Anaes SAQs · Anaesthetic adjuncts

Metaraminol — mixed-acting vasopressor: pharmacology, indications and the push-dose role

Metaraminol is a SYNTHETIC SYMPATHOMIMETIC AMINE with a DUAL mechanism: DIRECT alpha-1 agonism (Gq-coupled receptor, phospholipase C to IP3 and DAG, raised intracellular calcium, arteriolar and venous vasoconstriction, raised SVR and blood pressure) PLUS INDIRECT release of noradrenaline from sympathetic nerve terminals (taken up by uptake-1, displaces noradrenaline from vesicles; the released noradrenaline stimulates cardiac beta-1 receptors). The crucial haemodynamic consequence: metaraminol does NOT produce a reflex bradycardia (unlike phenylephrine) because the released noradrenaline offsets the baroreceptor reflex, so the cardiac output is MAINTAINED or slightly INCREASED. Duration of a bolus about 20 to 30 minutes (taught kinetics); metabolised by monoamine oxidase (taught); IV bolus 0.5 to 2 mg (taught formulary; local protocol governs). Indications: perioperative vasodilatory hypotension (common first-line bolus, especially in ANZ practice — taught); push-dose for post-intubation arterial hypotension (da Silveira retrospective cohort, n-349 septic ICU, association only, no mortality difference); obstetric spinal hypotension (continuous-infusion ED50 1.84 versus 2.28 mg/h by BMI per Gao); where maintaining cardiac output is desirable. Adverse effects: tachycardia and arrhythmias (beta effect of released noradrenaline), tissue necrosis on extravasation (phentolamine used for 0.8% of peripheral starts — Shyu 2025), reduced splanchnic and renal perfusion at high doses, TACHYPHYLAXIS (noradrenaline stores depleted), potentiation by monoamine oxidase inhibitors. Less expensive than some alternatives with a competitive environmental footprint (Parkinson stewardship scope). Built on the da Silveira push-dose metaraminol cohort, the Gao maternal BMI infusion-ED50 study, the Dong vasopressor selection and delirium study (null scope; metaraminol not studied), the Shyu peripheral-vasopressor cohort, and the Lee 2002 obstetric systematic review.