Emergency and Critical Care

Phenylephrine Infusion Rate Calculator (mcg/kg/min to mL/hr)

Converts phenylephrine dose (mcg/kg/min) to infusion rate (mL/hr) for hypotension.

Primary source: Российские КР по анестезиологии и интенсивной терапии.

Aleksandr A. Aulov, MD, internal medicine Last clinical review: April 28, 2026

Online calculation

Patient weight

Body weight

Phenylephrine dose

Therapeutic range 0.5–9 mcg/kg/min

mcg/kg/min

Drug amount in solution

Standard: 10 mg (1 ampule 1% – 1 mL) in 100 mL

Solution volume

Standard: 100 mL normal saline

About this score

Phenylephrine is a selective α1-agonist. Unlike mixed agonists, it lacks β-activity, so it does not cause tachycardia and is well suited for patients with tachyarrhythmias.

Main indications: hypotension after spinal/epidural anesthesia (standard in obstetric anesthesia), vasoplegic shock in patients with atrial fibrillation with rapid ventricular response or other tachyarrhythmias (where norepinephrine and epinephrine would worsen the arrhythmia), and symptomatic treatment of hypotension in general anesthesia practice.

Therapeutic infusion range: 0.5–9 mcg/kg/min (or 50–300 mcg as a bolus during anesthesia). Standard dilution: 10 mg (1 ampule 1% – 1 mL) in 100 mL normal saline, yielding a concentration of 100 mcg/mL.

When to use

Hypotension after spinal/epidural anesthesia — standard in obstetric anesthesia (preferred over ephedrine).
Vasoplegic shock in a patient with tachyarrhythmias (atrial fibrillation with rapid ventricular response, sinus tachycardia).
Symptomatic hypotension in general anesthesia practice (acute episodes).
Alternative to norepinephrine in severe cardiac ischemia (no β1 stimulation, does not increase myocardial O2 demand).

Clinical example

Case

70 kg patient after spinal anesthesia for cesarean section. BP drops to 80/50, HR 60. Phenylephrine infusion started at 1 mcg/kg/min. Dilution: 10 mg in 100 mL.

Calculation

Concentration: 10×1000/100 = 100 mcg/mL. Rate: 1×70×60/100 = 42 mL/hr.

Interpretation and management

Therapeutic dose. Target SBP ≥ 90 if normotensive before anesthesia. Monitor heart rate — if it falls below 50 (reflex bradycardia from α-vasoconstriction), consider atropine or switching to ephedrine/epinephrine. Concurrently, use Trendelenburg positioning and crystalloid infusion.

Limitations and cautions

Does not increase cardiac output (no β-effect) — in patients with low cardiac output, may worsen organ perfusion despite raising blood pressure.
Reflex bradycardia due to increased blood pressure — may cause ischemia in elderly patients.
Extravasation causes tissue necrosis. Use central access only for prolonged infusion.
Not first-line in septic shock — norepinephrine is preferred there.

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Frequently asked questions

Phenylephrine or ephedrine for hypotension after spinal anesthesia?

Current guidelines (RCOG, ASA) favor phenylephrine — it crosses the placental barrier to a lesser extent and does not cause fetal acidosis. Ephedrine is reserved for pronounced bradycardia.

Bolus vs infusion administration?

Bolus 50–200 mcg — for rapid correction of a hypotensive episode during anesthesia. Infusion 0.5–9 mcg/kg/min — for sustained support in critical care.

Can it be given peripherally?

As a bolus — yes (standard in anesthesia practice). For infusions lasting more than 6 hours, use central access.

What to do in case of reflex bradycardia?

If heart rate falls below 50, assess blood pressure. If BP is at target and heart rate is stable, continue. If BP drops along with bradycardia, give atropine 0.5 mg IV and consider switching to a mixed agonist.

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Aleksandr A. Aulov Physician, internal medicine, Sechenov School — independent medical education platform. Calculators are compiled from the original publications and current clinical guidelines. Interpretation thresholds follow the source study unless stated otherwise.

This calculator is intended for healthcare professionals. Do not use it for self-diagnosis or self-treatment. Management decisions are made by the treating clinician based on the full clinical picture of the individual patient.