November 2023

In this VETgirl blog, Tiffany Gendron, CVT, VTS (ECC) discusses how to calculate a constant rate infusion (CRI) in veterinary medicine. Being able to comfortable calculate a CRI (e.g., for metoclopramide, fentanyl, or other drugs) is very important, as an erroneous calculation can result in a life-threatening toxicity or the opposite – an ineffective drug delivery. The veterinary technician, veterinary student, and veterinarian must feel comfortable calculating a CRI. If you missed Part 1 of “Don’t Let a CRI Make You CRI: Why & How to Calculate a CRI,” check it out HERE.

Don’t Let a CRI Make You Cry: Why & How to Calculate a CRI Part 2

by Tiffany Gendron, CVT, VTS (ECC)
VETgirl Veterinary CE Coordinator

Previously, we reviewed the importance of veterinary technicians knowing how to calculate a constant rate infusion (CRI). As a veterinary technician, calculating a CRI for drugs is an essential skill that you need to master. Whether you are working in a clinic, an emergency hospital, or any other veterinary setting, understanding the calculations involved in a CRI can help you provide the best care possible to your patients. In this blog, we will take a comprehensive look at how to calculate a CRI in veterinary medicine, step-by-step. If you missed Part 1 of this blog, make sure to check out “Don’t Let a CRI Make You Cry” HERE.

How to Calculate a CRI for Drugs
A CRI is a method of administering a drug to a patient over a set period of time, typically 24 hours. This keeps the drug at a steady level in the patient’s system and can help maintain stable levels of pain relief or other benefits. To calculate a CRI, you will need several pieces of information, including the desired rate of infusion, the concentration of the medication, and the patient’s weight.

As we consider the use of CRIs, the importance of math and calculations comes into play!

Let’s review a few common CRI medication calculations:

Example: Fentanyl CRI
Loading dose: 3 mcg/kg
CRI dose: 5 mcg/kg/hr
Patient Weight: 38.2 kg
Concentration of fentanyl: 50 mcg/mL

To calculate the loading dose of fentanyl at 3 mcg/kg, you want to make sure that algebraically, the “units” cross out correctly (e.g., denominator) in the example below:

VETgirl blog CRI fentanyl CRI

To calculate the CRI of fentanyl at 5 mcg/kg/hour, you want to make sure that algebraically, the “units” cross out correctly as in the example below:

Desired calculation: CRI (5 mcg/kg/hr)

How do you physically deliver the CRI?
In veterinary medicine, syringe pumps are widely used, particularly in the emergency or specialty setting. Specific settings on the syringe pump can improve the accuracy of delivery, including a customizable drug library, auto-calibration, load syringe failure, and occlusion warnings. It is best to use low-volume, high-pressure lines when using syringe pumps; in general, manufacturer requirements/recommendations should be utilized. The accuracy of delivery volume is dependent upon the type of pump used as well as the rate of infusion. When in doubt, a low concentration drug should be potentially diluted into a larger amount of saline to allow for a larger volume to be delivered per hour to increase accuracy. The use of programmable syringe pumps providing therapeutic doses at lower flow rates has been shown to provide inconsistent delivery rates. The Food and Drug Administration (FDA) warns when using programmable syringe pumps to infuse therapies at low rates (e.g., less than 5 mL per hour, and especially at flow rates of less than 0.5 mL per hour), a lack of flow continuity (i.e., inconsistent rate of delivery) can result in serious clinical consequences.

Labeling, Preparation, and Administration
You have options when it comes to administering a CRI. CRIs can be delivered via a syringe pump (small amount to be delivered, burette (medium amount to be delivered), or in a fluid bag (large amount to be delivered, typically diluted in saline like a morphine-lidocaine-ketamine CRI or metoclopramide CRI diluted in fluids). Consider the fluid rate, duration of administration, total volume of solution, and the need for titration or reformulation.

Titration and reformulation are common in cases like diabetic ketoacidosis (DKA), where electrolyte CRIs are adjusted more frequently as the status of the patient changes. When preparing a CRI, you should always review the package insert or a formulary to determine the appropriate carrier fluid, stability, compatibility, and storage specifications. For quick references of compatibility and stability, there are a number of resources available, such as wall charts and online references (including desktop and mobile access). Drug formularies are also great resources! For best practices, admixtures or secondary infusions should adhere to a hospital protocol for preparation and administration.

