Phenylephrine

Phenylephrine is a selective alpha 1 adrenergic agonist. It raises blood pressure by vasoconstriction without direct cardiac stimulation, and it causes reflex bradycardia. Its main uses are prevention and treatment of hypotension under anaesthesia (especially after spinal anaesthesia), vasodilatory shock when other agents are unavailable, topical mydriasis and nasal decongestion.

Classification. Sympathomimetic; direct acting selective alpha 1 agonist.

Mechanism. Stimulates alpha 1 receptors on vascular and iris dilator smooth muscle, increasing systemic vascular resistance and venous tone, and dilating the pupil without cycloplegia.

Pharmacodynamics. IV onset within 1 minute, duration 15 to 20 minutes after a bolus. Mean arterial pressure rises; heart rate and cardiac output often fall. Renal and splanchnic flow may fall at high doses.

Pharmacokinetics. Oral bioavailability is about 38% because of first pass metabolism by monoamine oxidase, so oral decongestant doses are ineffective. Hepatic metabolism and urinary excretion. IV half life about 2 to 3 hours with an effective action of 15 to 20 minutes.

Hypotension during anaesthesia, adult.

• Bolus 50 to 100 micrograms IV, repeated every 1 to 2 minutes as needed; usual maximum 200 micrograms in one bolus.

• Infusion 25 to 50 micrograms/minute (about 0.25 to 0.5 micrograms/kg/min), titrated to the target pressure; prophylaxis at spinal anaesthesia for caesarean delivery is commonly 25 to 50 micrograms/minute from induction, reduced if bradycardia appears.

Vasodilatory shock, adult (second line). Noradrenaline is preferred. Where only phenylephrine is available: start 0.5 micrograms/kg/min IV, titrate every 5 to 10 minutes to mean arterial pressure at least 65 mmHg; doses over 3 micrograms/kg/min rarely add benefit. Phenylephrine is useful when tachyarrhythmia limits catecholamine use or in hypertrophic cardiomyopathy with hypotension.

Mydriasis, adult and child. Phenylephrine 2.5% eye drops, one drop, repeated once after 10 minutes if needed. The 10% concentration is not advised in older adults, infants or those with cardiovascular disease because of severe hypertension.

Nasal decongestion, adult. 0.25% to 0.5% nasal drops or spray, 1 to 2 sprays in each nostril every 4 hours, for no more than 3 days to avoid rebound congestion. Oral phenylephrine has no reliable decongestant effect and is not advised.

Ischaemic priapism, specialist. Intracavernosal phenylephrine 100 to 500 micrograms every 3 to 5 minutes, up to 1 mg, with blood pressure and ECG monitoring.

Children, hypotension. Bolus 1 to 10 micrograms/kg IV, repeated as needed; infusion 0.1 to 0.5 micrograms/kg/min; specialist.

Contraindications. Severe hypertension; ventricular tachycardia or other tachyarrhythmias; narrow angle glaucoma (topical use); hypersensitivity.

Warnings. Reflex bradycardia and reduced cardiac output, which can be harmful in heart failure or low output states. Extravasation causes severe tissue necrosis. Rebound hypertension, pulmonary oedema and ischaemia at high doses. Contains sulphite in some products.

Adverse effects. Hypertension, bradycardia, headache, nausea, anxiety, piloerection, urinary retention, reduced renal blood flow.

Interactions. Monoamine oxidase inhibitors and tricyclic antidepressants cause severe pressor responses. Oxytocic drugs and ergometrine add to the pressor effect. Beta blockers worsen bradycardia. Halogenated anaesthetics may increase arrhythmia risk. Antimuscarinics block reflex bradycardia and add to hypertension.

Special groups.

• Pregnancy: widely used to treat hypotension at caesarean delivery; limits foetal acidosis compared with ephedrine.

• Breastfeeding: compatible with short term use.

• Older adults: avoid the 10% eye drop; risk of hypertension and arrhythmia.

• Kidney and liver impairment: no adjustment, though high doses may reduce renal flow.

Extravasation. Infiltrate the area with phentolamine 5 to 10 mg in 10 mL 0.9% sodium chloride as soon as possible.

Formulations. Injection 10 mg/mL (must be diluted) and 100 micrograms/mL prefilled; eye drops 2.5% and 10%; nasal sprays.

Preparation. For boluses, dilute 10 mg in 100 mL to 100 micrograms/mL. For infusion, a concentration of 100 micrograms/mL in 0.9% sodium chloride or 5% glucose, run through a large vein; central access is preferred for prolonged infusion.

Baseline. Blood pressure, heart rate, ECG, volume status.

Monitoring. Continuous arterial or frequent cuff blood pressure, heart rate, perfusion, urine output, infusion site. Titrate down if heart rate under 50/min.

Stopping. Wean gradually once blood pressure is stable; abrupt cessation can cause rebound hypotension.

Common prescribing errors. Giving the undiluted 10 mg/mL solution as a bolus; using 10% eye drops in infants or older adults; using it in low output states where reflex bradycardia lowers cardiac output.