Note: Calculated flow is very low. Most vaporizers require a minimum flow of 0.5 to 1.0 L/min for accuracy. Please verify equipment minimums.
function updateCircuitInfo() {
var circuit = document.getElementById('circuitType').value;
var weightInput = document.getElementById('patientWeight');
// Optional: Could add visual hints about weight limits per circuit
}
function calculateVetFlow() {
// 1. Get Inputs
var weightVal = document.getElementById('patientWeight').value;
var unit = document.getElementById('weightUnit').value;
var circuit = document.getElementById('circuitType').value;
var stage = document.getElementById('anesthesiaStage').value;
// 2. Validate
if (weightVal === "" || isNaN(weightVal) || weightVal <= 0) {
alert("Please enter a valid positive weight.");
return;
}
var weightKg = parseFloat(weightVal);
if (unit === 'lb') {
weightKg = weightKg / 2.20462;
}
// 3. Define Logic Variables
var flowRateMl = 0;
var tidalVolMl = weightKg * 15; // Using 15ml/kg as conservative estimate for bag sizing
var bagSizeL = 0;
// 4. Calculate Flow Rate based on Circuit and Stage
if (circuit === 'nonrebreathing') {
// Non-rebreathing requires high flow to flush CO2
// Typical Bain factor: 200-300 ml/kg/min usually regardless of stage,
// though sometimes slightly less for maintenance.
// We will use 200 ml/kg/min as a standard baseline.
flowRateMl = weightKg * 200;
} else {
// Rebreathing (Circle)
if (stage === 'induction') {
// Induction/Recovery: 50-100 ml/kg/min. Using 50 ml/kg/min (semi-closed).
flowRateMl = weightKg * 50;
} else {
// Maintenance: 20-40 ml/kg/min. Using 30 ml/kg/min.
flowRateMl = weightKg * 30;
}
}
// 5. Convert to Liters
var flowRateL = flowRateMl / 1000;
// 6. Minimum Flow Safety Check (Soft Limit logic)
// Many clinicians set a hard minimum of 0.5L or 1.0L regardless of calculation
// This calculator shows the math but warns if below common minimums.
var minFlowAlert = document.getElementById('minFlowWarning');
if (flowRateL < 0.5) {
minFlowAlert.style.display = 'block';
} else {
minFlowAlert.style.display = 'none';
}
// 7. Calculate Bag Size (6 x Tidal Volume)
// Result in mL, convert to L
var reqBagVolMl = tidalVolMl * 6;
var reqBagVolL = reqBagVolMl / 1000;
// Standard Bag Sizes: 0.5, 1, 2, 3, 4, 5, 30L
var standardBags = [0.5, 1, 2, 3, 4, 5, 30];
var recommendedBag = 0.5; // default min
for (var i = 0; i = reqBagVolL) {
recommendedBag = standardBags[i];
break;
}
// If larger than biggest, take biggest
recommendedBag = standardBags[standardBags.length – 1];
}
// 8. Display Results
document.getElementById('resFlowRate').innerText = flowRateL.toFixed(2) + " L/min";
document.getElementById('resTidalVol').innerText = tidalVolMl.toFixed(0) + " mL";
document.getElementById('resBagSize').innerText = recommendedBag + " L";
document.getElementById('vet-results').style.display = 'block';
}
How to Calculate Veterinary Oxygen Flow Rates
Administering anesthesia safely in veterinary medicine requires precise calculation of oxygen flow rates. The flow rate determines how much fresh gas (oxygen and inhalant anesthetic) is delivered to the patient per minute. This calculation depends heavily on the patient's weight, the type of breathing circuit used, and the stage of the anesthetic procedure.
1. Choosing the Right Breathing System
Before calculating the flow rate, you must identify which breathing system fits your patient. This is generally determined by weight.
System Type
Patient Weight
Characteristics
Non-Rebreathing (Bain, Mapleson)
Under 7-10 kg
Low resistance, relies on high fresh gas flow to flush out exhaled CO2.
Rebreathing (Circle System)
Over 7-10 kg
Uses CO2 absorbent (soda lime), allows lower flow rates, conserves heat and moisture.
2. Formulas for Oxygen Flow Rate
The flow rate is calculated in milliliters per kilogram per minute (ml/kg/min) and then converted to Liters per minute (L/min) for the flowmeter.
For Rebreathing (Circle) Systems:
In a circle system, the gases are recirculated. The flow rate varies by stage to ensure nitrogen is flushed out initially and adequate anesthetic depth is maintained.
Induction & Recovery: Higher flow is needed to rapidly change anesthetic concentration. Formula: 50–100 ml/kg/min
Maintenance: Once depth is stable, flow can be reduced. Formula: 20–40 ml/kg/min
Example: 20kg Dog (Maintenance)
20 kg × 30 ml/kg/min = 600 ml/min Flow Rate = 0.6 L/min
For Non-Rebreathing Systems:
These systems do not use chemical CO2 absorption. They rely on a high flow of fresh gas to push exhaled breath out of the tube (washout). If the flow is too low, the animal will re-breathe carbon dioxide.
Factor: High flow factor required. Formula: 200–300 ml/kg/min
The reservoir bag (rebreathing bag) allows the animal to breathe easier and provides a way for the anesthetist to deliver manual breaths ("bagging").
To calculate the correct bag size, you first estimate the patient's Tidal Volume (the amount of air in a normal breath), usually estimated at 10–15 ml/kg.
The bag should be approximately 6 times the Tidal Volume to prevent over-inflation or emptying completely during inspiration.
Bag Calculation Formula:
Weight (kg) × 15 ml/kg × 6 = Bag Volume (ml)
Always round up to the next standard bag size (0.5L, 1L, 2L, 3L, 4L, 5L).
4. Important Safety Margins
Regardless of the calculated math, equipment limitations exist. Most veterinary vaporizers (e.g., Tec 3, Tec 4) are calibrated to be accurate at specific flow rates.
Minimum Flow Rule: Many veterinary anesthesiologists recommend a minimum oxygen flow rate of 0.5 to 1.0 L/min, even if the calculated weight-based requirement is lower. This ensures the vaporizer functions correctly and there is enough flow to drive the anesthetic gas through the circuit.
Disclaimer: This calculator is a tool for estimation. Always follow the specific protocols of your veterinary clinic and the advice of the attending veterinarian.