Vasopressor · Alpha agonist
Phenylephrine (Neo-Synephrine)
Also called Neo-Synephrine
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Indications
- Hypotension from vasodilation during anesthesia, neuraxial or general (label)
- Spinal hypotension at cesarean delivery: preferred over ephedrine for better fetal acid–base status
- Dynamic LVOT obstruction (HOCM, systolic anterior motion), where inotropes worsen the gradient (off-label)
- Push-dose boluses for brief hypotension around intubation outside the OR (off-label)
- Septic shock only when norepinephrine causes serious tachyarrhythmias, or as salvage (off-label)
Dosing
Phenylephrine drip card and mL/h converterStart: 10–35 mcg/min
| Where | Dose |
|---|---|
| Blood pressure boluses | Phenylephrine: 50–100 mcg (0.5–1 mL of 100 mcg/mL) every 1–2 min |
On the quick cards
Mechanism of action
Phenylephrine is a synthetic, direct-acting α1 agonist with little β activity at clinical doses. α1 receptors couple to Gq: phospholipase C makes IP3, which releases calcium from the sarcoplasmic reticulum, and DAG, which activates protein kinase C, so arterioles and veins constrict. The rise in pressure triggers the baroreflex, which slows the heart through vagal output.
Pharmacology
- Cardiovascular: raises SVR, MAP and venous return, with reflex bradycardia; cardiac output falls when the ventricle is weak or afterload-sensitive
- Cardiovascular: raises diastolic (coronary perfusion) pressure without tachycardia, which suits aortic stenosis and LVOT obstruction
- Respiratory: raises PVR and pulmonary artery pressure, a poor choice in RV failure or pulmonary hypertension
- Renal: constricts renal and splanchnic arteries; the label warns of more renal replacement therapy in septic shock
- Other: little crosses the placenta (umbilical vein to maternal ratio 0.17, against 1.13 for ephedrine), so less fetal acidosis (Ngan Kee 2009)
Pharmacokinetics
| Onset | Within minutes IV (label) |
|---|---|
| Duration | A bolus lasts up to about 20 min (label) |
| Half-life | Effective about 5 min after an infusion; terminal 2.5 h (label) |
| Distribution | Vd about 340 L, wide tissue distribution (label) |
| Metabolism | MAO-A and MAO-B to inactive m-hydroxymandelic acid (57%) and sulfation (8%), partly in the gut wall; not a COMT substrate |
| Clearance | About 2,100 mL/min (label) |
| Elimination | Urine: 80% within 12 h and 86% by 48 h; 16% unchanged (label) |
Chemistry
- A phenylethanolamine: epinephrine without the 4-hydroxyl, so one meta (3) hydroxyl instead of a catechol; this cuts β activity and spares it from COMT, so it outlasts catecholamines
- Single (R)-(−) isomer; amine pKa about 9.0 (PubChem), so it is ionized and water-soluble at pH 7.4
- Ready-to-use 100 mcg/mL, or 10 mg/mL vials that must be diluted (100 times the bolus strength); pH 3.0–5.0; some 10 mg/mL vials contain sodium metabisulfite
Adverse effects and cautions
In the one-pagers
- Vasopressors and InotropesReceptors and effects: Phenylephrine: α1 only: ↓ reflex / ↔ / ↑↑
- Endocarditis and Valve EmergenciesValve emergencies: Critical AS: Syncope, HF; AF or low preload crashes it: Phenylephrine; cardiovert AF; TAVR or BAV
- CEN: Genitourinary, Gynecologic and Early-Pregnancy EmergenciesThe acute scrotum and penis: Ischemic priapism: Over 4 h, rigid, painful; injections, sickle cell, drugs: Aspiration and phenylephrine 100 to 500 mcg every 3 to 5 min; BP and HR
- Right Heart Failure and Pulmonary HypertensionManagement priorities: Phenylephrine: Avoid alone: Raises PVR
- After Cardiac SurgeryValves, TAVR, off-pump and minimally invasive surgery: Mitral repair with SAM: About 5–10%: volume, phenylephrine, beta-blocker
- Airway and RSIThe physiologically difficult airway: resuscitate first: Fluids alone do not prevent collapse: in PREPARE II (Russell, JAMA 2022; 1,065 patients) a 500 mL bolus at induction did not reduce cardiovascular collapse (21.0% vs 18.2%). For most patients the more reliable step is a vasopressor started before induction. Push-dose phenylephrine (50–200 mcg) or epinephrine (10–20 mcg) covers the gap; label
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Sources
- FDA label: Biorphen (2026)
- Ngan Kee 2009 (placental transfer)
- PubChem: phenylephrine
For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow the order, the label and your institution's protocols.