Phenylephrine (Neosynephrine) Dosing Guidelines
For the treatment of clinically important hypotension during anesthesia, phenylephrine hydrochloride should be administered as an intravenous bolus with an initial dose of 40-100 mcg, with additional boluses up to 200 mcg every 1-2 minutes as needed, titrated to the desired blood pressure response. 1
Intravenous Administration
Phenylephrine hydrochloride injection is available as a ready-to-use formulation that must not be diluted before administration as an intravenous bolus. The dosing regimen is as follows:
- Initial bolus dose: 40-100 mcg IV
- Additional boluses: Up to 200 mcg every 1-2 minutes as needed
- Titration: Adjust dose according to the pressor response
The 500 mcg/5 mL (100 mcg/mL) formulation is supplied as a 5 mL single-dose ampule containing 0.5 mg of phenylephrine hydrochloride per ampule.
Clinical Considerations
Patient Monitoring
When administering phenylephrine, careful monitoring is essential:
- Blood pressure
- Heart rate
- Signs of peripheral vasoconstriction
- Cardiac status
Potential Adverse Effects
Common adverse reactions during treatment include:
- Nausea
- Vomiting
- Headache
Special Precautions
Exercise caution in patients with:
- Severe arteriosclerosis or history of angina
- Underlying heart failure
- Pulmonary arterial hypertension
- Risk of peripheral and visceral ischemia
Drug Interactions
Medications that Increase Phenylephrine Effects:
- Monoamine oxidase inhibitors (MAOIs)
- Oxytocin and oxytocic drugs
- Tricyclic antidepressants
- Angiotensin and aldosterone
- Atropine
- Steroids
- Norepinephrine transporter inhibitors
- Ergot alkaloids
Medications that Decrease Phenylephrine Effects:
- α-adrenergic antagonists
- Phosphodiesterase Type 5 inhibitors
- Mixed α-and β-receptor antagonists
- Calcium channel blockers
- Benzodiazepines
- ACE inhibitors
- Centrally acting sympatholytic agents
Important Warnings
- Extravasation risk: Extravasation during IV administration may cause necrosis or sloughing of tissue
- Bradycardia: Can cause severe bradycardia and decreased cardiac output
- Ischemia: Can cause excessive peripheral and visceral vasoconstriction and ischemia to vital organs
- Cardiac effects: Can exacerbate underlying heart failure and increase pulmonary arterial pressure
Phenylephrine should be used with caution in patients with cardiac disease, hypertension, aneurysms, long-standing insulin-dependent diabetes, or advanced arteriosclerosis 2. It is contraindicated in patients taking monoamine oxidase inhibitors or tricyclic antidepressants due to potential dangerous drug interactions.
Remember that phenylephrine is an alpha-1 adrenergic receptor agonist indicated specifically for the treatment of clinically important hypotension resulting primarily from vasodilation in the setting of anesthesia 1. Its use should be carefully monitored and the dose adjusted according to the individual patient's response.