How to Calculate Constant Rate Infusion

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Constant Rate Infusion (CRI) Calculator

mg/kg/hr mcg/kg/min mcg/kg/hr mg/kg/min

Calculation Results:

function calculateCRI() { var weight = parseFloat(document.getElementById("weight").value); var concentration = parseFloat(document.getElementById("concentration").value); var doseRate = parseFloat(document.getElementById("doseRate").value); var doseUnit = document.getElementById("doseUnit").value; var resultBox = document.getElementById("cri-result-box"); var resultText = document.getElementById("resultText"); if (isNaN(weight) || isNaN(concentration) || isNaN(doseRate) || weight <= 0 || concentration <= 0 || doseRate <= 0) { alert("Please enter valid positive numbers for all fields."); return; } var rateMLhr = 0; var totalDosePerHourMg = 0; // Convert all dose rates to mg/kg/hr for internal calculation if (doseUnit === "mgkg hr") { totalDosePerHourMg = doseRate * weight; } else if (doseUnit === "mcgkg min") { totalDosePerHourMg = (doseRate * weight * 60) / 1000; } else if (doseUnit === "mcgkg hr") { totalDosePerHourMg = (doseRate * weight) / 1000; } else if (doseUnit === "mgkg min") { totalDosePerHourMg = doseRate * weight * 60; } rateMLhr = totalDosePerHourMg / concentration; resultBox.style.display = "block"; resultText.innerHTML = "Infusion Rate: " + rateMLhr.toFixed(3) + " mL/hr" + "Total Dose Delivery: " + totalDosePerHourMg.toFixed(3) + " mg/hr" + "Based on a " + weight + " kg patient and " + concentration + " mg/mL concentration."; }

Understanding Constant Rate Infusion (CRI)

A Constant Rate Infusion (CRI) is a method of drug delivery where a medication is administered intravenously at a consistent, steady rate over a specific period. This technique is widely used in veterinary and human medicine to maintain stable plasma levels of drugs such as analgesics, anesthetics, or electrolytes, avoiding the "peaks and valleys" associated with bolus dosing.

The Basic CRI Formula

Calculating the infusion rate requires four primary variables: the patient's body weight, the desired dosage rate, the drug's concentration, and the time unit. The standard formula used in our calculator is:

Rate (mL/hr) = [Dose (mg/kg/hr) × Weight (kg)] / Concentration (mg/mL)

When to Use a CRI

  • Pain Management: Drugs like Fentanyl, Lidocaine, or Ketamine are often delivered via CRI to provide consistent analgesia during surgery or recovery.
  • Seizure Control: Maintaining therapeutic levels of anticonvulsants in critical patients.
  • Fluid Therapy: Administering specific electrolytes or supplements at a fixed hourly volume.
  • Sedation: Maintaining a "light" plane of anesthesia for diagnostic procedures.

Practical Example

Imagine you have a 20 kg canine patient that requires a Fentanyl CRI at a dose of 5 mcg/kg/min. You have a Fentanyl solution with a concentration of 0.05 mg/mL.

  1. Convert Dose: 5 mcg/kg/min × 60 minutes = 300 mcg/kg/hr.
  2. Convert to mg: 300 mcg / 1000 = 0.3 mg/kg/hr.
  3. Total mg per hour: 0.3 mg/kg/hr × 20 kg = 6 mg/hr.
  4. Calculate mL/hr: 6 mg/hr ÷ 0.05 mg/mL = 120 mL/hr.

Safety Considerations

Always double-check your math when dealing with high-potency drugs. Ensure the units (mcg vs mg) are consistent throughout the calculation. If adding a drug to a fluid bag, remember that the total volume of the bag changes slightly when you inject the drug, which can affect the final concentration in very precise clinical scenarios.

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