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Alpha-1 Adrenergic Agonist (Pure Vasoconstrictor) Pregnancy: Used routinely in obstetric regional anaesthesia — preferred for spinal hypotension in obstetrics

Phenylephrine

Brand names: Metaraminol (different drug, sometimes confused)

Adult dose

Dose: Spinal hypotension: 50–100 mcg IV bolus (repeat every 2–5 min). Infusion: 25–100 mcg/min. Alternative for hypotension: 50 mcg IV bolus titrated.
Route: IV
Frequency: Bolus or infusion
Max: Titrate to blood pressure response
Pure alpha-1 agonist — increases SVR without direct cardiac stimulation. Used for spinal anaesthesia hypotension in obstetrics. Reflex bradycardia occurs. Preferred over ephedrine in non-fetal-bradycardic obstetric settings.

Paediatric dose

Dose: 5 mcg/kg
Route: IV
Frequency: Bolus PRN
Max: 20 mcg/kg per bolus
Concentration: 10 mg/mL (concentrated) — must be diluted mcg/ml
Specialist paediatric anaesthesia use only. Always dilute before administration.

Dose adjustments

Renal

No dose adjustment

Hepatic

No dose adjustment

Paediatric weight-based calculator

Specialist paediatric anaesthesia use only. Always dilute before administration.

Clinical pearls

  • Obstetric spinal anaesthesia: phenylephrine preferred over ephedrine as first-line vasopressor for spinal hypotension (Ngan Kee et al: less fetal acidosis with phenylephrine)
  • Pure vasoconstrictor — no direct cardiac effect, so heart rate and inotropy unchanged (contrast with ephedrine)
  • Reflex bradycardia: if HR <60 bpm during phenylephrine infusion, reduce infusion and consider ephedrine instead
  • Always dilute concentrated preparation — errors with undiluted phenylephrine have caused fatal hypertension

Contraindications

  • Severe peripheral vascular disease
  • Bradycardia (relative — causes reflex bradycardia)
  • Angle-closure glaucoma (topical use)

Side effects

  • Reflex bradycardia (can be significant)
  • Hypertension (excess)
  • Reduced cardiac output
  • Peripheral vasoconstriction
  • Nausea

Interactions

  • MAOIs — severe hypertension
  • Beta-blockers — exaggerated bradycardia from reflex response
  • Ergot alkaloids — additive vasoconstriction

Monitoring

  • Blood pressure (frequent/continuous)
  • Heart rate (reflex bradycardia monitoring)
  • Fetal heart rate (obstetric use)

Reference: BNFc; BNF; Ngan Kee (Anesthesiology 2009); OAA/AAGBI Obstetric Anaesthesia Guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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