1 ml clear pale-yellow solution.

The intravenous dose is 1–5 mg and should be titrated to effect. The onset of action

is 1–2 minutes and its duration of action is 5–20 minutes.

Uses

Phentolamine is used in the treatment of hypertensive crises due to excessive sym-

pathomimetics, MAOI reactions with tyramine and phaeochromocytoma, especially

during tumour manipulation. It has a role in the assessment of sympathetically medi-

ated chronic pain and has previously been used to treat pulmonary hypertension.

Injection into the corpus cavernosum has been used to treat impotence due to erec-

tile failure.

Effects


Cardiovascular –α 1 -blockade results in vasodilatation and hypotension whileα 2 -

blockade facilitates noradrenaline release leading to tachycardia and a raised