How to Calculate a Constant Rate Infusion (CRI) in Veterinary Patients

Name: How to Calculate a Constant Rate Infusion (CRI) in Veterinary Patients
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-27

A constant rate infusion (CRI) calculation converts a prescribed dose in mcg/kg/min or mg/kg/h into a volume per hour that a fluid pump or syringe pump can deliver. You multiply the dose by the patient's body weight, convert the time base to hours, then divide by the drug concentration to get a flow rate.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Key Takeaways

A constant rate infusion (CRI) calculation converts a prescribed dose in mcg/kg/min or mg/kg/h into a volume per hour that a fluid pump or syringe pump can deliver.
CRI doses in mcg/kg/min must be multiplied by the patient's weight, then by 60, then divided by 1,000 to convert to mg/h before dividing by drug concentration.
For bag-based CRIs, the drug dose is tied to the fluid rate, so any change in fluid rate changes the delivered drug dose and requires recalculation or a separate pump.
A dose of 50 mcg/kg/min equals 3 mg/kg/h, so treating 50 mcg/kg/min as 50 mg/kg/h would overdose the patient by a factor of 1,000.
When the calculated flow rate is too small for the pump, such as 0.18 mL/h for ketamine at 0.6 mg/kg/h in a 30 kg dog, dilute the drug and recalculate the rate.

What a CRI Is and Why It Is Used

A CRI is a continuous intravenous infusion of a drug delivered at a fixed rate over time. Instead of repeated boluses that spike and crash, a CRI holds a steadier plasma concentration. That steady concentration is the clinical goal. It keeps drug effects predictable and avoids the peaks that cause side effects and the troughs that allow pain, arrhythmias, or hypotension to break through.

Veterinary anesthesia and critical care use CRIs for several drug classes. Analgesics such as lidocaine, ketamine, fentanyl, and dexmedetomidine are common. Antiarrhythmics such as lidocaine are used to suppress ventricular ectopy. Vasopressors and inotropes such as dopamine and dobutamine support blood pressure and cardiac output. Anesthetics such as propofol can be maintained as a CRI. Each class has its own dose range, and the calculation method is the same.

The evidence base for CRIs in dogs is substantial. Lidocaine CRIs at 50 mcg/kg/min are used during balloon valvuloplasty for pulmonic stenosis to reduce catheter-induced ventricular ectopic complexes [1]. The same 50 mcg/kg/min rate has been studied for balanced anesthesia in surgical dogs, where lidocaine reduced the need for supplemental intraoperative analgesia compared with placebo [2]. In dogs with septic peritonitis, an intraoperative lidocaine infusion at 50 mcg/kg/min was associated with improved short-term survival compared with opioid alone [3]. Ketamine CRIs at 10 mcg/kg/min have been studied for antinociceptive effects in conscious dogs [4], and ketamine at 0.6 mg/kg/h has been used as part of multimodal MAC-sparing protocols [5][6]. Medetomidine and dexmedetomidine CRIs have been studied for opioid sparing and nerve block prolongation [7][8][9][10]. Fentanyl CRIs at 5 mcg/kg/h have been compared with erector spinae plane blocks for hemilaminectomy analgesia [11].

The pattern across these studies is consistent. CRIs allow precise, titratable drug delivery that supports multimodal care. The calculation is the bridge between the prescription and the pump.

The Units You Will See

CRI doses appear in several unit formats. The two most common are mcg/kg/min and mg/kg/h. You will also see mcg/kg/h and mg/kg/min, though these are less frequent. The unit tells you the mass of drug per kilogram of body weight per unit of time.

mcg/kg/min: micrograms per kilogram per minute. Common for lidocaine, ketamine, fentanyl, and dexmedetomidine.
mg/kg/h: milligrams per kilogram per hour. Common for ketamine, morphine, and some antiarrhythmics.
mcg/kg/h: micrograms per kilogram per hour. Common for dexmedetomidine and fentanyl in some protocols.

You must know which unit the prescriber used. A dose written as 50 mcg/kg/min is not the same as 50 mcg/kg/h. The first is 60 times larger. Unit confusion is one of the most dangerous errors in CRI calculation.

Fluid rates are usually written in mL/h. Drug concentrations are written in mg/mL or mcg/mL. The calculation converts the dose unit into a volume unit that the pump can deliver.

The Core Formulas

Every CRI calculation follows the same logic. Convert the dose to an hourly mass, then divide by the concentration to get a volume per hour.

