What a CRI calculation needs
The core inputs are patient weight, target dose, dose time unit, drug concentration, and desired fluid or pump rate. Units must match before calculation: mg, mcg, kg, minutes, hours, mL, and percent solutions cannot be mixed casually.
Veterinarians, veterinary technicians, emergency teams, anesthesia teams, and specialty hospitals use CRI calculations for analgesics, anesthetic adjuncts, antiarrhythmics, vasopressors, insulin, and other medications. The prescribed dose and monitoring plan must come from the treating veterinarian.
- Patient weight in kg.
- Dose in mg/kg/hr or mcg/kg/min.
- Drug concentration in mg/mL or mcg/mL.
- Pump rate or bag volume.
- Start time, changes, and stop time.
How to calculate
For direct syringe or pump delivery, use mL/hr = dose x weight x time conversion / concentration. If the dose is in mcg/kg/min and concentration is mcg/mL, multiply by 60 to convert minutes to hours.
For example, a 20 kg dog needs 5 mcg/kg/min from a 1,000 mcg/mL solution. Required mcg/hr = 5 x 20 x 60 = 6,000 mcg/hr. Infusion rate = 6,000 / 1,000 = 6 mL/hr.
The bag method
The bag method adds a calculated amount of drug to a fluid bag so the patient receives the target dose at a planned fluid rate. First calculate the drug amount needed per hour, then multiply by the number of hours the bag will run, or calculate the concentration needed in the bag from the fluid rate.
Example: a 10 kg patient needs 2 mcg/kg/min and fluids run at 25 mL/hr. Dose per hour = 2 x 10 x 60 = 1,200 mcg/hr. Bag concentration needed = 1,200 / 25 = 48 mcg/mL. For a 250 mL bag, add 48 x 250 = 12,000 mcg, or 12 mg, adjusting for any fluid removed if volume accuracy matters.
Common mistakes
The most dangerous mistakes are unit mistakes: mg versus mcg, minutes versus hours, pounds versus kilograms, and percent concentration versus mg/mL. A 1,000-fold error can occur when mcg and mg are confused.
Another common issue is changing the fluid rate without recalculating a drug-added bag. If medication is mixed into the bag, the dose changes whenever the mL/hr changes. Direct syringe CRIs can be easier to adjust independently.
- Failing to convert lb to kg.
- Using mg/mL with a mcg dose without conversion.
- Forgetting to remove drug volume from a small bag.
- Not documenting rate changes and patient response.
Safe workflow
Write the prescription with patient weight, dose, route, concentration, final rate, and monitoring parameters. Have a second trained person check the math for high-risk drugs, pediatric patients, exotic species, or very small infusion rates.
Keep a dosing log that records preparation time, concentration, pump channel, start rate, adjustments, and stop time. CRI math is only useful if the bedside setup matches the calculation.
Frequently asked questions
How do I convert pounds to kilograms for veterinary dosing?
Divide pounds by 2.2 to estimate kilograms. Use the clinic's required rounding policy for medication calculations.
What is the difference between a syringe CRI and a bag CRI?
A syringe CRI delivers a known drug concentration directly at a pump rate. A bag CRI mixes drug into fluids, so the dose depends on the fluid rate.
Why multiply by 60 in mcg/kg/min calculations?
The dose is per minute, but many pumps are set in mL/hr. Multiplying by 60 converts the required dose from per minute to per hour.
Can I change the fluid rate after adding drug to the bag?
Only after confirming the new drug dose. In a drug-added bag, changing mL/hr changes medication delivery.