Anaes · Anaesthetic adjuncts

Phenylephrine — the pure alpha-1 vasopressor

Also known as Phenylephrine · Neosynephrine · Pure alpha-1 agonist · Selective alpha-1 vasopressor · Obstetric spinal hypotension vasopressor · Non-catecholamine alpha-1 agonist

Phenylephrine is the prototype PURE ALPHA-1 ADRENERGIC AGONIST with no beta-1, no beta-2 and no alpha-2 activity at clinical doses, and this single pharmacological fact shapes everything it does. It is a synthetic phenylethylamine that is NOT a catecholamine (it lacks the 3,4-dihydroxyl catechol ring) and is therefore not a substrate for catechol-O-methyltransferase (COMT) — it is instead metabolised by monoamine oxidase (MAO), giving it a longer duration than the catecholamines and the unusual property of being orally active. It activates the alpha-1 receptor (Gq-coupled), driving phospholipase C, inositol triphosphate (IP3) and diacylglycerol (DAG), raising intracellular calcium and producing arteriolar and venous vasoconstriction, an increased systemic vascular resistance (SVR) and a rise in blood pressure. Because it has no beta-1 effect, the rise in blood pressure is met by a BARORECEPTOR-MEDIATED REFLEX BRADYCARDIA and the cardiac output (CO) may fall. The exam-critical roles follow directly: phenylephrine is the FIRST-LINE vasopressor for obstetric spinal-induced hypotension in caesarean section because it produces a higher umbilical artery pH than ephedrine (Lee 2002 WMD +0.03; Ngan Kee 2009 UV/MA 0.17 vs 1.13; Singh 2020 rank-possibilities) — unlike ephedrine, it crosses far less so drives less fetal beta stimulation and metabolic acidosis. It is the vasopressor of choice whenever tachycardia is undesirable (aortic stenosis, hypertrophic obstructive cardiomyopathy, ischaemic heart disease — taught), and it is used as a nasal decongestant, a mydriatic and (classically) to terminate supraventricular tachycardia through its reflex vagal effect (taught). Onset is less than 1 minute after IV bolus, duration 15 to 20 minutes (taught kinetics); bolus and infusion doses per local formulary and protocol.

Checked against its sources on 11 Sept 2026