Formula 1: Dose in mcg/kg/min to mL/h

Dose (mcg/kg/min) x weight (kg) = mcg/min
mcg/min x 60 = mcg/h
mcg/h ÷ 1,000 = mg/h
mg/h ÷ concentration (mg/mL) = mL/h

Formula 2: Dose in mg/kg/h to mL/h

Dose (mg/kg/h) x weight (kg) = mg/h
mg/h ÷ concentration (mg/mL) = mL/h

Formula 3: Adding drug to a fluid bag

Calculate the patient's hourly dose in mg/h using Formula 1 or 2.
Calculate how long the bag will last: bag volume (mL) ÷ fluid rate (mL/h) = hours.
Multiply the hourly dose by the bag duration: mg/h x hours = total mg to add.
Divide total mg by the stock concentration: total mg ÷ stock (mg/mL) = mL of stock to add.
Remove the same volume of fluid from the bag before adding the drug.

Formula 4: Diluting a concentrated drug for a syringe pump

Calculate the required dose in mg/h.
If the resulting mL/h is too small for the pump to deliver accurately, dilute the drug.
Choose a dilution that produces a practical flow rate. For example, dilute 1 mL of a 100 mg/mL drug into 99 mL of saline to make 1 mg/mL.
Recalculate the flow rate using the new concentration.

Formula Table

Step	What you need	Formula	Example
1	Dose in mcg/kg/min to mcg/min	Dose x weight (kg)	5 x 20 = 100 mcg/min
2	mcg/min to mcg/h	mcg/min x 60	100 x 60 = 6,000 mcg/h
3	mcg/h to mg/h	mcg/h ÷ 1,000	6,000 ÷ 1,000 = 6 mg/h
4	mg/h to mL/h (undiluted)	mg/h ÷ concentration (mg/mL)	6 ÷ 40 = 0.15 mL/h
5	Bag duration	Bag volume ÷ fluid rate	1,000 ÷ 50 = 20 hours
6	Total drug for bag	mg/h x bag duration	6 x 20 = 120 mg
7	Volume of stock to add	Total mg ÷ stock (mg/mL)	120 ÷ 40 = 3 mL
8	Fluid to remove	Same volume as drug added	Remove 3 mL

Worked Example A: Adding Drug to a Fluid Bag

This is the most common CRI setup in general practice. The drug is added to a maintenance fluid bag, and the bag is run at a fixed rate. The drug dose is tied to the fluid rate.

Patient: 20 kg dog. Drug dose: 5 mcg/kg/min. Stock concentration: 40 mg/mL. Fluid rate: 50 mL/h. Bag size: 1 liter (1,000 mL).

Step 1: Convert dose to mcg/min. 5 mcg/kg/min x 20 kg = 100 mcg/min.

Step 2: Convert to mcg/h. 100 mcg/min x 60 = 6,000 mcg/h.

Step 3: Convert to mg/h. 6,000 mcg/h ÷ 1,000 = 6 mg/h.

Step 4: Calculate how long the bag lasts. 1,000 mL ÷ 50 mL/h = 20 hours.

Step 5: Calculate total drug needed for the bag. 6 mg/h x 20 hours = 120 mg.

Step 6: Calculate volume of stock to add. 120 mg ÷ 40 mg/mL = 3 mL.

Step 7: Remove fluid before adding drug. Remove 3 mL of fluid from the 1 liter bag, then add 3 mL of the 40 mg/mL stock. The bag now contains 120 mg of drug in 1,000 mL. At 50 mL/h, the patient receives 6 mg/h, which is 5 mcg/kg/min.

This method works only if the fluid rate stays at 50 mL/h. If the fluid rate changes, the drug dose changes. A rate increase delivers more drug per hour. A rate decrease delivers less. This is the single most important safety point for bag-based CRIs.

Worked Example B: Syringe Pump with Undiluted Drug

A syringe pump delivers a small volume at a precise rate. This is common for potent drugs that need tight control.

Patient: 10 kg dog. Drug: Lidocaine at 50 mcg/kg/min. Stock: 2 percent lidocaine, which is 20 mg/mL.

Step 1: Convert dose to mcg/min. 50 mcg/kg/min x 10 kg = 500 mcg/min.

Step 2: Convert to mcg/h. 500 mcg/min x 60 = 30,000 mcg/h.

Step 3: Convert to mg/h. 30,000 mcg/h ÷ 1,000 = 30 mg/h.

Step 4: Calculate mL/h using the undiluted concentration. 30 mg/h ÷ 20 mg/mL = 1.5 mL/h.

The syringe pump runs at 1.5 mL/h. This is a practical rate for most syringe pumps. No dilution is needed.

