Veterinary CRI Calculator (Fentanyl, Ketamine, Lidocaine, MLK)
Calculate the mL/hr pump rate for common veterinary CRIs - fentanyl, ketamine, lidocaine, morphine, maropitant, dexmedetomidine, and diazepam. Uses standard stock concentrations and outputs the rate you set directly on the pump.
Species
Drug
Stock concentration: 0.05 mg/mL
Patient weight (kg)
Dose (mcg/kg/hr)
Recommended range: 2-10 mcg/kg/hr
Surgical analgesia 5-10 mcg/kg/hr; post-op 2-5 mcg/kg/hr.
How to use this tool
What is a CRI and when is it used?
A Constant Rate Infusion delivers a drug continuously at a fixed rate per unit time, keeping plasma concentrations steady. Compared to repeated boluses, CRIs reduce peak/trough swings, lower the total drug needed, and provide more even analgesia or arrhythmia control.
How the maths works
Total drug per hour = dose (mg/kg/hr) × weight (kg). Divide by stock concentration (mg/mL) to get the pump rate in mL/hr. For mcg/kg/min doses, multiply by 60 to get mcg/kg/hr first, then ÷ 1000 to get mg.
Common CRI recipes
MLK (morphine + lidocaine + ketamine) for multimodal surgical analgesia in dogs: morphine 0.12 mg/kg/hr, lidocaine 3 mg/kg/hr (50 mcg/kg/min), ketamine 0.6 mg/kg/hr (10 mcg/kg/min). FLK swaps fentanyl for morphine. Lidocaine alone is the workhorse for ventricular arrhythmias - load 1-2 mg/kg slow IV, then 25-50 mcg/kg/min.
Cat-specific cautions
Lidocaine CRIs are best avoided in cats - narrower therapeutic window and slower hepatic clearance. Ketamine, fentanyl, and dexmedetomidine are all useable at conservative doses with monitoring.
Frequently asked questions
What is a CRI in veterinary medicine?
A Constant Rate Infusion delivers a drug continuously at a fixed mg/kg or mcg/kg per unit time, instead of intermittent boluses. Used for analgesia (fentanyl, ketamine, lidocaine), arrhythmia control (lidocaine, esmolol), and anti-emesis (maropitant).
How is a CRI dose converted to a drip rate?
Multiply the dose (e.g. 3 mcg/kg/min fentanyl) by the body weight to get drug-per-minute, multiply by 60 to get drug-per-hour, then divide by the bag concentration (mg/mL or mcg/mL) to get mL/hr. This calculator does the maths for you and outputs the final mL/hr you set on the pump.
What are typical CRI doses for common drugs?
Fentanyl 2-10 mcg/kg/hr (analgesia). Ketamine 2-10 mcg/kg/min (analgesia, with bolus 0.5 mg/kg). Lidocaine 25-50 mcg/kg/min in dogs (avoid in cats). MLK = morphine 0.12 mg/kg/hr + lidocaine 3 mg/kg/hr + ketamine 0.6 mg/kg/hr in dogs. Always titrate to effect.
Why are some CRIs dangerous in cats?
Lidocaine has a much narrower safety margin in cats due to slower hepatic clearance and CNS sensitivity - most clinicians avoid it as a CRI in cats. Ketamine and fentanyl are generally safe at conservative doses. Always reduce the dose when combining drugs.