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Vasopressors & Inotropes

Vasopressin CRI

Refractory hypotension and vasodilatory shock in dogs and cats. Used as a second-line agent when catecholamines (norepinephrine, epinephrine) have not restored MAP, or first-line in catecholamine-refractory vasoplegia (sepsis, post-CPB). Mechanism is V1-mediated, so activity is preserved in acidotic and catecholamine-down-regulated states. Continuous BP monitoring required; central venous access preferred given extravasation risk.

Stock: 20 U/mL (20 000 mU/mL), 1 mL vial (20 units per vial)
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 20 U vial 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: 0.5–5.0 mU/kg/min (dogs and cats)
100 mL preparation suggested · uses one full vial
Draw 1 mL of 20 mg/mL vasopressin stock (1 vial = 20 mg)
Add to 100 mL bag of 0.9% NaCl or 5% dextrose
Final 200 mU/mL vasopressin in 100 mL
250 mL preparation
Draw 2.5 mL of 20 mg/mL vasopressin stock (2.50 vials = 50 mg)
Add to 250 mL bag of 0.9% NaCl or 5% dextrose
Final 200 mU/mL vasopressin in 250 mL
500 mL preparation
Draw 5 mL of 20 mg/mL vasopressin stock (5 vials = 100 mg)
Add to 500 mL bag of 0.9% NaCl or 5% dextrose
Final 200 mU/mL vasopressin in 500 mL
100 mL preparation
Draw 0.4 mL of 20 mg/mL vasopressin stock (0.40 of a vial; discard the rest = 8 mg)
Add to 100 mL bag of 0.9% NaCl or 5% dextrose
Final 80 mU/mL vasopressin in 100 mL
250 mL preparation suggested · uses one full vial
Draw 1 mL of 20 mg/mL vasopressin stock (1 vial = 20 mg)
Add to 250 mL bag of 0.9% NaCl or 5% dextrose
Final 80 mU/mL vasopressin in 250 mL
500 mL preparation
Draw 2 mL of 20 mg/mL vasopressin stock (2 vials = 40 mg)
Add to 500 mL bag of 0.9% NaCl or 5% dextrose
Final 80 mU/mL vasopressin in 500 mL
100 mL preparation
Draw 0.2 mL of 20 mg/mL vasopressin stock (0.20 of a vial; discard the rest = 4 mg)
Add to 100 mL bag of 0.9% NaCl or 5% dextrose
Final 40 mU/mL vasopressin in 100 mL
250 mL preparation
Draw 0.5 mL of 20 mg/mL vasopressin stock (half-vial; discard the rest = 10 mg)
Add to 250 mL bag of 0.9% NaCl or 5% dextrose
Final 40 mU/mL vasopressin in 250 mL
500 mL preparation suggested · uses one full vial
Draw 1 mL of 20 mg/mL vasopressin stock (1 vial = 20 mg)
Add to 500 mL bag of 0.9% NaCl or 5% dextrose
Final 40 mU/mL vasopressin in 500 mL
Awaiting input

Enter a patient weight to see the result.

Reference

How the calculation works

Vasopressin CRI is dosed in µg/kg/hr. The CRI rate calculation is:

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

Patient weight (kg) times dose (mU per kg per hour) gives total mU per hour. Dividing by concentration (mU per mL) yields mL per hour. No unit-time conversion needed because dose and rate are both 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:

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

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
200 mU/mL 1 vial (20 U) into a 100 mL bag of 0.9% NaCl or 5% dextrose Standard preparation for patients ≥7 kg. Pump rate stays above 2 mL/hr at typical doses (0.5–2.5 mU/kg/min).
80 mU/mL 1 vial (20 U) into a 250 mL bag of 0.9% NaCl or 5% dextrose Recommended for patients 3–7 kg, or prolonged ICU infusions where bag-exchange frequency matters. More dilute than the 100 mL preparation, so pump rate stays in the precision range for smaller patients.
40 mU/mL 1 vial (20 U) 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 80 mU/mL preparation would still drop pump rate below 2 mL/hr.

Standard preparation is the contents of one 20 U vial (1 mL of 20 U/mL stock) added to a single bag of carrier fluid. The resulting concentration depends on bag size: 100 mL gives 200 mU/mL (0.2 U/mL), 250 mL gives 80 mU/mL (0.08 U/mL), and 500 mL gives 40 mU/mL (0.04 U/mL). 100 mL is the textbook preparation and the right default for most patients. Carrier fluid: 0.9% NaCl and 5% dextrose are both compatible (Plumb's). For prolonged ICU infusions, dextrose-containing diluents are sometimes preferred. Monitor serum sodium on infusions running > 12 hours: vasopressin retains free water via V2 receptors and can produce or worsen hyponatremia.

Dilution helper — Vasopressin CRI

Work out how many mL of stock drug and diluent to combine for any target concentration. Stock is pre-filled for Vasopressin 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.