Lidocaine is the most studied CRI drug in veterinary anesthesia. The 50 mcg/kg/min rate appears across multiple studies, including balanced anesthesia [2], balloon valvuloplasty [1], and septic peritonitis [3]. Lower rates such as 25 mcg/kg/min have also been studied for gastrointestinal effects [12]. The dose used in any patient comes from the veterinarian and current references, not from this calculation example.

Cats are more sensitive to lidocaine than dogs. The dose and the decision to use lidocaine in a cat come from a veterinarian. The calculation method is the same, but the numbers are different.

Worked Example C: Diluting a Concentrated Drug

Some drugs are supplied at concentrations so high that the undiluted flow rate is too small for the pump to deliver accurately. Dilution solves this.

Patient: 30 kg dog. Drug: Ketamine at 0.6 mg/kg/h. Stock: Ketamine 100 mg/mL.

Step 1: Convert dose to mg/h. 0.6 mg/kg/h x 30 kg = 18 mg/h.

Step 2: Calculate mL/h using the undiluted concentration. 18 mg/h ÷ 100 mg/mL = 0.18 mL/h.

A rate of 0.18 mL/h is too small for many syringe pumps to deliver accurately. Small errors in pump calibration become large errors in dose at these low rates. Dilution fixes this.

Step 3: Dilute the drug. Add 1 mL of ketamine (100 mg) to 99 mL of saline. The total volume is 100 mL, and the concentration is 100 mg ÷ 100 mL = 1 mg/mL.

Step 4: Recalculate the flow rate. 18 mg/h ÷ 1 mg/mL = 18 mL/h.

The syringe pump now runs at 18 mL/h. This is a practical, accurate rate. The dilution factor is 100, so the flow rate is 100 times larger than the undiluted rate. The dose delivered is the same.

Ketamine at 0.6 mg/kg/h is a recognized rate in multimodal protocols. It has been used with lidocaine and morphine or fentanyl to reduce isoflurane MAC in dogs [5][6]. The same rate written as 10 mcg/kg/min (0.6 mg/kg/h) has been studied for antinociception in conscious dogs [4], and a higher rate of 30 mcg/kg/min has been studied for cardiac effects in propofol-anesthetized dogs [13]. The dose for any patient comes from the veterinarian.

Comparing the Three Examples

Feature	Example A	Example B	Example C
Patient weight	20 kg	10 kg	30 kg
Drug dose	5 mcg/kg/min	50 mcg/kg/min	0.6 mg/kg/h
Stock concentration	40 mg/mL	20 mg/mL (2 percent)	100 mg/mL
Delivery method	Fluid bag	Syringe pump	Syringe pump
Dilution needed	No	No	Yes, 1 mg/mL
Final flow rate	50 mL/h (bag rate)	1.5 mL/h	18 mL/h
Drug per hour	6 mg/h	30 mg/h	18 mg/h

The three examples show the same logic applied to different delivery systems. The dose always converts to mg/h first. The concentration always converts mg/h to mL/h. The delivery system determines whether dilution is needed.

Why Steady Plasma Concentration Matters

A bolus produces a sharp peak in plasma concentration followed by a rapid decline. The peak may cause side effects. The decline may allow the clinical effect to fade. A CRI avoids both problems by matching the rate of drug delivery to the rate of drug elimination.

When the infusion rate equals the clearance rate, plasma concentration reaches a steady state. The time to steady state depends on the drug's half-life. Drugs with short half-lives reach steady state quickly. Drugs with long half-lives take longer. This is why some CRIs start with a loading dose. The loading dose fills the body's volume of distribution, and the CRI maintains the concentration.

The clinical implication is straightforward. A CRI is not just a slow bolus. It is a delivery method designed to hold a target concentration for as long as the infusion runs. The calculation ensures that the target dose is delivered at the target rate.

Common Errors in CRI Calculation

Unit confusion. Mixing up mcg/kg/min and mg/kg/h is the most dangerous error. A dose of 50 mcg/kg/min is 3,000 mcg/kg/h, which is 3 mg/kg/h. If you treat 50 mcg/kg/min as 50 mg/kg/h, you will overdose by a factor of 1,000. Always write the unit next to the number.

Forgetting the 60-minute conversion. Doses in mcg/kg/min must be multiplied by 60 to get mcg/kg/h. Skipping this step underdoses by a factor of 60.

Forgetting the 1,000 conversion. Micrograms and milligrams differ by a factor of 1,000. Skipping this step overdoses by a factor of 1,000.

Not removing fluid from the bag. If you add 3 mL of drug to a 1 liter bag without removing 3 mL of fluid, the total volume becomes 1,003 mL. The concentration is slightly lower than intended. The error is small but avoidable.

