About This Calculator

The Phenylephrine Infusion Calculator is a clinical tool designed for healthcare professionals to accurately determine the correct infusion rate or dose of phenylephrine, a potent vasopressor used to treat hypotension. By inputting key patient and medication parameters, this tool helps ensure precise administration, which is critical for managing hemodynamics effectively and safely.

Outputs Explained

The calculator provides one of two primary outputs based on the selected mode:

• Infusion Rate (mL/hr): When you input the desired dose (in mcg/kg/min or mcg/min), the tool calculates the corresponding pump rate required to deliver that dose based on the specified drug concentration.
• Dose (mcg/min and mcg/kg/min): When you input the current infusion rate (in mL/hr), the tool calculates the exact dose the patient is receiving. It provides this in both a weight-based (mcg/kg/min) and non-weight-based (mcg/min) format.

How to Use the Calculator

Follow these steps for accurate calculations:

1. Select Calculation Mode: Choose whether you need to calculate the 'Rate' from a known dose or calculate the 'Dose' from a known rate.
2. Enter Patient Weight: Input the patient's weight and select the unit (kg or lbs). This is mandatory for mcg/kg/min calculations.
3. Provide Dose or Rate: Depending on the mode, enter either the desired dose or the current infusion rate.
4. Set Drug Concentration: Select a standard concentration from the dropdown menu (e.g., 40 mg in 250 mL) or choose 'Custom' to enter the specific milligrams of phenylephrine and total volume in milliliters (mL) for your institution's preparation.
5. Calculate: The tool will instantly display the calculated results, which should be verified before clinical use.

Dosing Overview

Phenylephrine is a pure alpha-1 adrenergic agonist that increases blood pressure through vasoconstriction. Dosing must be carefully titrated to achieve the target mean arterial pressure (MAP) while monitoring for adverse effects.

• IV Bolus: A common initial approach for severe hypotension is an IV push of 50-100 mcg, which can be repeated every 1-2 minutes as needed.
• Continuous IV Infusion: A continuous infusion is used for sustained blood pressure support. The typical starting dose is 0.5 mcg/kg/min or 40-60 mcg/min. The dose is then titrated up or down to maintain the desired MAP, usually within a maintenance range of 0.5 to 2 mcg/kg/min.

Switching To/From Other Vasopressors

Transitioning between phenylephrine and other vasopressors (e.g., norepinephrine) requires careful management. There is no standard conversion ratio. The process typically involves starting the new agent at a low dose while gradually titrating down the initial agent. This "overlapping" technique should be performed with continuous arterial blood pressure monitoring to ensure hemodynamic stability.

Managing Infusion Interruptions

Due to its short half-life (approximately 2-3 minutes), any interruption in a phenylephrine infusion will cause a rapid drop in blood pressure. If the infusion is stopped, it must be restarted promptly and re-titrated to the target blood pressure. A "missed dose" concept does not apply to continuous infusions; any interruption is a critical event requiring immediate action.

Safety Alerts

• Extravasation: Phenylephrine is a potent vasoconstrictor and can cause severe tissue necrosis if it leaks out of the vein (extravasates). Administration through a central venous catheter is strongly preferred, especially for high concentrations or prolonged infusions.
• Reflex Bradycardia: The increase in blood pressure caused by phenylephrine can trigger a baroreceptor-mediated reflex, leading to a decrease in heart rate (bradycardia). Monitor heart rate closely.
• Administration Site: If a peripheral IV must be used, it should be a large, stable vein, and the site must be monitored frequently for signs of infiltration.
• Verification: All calculations for critical medications like vasopressors should be independently double-checked by another qualified healthcare professional.

Frequently Asked Questions (FAQ)

Why is patient weight required for some calculations?

Patient weight is necessary to calculate or determine a weight-based dose (mcg/kg/min). This dosing strategy normalizes the dose to the patient's size, which is a common practice in critical care. If you are working with a non-weight-based dose (mcg/min), weight is not strictly required for the calculation but is still useful for context.

What is the difference between mcg/min and mcg/kg/min?

mcg/min is an absolute dose rate, delivering a fixed amount of drug per minute regardless of patient size. mcg/kg/min is a weight-based dose rate, where the total amount of drug delivered per minute depends on the patient's weight in kilograms.

What should I do if my hospital uses a concentration not listed in the presets?

Select "Custom..." from the Drug Concentration dropdown menu. This will reveal fields where you can enter the exact total amount of drug (in mg) and the total solution volume (in mL) for your specific preparation.

Can this calculator be used for IV push (bolus) doses?

No, this tool is specifically designed for continuous IV infusions. IV push doses are typically drawn up to a specific concentration and administered as a fixed volume (e.g., 1 mL of a 100 mcg/mL solution), not run as a rate on an infusion pump.

Why are there multiple standard concentrations available?

Hospitals and pharmacies prepare phenylephrine infusions in various standard concentrations depending on clinical needs. More concentrated solutions are used to limit the total volume of fluid administered to the patient, which is important in fluid-sensitive conditions like heart failure or kidney disease.

What happens if I enter weight in pounds (lbs)?

The calculator automatically converts the weight from pounds to kilograms (kg) for any weight-based calculations, using the conversion factor 1 kg ≈ 2.20462 lbs. The help text below the weight input shows the converted value for confirmation.

How is the final drug concentration (mcg/mL) determined?

The concentration is calculated by converting the total drug amount from milligrams (mg) to micrograms (mcg) and dividing by the total solution volume in milliliters (mL). For example, 40 mg in 250 mL is (40 mg * 1000 mcg/mg) / 250 mL = 160 mcg/mL.

Is this calculator a substitute for clinical judgment?

Absolutely not. This tool is for educational and calculation support purposes only. It does not replace the clinical judgment, experience, and responsibility of a qualified healthcare provider. All results must be confirmed before application in a clinical setting.

References

1. U.S. Food and Drug Administration (FDA). Neo-Synephrine (phenylephrine) Prescribing Information. Drugs@FDA Database.
2. DailyMed - National Library of Medicine. Phenylephrine Hydrochloride Injection Label.
3. Richards, E., & Holt, G. (2023). Phenylephrine. In: StatPearls [Internet]. StatPearls Publishing.
4. Panchal, A. R., et al. (2020). Part 3: Adult Basic and Advanced Life Support: 2020 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation, 142(16_suppl_2), S366-S468. (Provides context on vasopressor use in resuscitation).