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 / ↔ / ↑↑
- 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
- Pulmonary EmbolismSupporting the failing RV: fluids, drugs, oxygen and the airway: Norepinephrine restores the coronary perfusion gradient without raising PVR (ESC 2019); above about 15 mcg/min it may raise PVR, so AHA/ACC 2026 suggests adding vasopressin or phenylephrine rather than pushing it higher. Dobutamine (2–20 mcg/kg/min, ESC 2019) can raise a low cardiac index once pressure holds but alone can drop BP; AH
- Shock Types at a GlanceFindings and first treatments: Fluids worsen LV cardiogenic shock with a high wedge and crackles. RV infarct is preload-dependent: cautious fluids, no nitrates or diuretics. Bleeding: limit crystalloid; accept SBP 80–90 until controlled, except with brain injury. Neurogenic: phenylephrine can worsen bradycardia.
- Spinal Motion RestrictionSpinal shock and neurogenic shock: Cord injury hides bleeding (no pain or guarding below the level, blunted heart rate, warm skin), so image the chest, abdomen and pelvis before blaming the cord (Consortium 2008). Restore volume without overload, then start a vasopressor. Norepinephrine is the usual choice because it adds β1 rate and contractility to α1 tone; phenylephrine can deepen bradycardia;
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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 your local protocols and medical direction.