Veterinary CRI Calculator
This tool helps veterinary professionals calculate the correct amount of a drug to add to an IV fluid bag for a Constant Rate Infusion (CRI). Ensure all values are correct before administration.
Disclaimer: This calculator is intended for use by qualified veterinary professionals only. It is a tool to aid in calculations and does not replace clinical judgment, experience, or the need to double-check all calculations. The user assumes all risk for the use of this tool.
What is a Constant Rate Infusion (CRI)?
A Constant Rate Infusion, or CRI, is a veterinary medical technique used to administer medications intravenously at a steady, continuous rate over an extended period. This is achieved by adding a specific amount of a drug to a larger volume of IV fluids, which are then delivered to the patient via an infusion pump or a drip set.
The primary advantage of a CRI is its ability to maintain a stable and therapeutic concentration of a drug in the patient's bloodstream. This avoids the "peaks and troughs" associated with intermittent injections (e.g., every 4-6 hours), leading to more consistent effects, improved patient comfort, and potentially fewer side effects.
CRIs are commonly used for:
- Analgesia (Pain Control): Using drugs like Fentanyl, Lidocaine, or Ketamine to provide consistent pain relief.
- Anesthesia: As part of a balanced anesthetic protocol to reduce the amount of inhalant gas needed.
- Anti-arrhythmic Therapy: Administering drugs like Lidocaine to control cardiac arrhythmias.
- Seizure Control: Providing a continuous infusion of anti-convulsant medication.
How to Calculate a CRI
The calculation ensures the correct volume of a concentrated drug is added to a fluid bag, so the patient receives the desired dose based on their weight and the fluid administration rate. Here are the steps involved:
- Calculate Total Drug Dose per Hour (mg/hr): This determines how many milligrams of the drug the patient needs every hour.
Formula: Patient Weight (kg) × Drug Dose (mg/kg/hr) = Total Dose (mg/hr) - Calculate Bag Duration (hours): This determines how long the IV fluid bag will last at the current rate.
Formula: Fluid Bag Volume (mL) ÷ IV Fluid Rate (mL/hr) = Duration (hr) - Calculate Total Drug Needed for the Bag (mg): This is the total amount of drug required for the entire duration of the fluid bag.
Formula: Total Dose (mg/hr) × Duration (hr) = Total Drug Needed (mg) - Calculate Volume of Drug to Add (mL): This is the final, critical value—the volume of the stock drug solution to draw up and add to the fluid bag.
Formula: Total Drug Needed (mg) ÷ Drug Concentration (mg/mL) = Volume to Add (mL)
Example CRI Calculation
Let's calculate a Lidocaine CRI for post-operative pain management in a dog.
- Patient: A 20 kg dog
- Desired Dose: 2 mg/kg/hr of Lidocaine
- Drug on Hand: 2% Lidocaine, which is 20 mg/mL
- IV Fluid Rate: 40 mL/hr
- Fluid Bag: 1000 mL bag of Lactated Ringer's Solution
Step-by-Step Calculation:
- Total Dose per Hour: 20 kg × 2 mg/kg/hr = 40 mg/hr
- Bag Duration: 1000 mL ÷ 40 mL/hr = 25 hours
- Total Lidocaine Needed: 40 mg/hr × 25 hours = 1000 mg
- Volume of Lidocaine to Add: 1000 mg ÷ 20 mg/mL = 50 mL
Conclusion: You would add 50 mL of 2% Lidocaine to the 1000 mL bag of fluids.
Important Considerations
Accuracy is Critical: Always double-check your math, preferably with another veterinary professional. A misplaced decimal can have serious consequences.
Drug Compatibility: Ensure the drug you are adding is compatible with the type of IV fluids being used and any other medications being administered.
Labeling: Clearly label the IV fluid bag with the name and concentration of the added drug, the date and time it was mixed, and your initials.
Patient Monitoring: Continuously monitor the patient for therapeutic effects and any signs of adverse reactions. Be prepared to adjust the fluid rate or CRI as needed based on the patient's response.
Calculation Results
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