Changing the fluid rate. When the drug is in the maintenance bag, the drug dose is tied to the fluid rate. Increasing the fluid rate increases the drug dose. Decreasing the fluid rate decreases the drug dose. If the patient needs a fluid rate change, the drug bag must be recalculated or the drug must be delivered by a separate pump.

Using the wrong concentration. A 2 percent lidocaine solution is 20 mg/mL. A 1 percent solution is 10 mg/mL. A 0.5 percent solution is 5 mg/mL. Always check the label. Do not assume.

Not labeling the bag or syringe. An unlabeled bag or syringe is a patient safety hazard. Label the bag or syringe with the drug name, total dose, concentration, flow rate, and time of preparation.

Rounding errors. Rounding 0.18 mL/h to 0.2 mL/h changes the dose by about 11 percent. Rounding 1.5 mL/h to 2 mL/h changes the dose by 33 percent. Round only at the end, and round to a rate the pump can deliver accurately.

Assuming all pumps are accurate at all rates. Some syringe pumps are not accurate below a certain flow rate. If the calculated rate is very low, dilute the drug and recalculate. Example C shows this.

Forgetting that cats are different. Cats are more sensitive to lidocaine than dogs. The dose and the decision to use lidocaine in a cat come from a veterinarian. The calculation method is the same, but the numbers are different.

Practical Implications for the Veterinary Team

The CRI calculation is a team skill. The veterinarian prescribes the dose. The technician or assistant calculates the rate, prepares the bag or syringe, labels it, and sets the pump. The team double-checks the calculation before administration.

A useful workflow is:

Write the dose, unit, weight, and concentration on a calculation sheet.
Calculate the mg/h.
Calculate the mL/h.
If the mL/h is too small, dilute and recalculate.
Prepare the bag or syringe.
Label the bag or syringe with drug, dose, concentration, rate, and time.
Have a second person verify the calculation and the label.
Set the pump and confirm the rate.
Monitor the patient for the expected effect and for side effects.
Document the start time, rate, and any changes.

This workflow reduces the risk of the common errors listed above. It also creates a record that can be reviewed if the patient's condition changes.

What Is Still Uncertain

CRI dosing is not fully standardized across all drugs and all conditions. Different studies use different doses, different loading protocols, and different patient populations. The optimal dose for a specific patient depends on the clinical context, the patient's response, and the veterinarian's judgment.

The calculation itself is deterministic. The dose, the weight, and the concentration determine the rate. The uncertainty is in the dose selection, not the arithmetic. That is why the calculation examples in this article are calculation examples only. They are not dosing advice. Drug choice and dose come from a veterinarian and current references.

Limitations and When to Contact a Veterinarian

This article teaches the arithmetic of CRI calculation. It does not replace clinical training, institutional protocols, or the veterinarian's prescription. Individual patients need individual assessment.

Contact a veterinarian or the prescribing clinician if:

The calculated flow rate seems too high or too low for the pump.
The patient shows signs of drug side effects, such as sedation, nausea, tremors, or cardiac changes.
The fluid rate needs to change while a drug is in the maintenance bag.
The drug concentration on the label does not match the concentration used in the calculation.
The patient's weight has changed since the prescription was written.
You are unsure about the dose unit or the conversion.

When in doubt, stop and recalculate. A second check is faster than treating an overdose.

Frequently Asked Questions

What is a CRI in veterinary medicine?

A CRI is a constant rate infusion, which is a continuous intravenous infusion of a drug at a fixed rate. It is used to maintain a steady plasma concentration for analgesia, anesthesia, antiarrhythmic therapy, or cardiovascular support.

How do I calculate a CRI in mcg/kg/min?

Multiply the dose by the patient's weight in kg to get mcg/min. Multiply by 60 to get mcg/h. Divide by 1,000 to get mg/h. Divide by the drug concentration in mg/mL to get mL/h.

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

They are different unit scales. One mcg/kg/min equals 0.06 mg/kg/h. A dose of 50 mcg/kg/min equals 3 mg/kg/h. Always confirm which unit the prescriber used.

Why do some CRIs need a loading dose?

A loading dose fills the body's volume of distribution quickly so the drug reaches a therapeutic concentration sooner. The CRI then maintains that concentration. Not all CRIs use a loading dose.

Why does changing the fluid rate change the drug dose in a bag-based CRI?

The drug is mixed into the maintenance fluid. The drug dose is tied to the fluid rate. A higher fluid rate delivers more drug per hour, and a lower fluid rate delivers less.

Can I use a veterinary CRI calculator instead of doing the math?

A veterinary CRI calculator can reduce arithmetic errors, but you still need to verify the dose, unit, weight, and concentration. The calculator is only as accurate as the numbers you enter.

Why is lidocaine different in cats?