Pharmacology for Anaesthesia and Intensive Care

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12 Sympathomimetics

inhibition of proximal tubule Na+reabsorption and an improved cardiac output

and blood pressure.


Central nervous system – dopamine modulates extra-pyramidal movement and

inhibits the secretion of prolactin from the pituitary gland. It cannot cross the

blood–brain barrier, although its precursorl-dopa can.


Miscellaneous – owing to stimulation of the chemoreceptor trigger zone it causes

nausea and vomiting. Gastric transit time is also increased.


Interactions – despite being a direct-acting sympathomimetic amine the effects of

dopamine may be significantly exaggerated and prolonged during MAOI therapy.

Kinetics

Dopamine is only administered intravenously and preferably via a central vein. It acts

within 5 minutes and has a duration of 10 minutes. Metabolism is via MAO and COMT

in the liver, kidneys and plasma to inactive compounds (3,4-dihydroxyphenylacetic

acid and homovanillic acid (HVA)) which are excreted in the urine as sulphate and

glucuronide conjugates. About 25% of an administered dose is converted to nora-

drenaline in sympathetic nerve terminals. Its half-life is about 3 minutes.

Synthetic agents

Of the synthetic agents, only isoprenaline, dobutamine and dopexamine are classi-

fied as catecholamines as only they contain hydroxyl groups on the 3- and 4-positions

of the benzene ring.

α 1 - Agonists

Phenylephrine

Phenylephrine is a direct-acting sympathomimetic amine with potentα 1 -agonist

actions. It causes a rapid rise in systemic vascular resistance and blood pressure. It

has no effect onβ-adrenoceptors.

Presentation and uses

Phenylephrine is presented as a clear solution containing 10 mg in 1 ml. Phenyle-

phrine is presented as a clear solution containing 10 mg in 1 ml. Bolus doses of

50–100μgareused intravenously although 2–5 mg may be administered intramus-

cularly or subcutaneously for a more prolonged duration. It is used to increase a

low systemic vascular resistance associated with spinal anaesthesia or systemically

administered drugs. In certain patients, general anaesthesia may drop the systemic

vascular resistance and reverse a left-to-right intracardiac shunt, this may be reversed

byphenylephrine. It is also available for use as a nasal decongestant and mydriatic

agent. It may have a limited use in the treatment of supraventricular tachycardia

associated with hypotension.