Phenylephrine Infusion Dosing for Hypotension
For perioperative hypotension, initiate phenylephrine as a continuous infusion at 0.5-1.4 mcg/kg/min, or use bolus doses of 50-250 mcg; for vasodilatory shock, start at 0.5 mcg/kg/min and titrate up to 6 mcg/kg/min as needed. 1
FDA-Approved Dosing Regimens
The FDA label provides clear, context-specific dosing parameters that should guide clinical practice:
Perioperative Hypotension
- Continuous infusion: Start at 0.5 mcg/kg/min and titrate up to 1.4 mcg/kg/min based on blood pressure response 1
- Bolus administration: 50-250 mcg per dose for rapid correction 1
Vasodilatory Shock (Including Septic Shock)
- Continuous infusion: Start at 0.5 mcg/kg/min and titrate up to 6 mcg/kg/min to achieve target blood pressure 1
Critical Clinical Context: Phenylephrine vs. Norepinephrine
Important caveat: While phenylephrine has FDA-approved indications for hypotension, norepinephrine is strongly preferred over phenylephrine as first-line therapy in septic shock and most vasodilatory shock states. 2 The Surviving Sepsis Campaign guidelines explicitly recommend avoiding phenylephrine as a first-line vasopressor, reserving it only for specific circumstances such as when norepinephrine causes serious arrhythmias. 2
When Phenylephrine May Be Appropriate
- Perioperative hypotension during anesthesia (its primary FDA-approved indication) 1
- Situations where tachycardia is problematic and pure alpha-agonism is desired 1
- As a second-line agent when norepinephrine is contraindicated 2
Practical Dosing Considerations
Obstetric Anesthesia Context
For spinal anesthesia-induced hypotension during cesarean section, research provides more granular dosing guidance:
- Prophylactic infusion: 37.5-67 mcg/min (or 0.5 mcg/kg/min weight-adjusted) started immediately after spinal placement 3, 4
- Treatment bolus: The ED90 for treating established hypotension is approximately 150 mcg (95% CI: 98-222 mcg) 5
- Alternative prophylactic strategy: 500 mcg added to first liter of IV fluid in resource-limited settings 6
Weight-adjusted dosing (0.5 mcg/kg/min) reduces hypotension incidence compared to fixed-dose regimens (18.6% vs 35.2%, p=0.03) without increasing adverse effects. 4
Administration and Monitoring
Preparation
- Standard concentration: 10 mg/mL supplied in 1 mL vials; must be diluted before administration 1
- Administer via central line when possible to minimize extravasation risk 1
Monitoring Requirements
- Continuous blood pressure monitoring during titration 1
- Watch for reflex bradycardia and decreased cardiac output, which are common with phenylephrine 1
- Monitor for signs of extravasation; if occurs, infiltrate 5-10 mg phentolamine diluted in 10-15 mL saline at the site 7
Key Safety Considerations
Contraindications and Precautions
- Absolute contraindication: Hypersensitivity to phenylephrine or any component 1
- Relative contraindication: Hypovolemia—always correct volume depletion before or concurrent with vasopressor administration 7
- Use cautiously in ischemic heart disease due to increased myocardial oxygen demand 7
Common Adverse Effects
- Reflex bradycardia and decreased cardiac output (most common) 1
- Nausea, vomiting, headache, nervousness 1
- Reactive hypertension, especially with higher doses 4
- Tissue necrosis if extravasation occurs 1
Drug Interactions
- Potentiated effects with: MAOIs, beta-blockers, tricyclic antidepressants, oxytocic drugs 1
- Antagonistic effects with: alpha-adrenergic blocking agents 1
- Contains sulfites—risk of allergic reactions in susceptible individuals 1
Comparative Potency
Norepinephrine is approximately 11 times more potent than phenylephrine when used as bolus doses (95% CI: 8.1-16.9). 8 This means 100 mcg phenylephrine is roughly equivalent to 9 mcg norepinephrine. 8 This significant potency difference must be considered when switching between agents or calculating equivalent doses.