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Vasopressors & Inotropes

Phenylephrine CRI

Vasopressor CRI for vasodilatory hypotension in dogs and cats, particularly when β-stimulation is undesired or contraindicated. Common indications include hypotension during anesthesia in patients with hypertrophic cardiomyopathy (cats), obstructive cardiac disease, atrial fibrillation, or tachyarrhythmias. Also used as a second-line alternative to norepinephrine. Continuous BP and ECG monitoring required; central venous access preferred.

Stock: 10 mg/mL (10 000 µg/mL), 1 mL ampule (10 mg)
How this calculator works

Enter the patient's weight and dose. The calculator picks a bag concentration that keeps the pump in its accurate range (≥ 2 mL/hr for most volumetric pumps), and a bag size that uses one full 10 mg ampule of stock.

Both selections show a suggested tag that updates as you change the patient inputs. Click any tab to override; the override sticks for the rest of the session, and a notice will appear if your choice doesn't match the patient.

If even the most dilute preparation gives an unworkable rate (very small patient on a low dose), switch to Advanced: target pump rate; you pick the rate you want, the calculator derives the bag concentration to deliver it.

Typical range: 0.5–3.0 ug/kg/min (dogs and cats)
100 mL preparation suggested · uses one full vial
Draw 1 mL of 10 mg/mL phenylephrine stock (1 vial = 10 mg)
Add to 100 mL bag of 0.9% NaCl or 5% dextrose
Final 100 µg/mL phenylephrine in 100 mL
250 mL preparation
Draw 2.5 mL of 10 mg/mL phenylephrine stock (2.50 vials = 25 mg)
Add to 250 mL bag of 0.9% NaCl or 5% dextrose
Final 100 µg/mL phenylephrine in 250 mL
500 mL preparation
Draw 5 mL of 10 mg/mL phenylephrine stock (5 vials = 50 mg)
Add to 500 mL bag of 0.9% NaCl or 5% dextrose
Final 100 µg/mL phenylephrine in 500 mL
100 mL preparation
Draw 0.4 mL of 10 mg/mL phenylephrine stock (0.40 of a vial; discard the rest = 4 mg)
Add to 100 mL bag of 0.9% NaCl or 5% dextrose
Final 40 µg/mL phenylephrine in 100 mL
250 mL preparation suggested · uses one full vial
Draw 1 mL of 10 mg/mL phenylephrine stock (1 vial = 10 mg)
Add to 250 mL bag of 0.9% NaCl or 5% dextrose
Final 40 µg/mL phenylephrine in 250 mL
500 mL preparation
Draw 2 mL of 10 mg/mL phenylephrine stock (2 vials = 20 mg)
Add to 500 mL bag of 0.9% NaCl or 5% dextrose
Final 40 µg/mL phenylephrine in 500 mL
100 mL preparation
Draw 0.2 mL of 10 mg/mL phenylephrine stock (0.20 of a vial; discard the rest = 2 mg)
Add to 100 mL bag of 0.9% NaCl or 5% dextrose
Final 20 µg/mL phenylephrine in 100 mL
250 mL preparation
Draw 0.5 mL of 10 mg/mL phenylephrine stock (half-vial; discard the rest = 5 mg)
Add to 250 mL bag of 0.9% NaCl or 5% dextrose
Final 20 µg/mL phenylephrine in 250 mL
500 mL preparation suggested · uses one full vial
Draw 1 mL of 10 mg/mL phenylephrine stock (1 vial = 10 mg)
Add to 500 mL bag of 0.9% NaCl or 5% dextrose
Final 20 µg/mL phenylephrine in 500 mL
Awaiting input

Enter a patient weight to see the result.

