Pharmacology for Anaesthesia and Intensive Care

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12 Sympathomimetics

Effects


Cardiovascular – phenylephrine raises the systemic vascular resistance and blood

pressure and may result in a reflex bradycardia, all of which results in a drop in

cardiac output. It is not arrhythmogenic.


Central nervous system – it has no stimulatory effects.


Renal – blood flow falls in a manner similar to that demonstrated by noradrenaline.


Uterus – while its use in obstetrics results in a more favorable cord gas profile

it has not yet gained widespread acceptance due to the possibility of accidental

overdose.

Kinetics

Intravenous administration results in a rapid rise in blood pressure, which lasts

5–10 minutes, while intramuscular or subcutaneous injection takes 15 minutes to

work but lasts up to 1 hour. It is metabolized in the liver by MAO. The products of

metabolism and their route of elimination have not been identified.

Methoxamine

Methoxamine (no longer in production) is a direct-acting sympathomimetic amine

with specificα 1 -agonist actions. Consequently it has similar effects to phenylephrine.

Itis presented as a clear solution containing 20 mg.ml−^1.

An intravenous bolus of 1 mg usually produces a rapid rise in systemic vascular

resistance and, therefore, blood pressure, often with an accompanying reflex brady-

cardia. It may also be given intramuscularly or subcutaneously at a higher dose for

amore prolonged duration of action.

β-Agonists

Isoprenaline

Isoprenaline is a highly potent synthetic catecholamine with actions atβ 1 - and

β 2 -adrenoceptors. It has noαeffects. It is not available in the UK any longer.

Presentation and uses

Isoprenaline is presented as a clear solution containing 1 mg.ml−^1 for intravenous

infusion and as a metered dose inhaler delivering 80 or 400μg. It is no longer used to

treat reversible airway obstruction as this was associated with an increased mortality.

Morespecificβ 2 -agonists are now used (e.g. salbutamol). The 30 mg tablets are

very rarely used. It is used intravenously to treat severe bradycardia associated with

atrioventricular (AV) block orβ-blockers (dose range 0.5–10μg.min−^1 ).

Effects


Cardiovascular – stimulation ofβ 1 -adrenoceptors increases heart rate, myocardial

contractility, automaticity and cardiac output. The effects on blood pressure are