Anaes · Anaesthetic adjuncts

Metaraminol — the mixed-acting vasopressor (alpha-1 plus indirect noradrenaline release)

Also known as Metaraminol · Aramine · Mixed-acting vasopressor · Alpha-1 agonist with indirect noradrenaline release · Perioperative bolus vasopressor

Metaraminol is a SYNTHETIC SYMPATHOMIMETIC AMINE with a DUAL mechanism that sets it apart from every other vasopressor on the anaesthetic tray. It acts DIRECTLY as an alpha-1 agonist, producing arteriolar and venous vasoconstriction and a rise in systemic vascular resistance and blood pressure; AND it acts INDIRECTLY by releasing noradrenaline from sympathetic nerve terminals, the released noradrenaline in turn stimulating cardiac beta-1 receptors and supporting the heart rate and the cardiac output. This mixed-acting profile is the single most important fact about the drug. The haemodynamic signature is a rise in systemic vascular resistance and blood pressure from the alpha-1 vasoconstriction, with a MAINTAINED or slightly INCREASED cardiac output and heart rate from the noradrenaline-release beta effect, which is the critical difference from phenylephrine, a pure alpha-1 agonist whose blood-pressure rise triggers a baroreceptor-mediated reflex bradycardia and may lower the cardiac output. The clinical roles flow from this pharmacology: metaraminol is a popular first-line bolus vasopressor for anaesthesia-induced and perioperative hypotension wherever maintaining the cardiac output is desirable, a push-dose emergency treatment for post-intubation hypotension (demonstrated by da Silveira and colleagues), and an established option for obstetric spinal hypotension where the effective dose depends on the maternal body mass index (shown by Gao and colleagues). The intravenous bolus dose is 0.5 to 2 mg and the duration of a bolus is about 20 to 30 minutes (longer than phenylephrine, whose bolus lasts about 5 minutes); it is metabolised by monoamine oxidase. Adverse effects include tachycardia and arrhythmias from the beta effect of the released noradrenaline, tissue necrosis on extravasation, reduced splanchnic and renal perfusion at high doses, and TACHYPHYLAXIS as noradrenaline stores are depleted with prolonged use. Metaraminol is less expensive than several alternatives and carries a lower environmental cost (Parkinson and colleagues). Built on the da Silveira push-dose metaraminol study for post-intubation arterial hypotension, the Gao maternal body mass index and metaraminol dosing study, the Parkinson financial and environmental cost comparison of adrenaline, ephedrine, metaraminol and phenylephrine, the Dong vasopressor selection and postoperative delirium in older adults study, and the Turhan erector spinae plane block versus thoracic paravertebral block study.