Reference

How the calculation works

Phenylephrine CRI is dosed in µg/kg/min. To convert a per-minute dose into a per-hour CRI rate, multiply by 60. The full formula:

$$\text{mL/hr} = \frac{\text{weight}_{\text{kg}} \times \text{dose}_{\mu g/kg/min} \times 60}{\text{concentration}_{\mu g/mL}}$$

Why the 60? The patient's weight (kg) times the dose (µg per kg per minute) gives micrograms per minute. Multiplying by 60 converts to micrograms per hour. Dividing by the concentration (µg per mL of the prepared CRI) yields mL per hour.

In target-pump-rate mode the pump rate is fixed by the clinician and the bag concentration is the unknown. The preparation runs in three steps.

Step 1: bag concentration

Same numerator as standard-bag mode (the total drug delivered per hour for this patient), but divided by the chosen pump rate instead of by a chosen bag concentration:

$$\text{bag concentration}_{\mu g/mL} = \frac{\text{weight}_{\text{kg}} \times \text{dose}_{\mu g/kg/min} \times 60}{\text{pump rate}_{\text{mL/hr}}}$$

Step 2: total drug in the bag

The bag concentration times the chosen bag volume gives the total micrograms of drug to add. Convert to milligrams (divide by 1,000) for the size of stock you'll actually be drawing:

$$\text{total drug}_{\mu g} = \text{bag concentration}_{\mu g/mL} \times \text{bag volume}_{\text{mL}}$$
$$\text{total drug}_{mg} = \frac{\text{total drug}_{\mu g}}{1{,}000}$$

Step 3: volume of stock to draw

Total drug divided by the stock vial concentration gives the volume of stock to draw and add to the bag:

$$\text{stock volume}_{\text{mL}} = \frac{\text{total drug}_{mg}}{\text{stock}_{mg/mL}}$$

Worked through together: divide the total drug in the bag by the stock vial's mg/mL to get the mL of stock that contains that much drug. That is the volume to pull into the syringe and add to the bag.

Worked example with current inputs

Enter a patient weight to see the worked example.

Reference

Recommended dilutions

Target concentration Dilution When useful
100 µg/mL 1 ampule (10 mg / 1 mL) into a 100 mL bag of 0.9% NaCl or 5% dextrose Concentrated preparation for patients ≥10 kg or fluid-restricted cases (sepsis, CHF). Lowest carrier-fluid load.
40 µg/mL 1 ampule (10 mg / 1 mL) into a 250 mL bag of 0.9% NaCl or 5% dextrose Standard textbook preparation (Plumb's). Recommended for patients 3–10 kg and most general clinical use.
20 µg/mL 1 ampule (10 mg / 1 mL) into a 500 mL bag of 0.9% NaCl or 5% dextrose Recommended for patients <3 kg or any patient at very low doses where the 40 µg/mL preparation would drop pump rate below 2 mL/hr.

Standard preparation is the contents of one 10 mg ampule (1 mL of 10 mg/mL stock) added to a single bag of carrier fluid. The resulting concentration depends on bag size: 100 mL gives 100 µg/mL, 250 mL gives 40 µg/mL, and 500 mL gives 20 µg/mL. The 250 mL bag is the textbook preparation cited in Plumb's ("10 mg in 250 mL NaCl") and is the right default for most patients. Carrier fluid: 0.9% NaCl is the conventional diluent; 5% dextrose is also compatible. Phenylephrine is more stable in neutral-to-slightly-acidic solutions; avoid co-administration in lines containing sodium bicarbonate or other alkalinizing fluids. Watch for reflex bradycardia. Phenylephrine is a pure α-agonist with no β-1 inotropic effect, and the rise in afterload can trigger a baroreceptor-mediated drop in heart rate. If marked bradycardia develops, consider adding a low-dose β-agonist or switching to norepinephrine.

Dilution helper — Phenylephrine CRI

Work out how many mL of stock drug and diluent to combine for any target concentration. Stock is pre-filled for Phenylephrine CRI; change it if you're using a different vial.

Suggestions from the reference table above
Draw up

Enter stock, target concentration, and final volume.