Pharmacology for Anaesthesia and Intensive Care

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12 Sympathomimetics

Ritodrine

Ritodrine is aβ 2 -agonist that is used to treat premature labour. Tachycardia (β 1

effect) is often seen during treatment. It crosses the placenta and may result in foetal

tachycardia.

Ritodrine has been associated with fatal maternal pulmonary oedema. It also

causes hypokalaemia, hyperglycaemia and, at higher levels, vomiting, restlessness

and seizures.

Terbutaline

Terbutaline is aβ 2 -agonist with some activity atβ 1 -adrenoceptors. It is used in the

treatment of asthma and uncomplicated preterm labour. It has a similar side-effect

profile to other drugs in its class.

Mixed (αandβ)

Ephedrine

Ephedrine is found naturally in certain plants but is synthesized for medical use.

Presentation and uses

Ephedrine is formulated as tablets, an elixir, nasal drops and as a solution for injection

containing 30 mg.ml−^1 .Itcan exist as four isomers but only thel-isomer is active.

Itis used intravenously to treat hypotension associated with regional anaesthesia.

Inthe obstetric setting this is now known to result in a poorer cord gas pH when

compared to purerαagonists, but its widespread use persists due to the potential

for theαagonists to cause a significant maternal hypertension. It is also used to treat

bronchospasm, nocturnal enuresis and narcolepsy.

Mechanism of action

Ephedrine has both direct and indirect sympathomimetic actions. It also inhibits the

actions of MAO on noradrenaline.

Owing to its indirect actions it is prone to tachyphylaxis as noradrenaline stores

in sympathetic nerves become depleted.

Effects


Cardiovascular – it increases the cardiac output, heart rate, blood pressure, coro-

nary blood flow and myocardial oxygen consumption. Its use may precipitate

arrhythmias.


Respiratory – it is a respiratory stimulant and causes bronchodilation.


Renal – renal blood flow is decreased and the glomerular filtration rate falls.


Interactions – it should be used with extreme caution in those patients taking

MAOI.