Pro Tip: Clearly outlined/written protocols increase continuity in preparation.

Fluid Bag Preparation
Manufacturer solutions usually account for a 10% difference in bag volume due to overfill. As bags are stored, evaporative losses can occur. A protocol must be in place to determine if overfill volumes will be accounted for when preparing your mixture. It is also equally important to consider the volume of drug introduced to create the admixture and if that same volume should be removed from the carrier fluid. In some pharmacies, the 10% rule indicates that if the volume of drug to be added is 10% or greater than the total bag volume, the volume will be removed prior to the addition of the drug to the fluid bag. Not all hospitals follow this rule, and for this reason, it is best to have an established protocol for safety and continuity.

Example of how to create a 2 mg/kg/day CRI of metoclopramide:

Patient Weight: 5 kg
Concentration: Metoclopramide (5 mg/ml)
CRI Dose: 2 mg/kg/day
Fluid Rate: Doctor requests that this patient be on 12 mls/hr of IV fluids (0.9 % NaCl)
Total Volume bag that you’re using: 250 mls

VETgirl blog CRI metoclopramide CRI

Syringe Preparation
Choose a syringe size that best suits the desired volume to be extracted. The drug (e.g.., Fentanyl 50 mcg/ml) and carrier fluid (compatible diluent such as 0.9% NaCl or D5W) should be drawn up in separate syringes. The dead space that exists in the needle and the hub of the syringe should not be considered part of the final volume. This volume will account for what is left in the syringe during injection. Make sure to invert the syringe several times to achieve proper mixing.

Burette Preparation
Half of the desired volume of carrier fluid should be introduced into the chamber. The desired volume of drug is then introduced through the injection port followed by the other half of carrier fluid to achieve the desired final volume. The medication injection port should be flushed with a small volume of carrier fluid and swirled not inverted.

Best practices would suggest aseptic technique be used during the preparation of the CRI. Clearly labeling the contents is very important to identify the formulation, ensure consistency in preparation, and assist in error identification. When fluid quantifications are performed, the CRI label contents should be verified against the order located in the patients’ medical record. The fluid pump, syringe pump, or burette should be verified that they are running at the correct fluid rate and the approximate volume delivered matches the approximate volume remaining in the bag or syringe. The label is typically bright in color as an indication of an additive in the solution. The label should include the following information:

  • Time
  • Date
  • Initials of Individual who made the CRI
  • Drug name and volume in both mg (mcg) and ml
  • The carrier fluid volume in ml
  • The rate and dose

The use of the CRI is an invaluable tool that allows titratable and continuous therapy at therapeutic levels. When the use of the medication as CRI is going to be transitioned to intermittent bolus dosing, oral administration, or discontinued the drug half-life must be factored into the plan.

Monitoring a CRI
Once you have calculated the CRI rate, it is important to monitor the patient closely for any signs of adverse reactions or complications. Close monitoring of the patient’s response helps to dictate the need for continued therapy or consideration of weaning. You will need to keep precise records of the amount of medication administered, the rate of infusion, and any changes in the patient’s condition. The veterinarian should be notified immediately if any problems arise.

Monitoring parameters should include:

  • Temperature
  • Pulse
  • Respiration
  • Blood pressure
  • Pain assessment
  • Blood work evaluation (e.g., electrolytes when indicated)
  • Intravenous fluid monitoring, including catheter care
  • Signs of adverse drug reactions

Constant rate infusions can be an excellent tool that can be safely and effectively implemented into a patient’s treatment plan. Just remember there are many considerations that the veterinary team must review to ensure the maximum response and the intended outcome are achieved.

As a veterinary technician, understanding how to calculate a CRI for drugs is an essential skill that can help you provide the best care possible to your patients. By accurately calculating the appropriate dosage and CRI rate, you can help maintain stable levels of medication in the patient’s system and ensure their well-being. Remember to always consult with the veterinarian and monitor the patient closely for any potential complications. With consistent practice and careful attention to detail, you can master the art of calculating a CRI and excel in your career as a veterinary technician.

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