Induction / Change of Depth (100 ml/kg/min)
Initial Maintenance (50 ml/kg/min)
Low Flow Maintenance (10 ml/kg/min)
Standard Maintenance (20 ml/kg/min)
Bain / Lack System (K = 2-3)
Ayre's T-Piece / Jackson-Rees (K = 2.5-3)
Mapleson A (K = 1.5)
Calculated as: Weight × 200ml/kg/min × K-Factor
Recommended Fresh Gas Flow:
Understanding Veterinary Fresh Gas Flow Rates
Calculating the correct Fresh Gas Flow (FGF) is critical in veterinary anesthesia to ensure patient safety, maintain adequate anesthetic depth, and minimize waste. The flow rate determines how quickly anesthetic concentration changes in the circuit and how effectively carbon dioxide is removed.
Rebreathing (Circle) Systems
In a circle system, exhaled gases are scrubbed of CO2 and reused. Flow rates are generally lower, making them more economical for larger patients (typically >7-10kg). We categorize flows into three phases:
Induction/Wash-in: High flows (100 ml/kg/min) are used to quickly match the vaporizer setting with the concentration in the breathing circuit.
Maintenance: Standard flows (20-50 ml/kg/min) provide a safety margin and ensure the one-way valves function correctly.
Low Flow: Advanced monitoring allows flows as low as 10 ml/kg/min (metabolic requirement), but requires careful capnography.
Non-Rebreathing Systems
Non-rebreathing systems (Bain, T-Piece) rely entirely on high fresh gas flow to flush CO2 out of the circuit. If the flow is too low, the patient will re-inhale carbon dioxide. These are preferred for small patients (<7kg) due to lower resistance to breathing. The flow is calculated using a circuit factor (K) multiplied by the patient's estimated minute volume.
Calculation Examples
Example 1 (Rebreathing): A 20kg dog in maintenance on a circle system.
20kg × 30 ml/kg/min = 600 ml/min (0.6 L/min).
Example 2 (Non-Rebreathing): A 4kg cat on a Bain system.
4kg × 200 ml/kg/min × 2.5 (Factor) = 2,000 ml/min (2.0 L/min).
Important Safety Note: Most precision vaporizers require a minimum flow of 500ml/min to 1L/min to maintain accurate output. Always check your equipment's specific requirements.
function toggleSystemFields() {
var system = document.getElementById("systemType").value;
var rebFields = document.getElementById("rebreathingFields");
var nonRebFields = document.getElementById("nonRebreathingFields");
if (system === "rebreathing") {
rebFields.style.display = "block";
nonRebFields.style.display = "none";
} else {
rebFields.style.display = "none";
nonRebFields.style.display = "block";
}
}
function calculateFGF() {
var weight = parseFloat(document.getElementById("patientWeight").value);
var system = document.getElementById("systemType").value;
var resultDiv = document.getElementById("fgfResult");
var flowValue = document.getElementById("flowValue");
var flowSafety = document.getElementById("flowSafety");
if (isNaN(weight) || weight <= 0) {
alert("Please enter a valid patient weight.");
return;
}
var totalFlowMl = 0;
var safetyMsg = "";
if (system === "rebreathing") {
var phaseRate = parseFloat(document.getElementById("anesthesiaPhase").value);
totalFlowMl = weight * phaseRate;
// Safety floor for Circle Systems
if (totalFlowMl < 500) {
safetyMsg = "Note: Flow increased to 0.5 L/min minimum for vaporizer accuracy.";
totalFlowMl = 500;
}
} else {
var kFactor = parseFloat(document.getElementById("circuitFactor").value);
// Base formula for Non-rebreathing: Wt * 200ml/kg * K
totalFlowMl = weight * 200 * kFactor;
// Safety floor for Non-rebreathing
if (totalFlowMl < 1000) {
safetyMsg = "Note: Minimum 1.0 L/min recommended for most NRB circuits to ensure CO2 washout.";
totalFlowMl = 1000;
}
}
var flowLiters = (totalFlowMl / 1000).toFixed(2);
flowValue.innerHTML = flowLiters + " L/min";
flowSafety.innerHTML = safetyMsg;
resultDiv.style.display = "block";
}