Constant Rate Infusion Calculations

Constant Rate Infusion (CRI) Calculator

Calculation Summary:

Understanding Constant Rate Infusions (CRI)

A Constant Rate Infusion (CRI) is a technique used in clinical medicine, particularly in emergency and intensive care, to deliver a precise amount of medication over a specific period. Unlike bolus injections which cause "peaks and valleys" in drug plasma concentration, a CRI maintains a steady-state therapeutic level, improving efficacy and reducing the risk of toxicity.

The CRI Formula

To calculate how much of a drug should be added to a fluid bag, we use the following mathematical logic:

  1. Determine mg needed per hour: (Dosage in mcg/kg/min × Weight in kg × 60 minutes) / 1000
  2. Determine hours the bag will last: Total Bag Volume / Fluid Rate per hour
  3. Total drug required: (mg needed per hour / drug concentration) × total hours

Common Clinical Examples

CRI delivery is frequently used for analgesia, blood pressure management, and seizure control. Typical drugs include:

  • Lidocaine: Often used for cardiac arrhythmias or visceral pain management.
  • Fentanyl: A potent opioid used for severe intra-operative and post-operative pain.
  • Ketamine: Used at sub-anesthetic doses for NMDA antagonism and wind-up pain prevention.
  • Metoclopramide: Used for gastrointestinal motility disorders.

Calculated Case Study

Suppose you have a 20kg canine patient requiring a Lidocaine CRI at 50 mcg/kg/min. Your drug concentration is 20 mg/mL. You are using a 500 mL saline bag, and the patient is receiving fluids at 40 mL/hr.

Using the calculator above, you would find that you need to add 37.5 mL of Lidocaine to the 500 mL fluid bag to achieve the desired delivery rate.

Medical Disclaimer: This tool is intended for educational and reference purposes only. Veterinary and medical professionals should always double-check calculations manually before administering any medication. Ensure the volume of drug added is accounted for by removing an equal volume of fluid from the bag if precision is critical.
function calculateCRI() { var weight = parseFloat(document.getElementById('weight').value); var dosage = parseFloat(document.getElementById('dosage').value); var concentration = parseFloat(document.getElementById('concentration').value); var fluidVolume = parseFloat(document.getElementById('fluidVolume').value); var fluidRate = parseFloat(document.getElementById('fluidRate').value); var resultArea = document.getElementById('cri-result-area'); var drugToAddDisplay = document.getElementById('drug-to-add'); var totalDoseHrDisplay = document.getElementById('total-dose-hr'); var durationDisplay = document.getElementById('infusion-duration'); if (isNaN(weight) || isNaN(dosage) || isNaN(concentration) || isNaN(fluidVolume) || isNaN(fluidRate) || fluidRate <= 0 || concentration <= 0) { alert("Please enter valid positive numbers in all fields."); return; } // 1. Calculate mg required per minute var mgPerMin = (dosage * weight) / 1000; // 2. Calculate mg required per hour var mgPerHour = mgPerMin * 60; // 3. Calculate how long the bag will last (hours) var bagDurationHours = fluidVolume / fluidRate; // 4. Calculate total mg needed for the entire bag duration var totalMgNeeded = mgPerHour * bagDurationHours; // 5. Convert total mg to mL of drug var totalMlToAdd = totalMgNeeded / concentration; // Display Results resultArea.style.display = "block"; drugToAddDisplay.innerHTML = "Add " + totalMlToAdd.toFixed(2) + " mL of drug to the bag"; totalDoseHrDisplay.innerHTML = "Patient receives: " + mgPerHour.toFixed(2) + " mg of drug per hour"; durationDisplay.innerHTML = "This fluid bag will last: " + bagDurationHours.toFixed(1) + " hours at the current rate"; resultArea.scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }

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