Anaes SAQs · Anaesthetic adjuncts

Phenylephrine — pure alpha-1 vasopressor: pharmacology, indications and obstetric use

Phenylephrine is the prototype PURE ALPHA-1 ADRENERGIC AGONIST (no beta-1, no beta-2, no alpha-2). The exam-critical content: Gq-coupled alpha-1 receptor activates phospholipase C, IP3 and DAG, raises intracellular calcium, produces arteriolar and venous vasoconstriction, raises the SVR and the blood pressure, and triggers a baroreceptor-mediated reflex bradycardia with a fall (or no rise) in cardiac output (Lee bradycardia RR 4.79). Short half-life about 5 minutes (taught; metabolised by monoamine oxidase); bolus and infusion doses per local formulary and protocol. Indications: vasodilatory perioperative hypotension (spinal, epidural, anaesthesia-induced); FIRST-LINE for obstetric spinal-induced hypotension in caesarean section on the basis of a higher umbilical artery pH than ephedrine (Lee WMD +0.03; ephedrine UV/MA 1.13 vs 0.17 with fetal beta stimulation and metabolic acidosis, Ngan Kee 2009); the vasopressor of choice where tachycardia is undesirable (aortic stenosis, HOCM, ischaemic heart disease — taught). Adverse effects: reflex bradycardia (can be severe), reduced cardiac output, tissue necrosis on extravasation (antidote phentolamine per protocol; Shyu 0.8% phentolamine use), reduced renal and splanchnic perfusion at high doses (taught), pulmonary vasoconstriction (caution in pulmonary hypertension — taught), potentiation by monoamine oxidase inhibitors (taught). Dong delirium and Ghaffar airway contexts are scoped adjuncts, not the pharmacology owners (see model answer).