Calculate Total Flow and Delivered Oxygen Fraction (FdO2)
L/min
L/min
L/min
Total Fresh Gas Flow (FGF):0 L/min
Oxygen Volume (Pure):0 L/min
Delivered Oxygen Fraction (FdO2):0%
Nitrous Oxide Fraction:0%
WARNING: HYPOXIC MIXTURE (< 21% O2)
function calculateFGF() {
// 1. Get input values
var o2Flow = parseFloat(document.getElementById('flow_o2').value);
var airFlow = parseFloat(document.getElementById('flow_air').value);
var n2oFlow = parseFloat(document.getElementById('flow_n2o').value);
// 2. Handle NaN (empty inputs treated as 0)
if (isNaN(o2Flow)) o2Flow = 0;
if (isNaN(airFlow)) airFlow = 0;
if (isNaN(n2oFlow)) n2oFlow = 0;
// 3. Validation: prevent negative numbers
if (o2Flow < 0) o2Flow = 0;
if (airFlow < 0) airFlow = 0;
if (n2oFlow 0) {
fio2 = (totalO2 / totalFlow) * 100;
n2oPercent = (n2oFlow / totalFlow) * 100;
}
// 7. Update UI
var resultContainer = document.getElementById('fgf_results');
resultContainer.style.display = 'block';
document.getElementById('result_total_flow').innerHTML = totalFlow.toFixed(2) + ' L/min';
document.getElementById('result_o2_vol').innerHTML = totalO2.toFixed(2) + ' L/min';
var fio2Element = document.getElementById('result_fio2');
fio2Element.innerHTML = fio2.toFixed(1) + '%';
// Color coding for FiO2
var warningDiv = document.getElementById('hypoxic_warning');
if (fio2 0) {
fio2Element.className = 'fgf-result-value fgf-highlight';
warningDiv.style.display = 'block';
} else {
fio2Element.className = 'fgf-result-value fgf-safe';
warningDiv.style.display = 'none';
}
document.getElementById('result_n2o_perc').innerHTML = n2oPercent.toFixed(1) + '%';
}
Understanding Fresh Gas Flow Calculations
In anesthesiology, managing the Fresh Gas Flow (FGF) is critical for maintaining adequate depth of anesthesia and ensuring patient safety. The fresh gas flow refers to the mixture of gases (typically Oxygen, Medical Air, and Nitrous Oxide) delivered from the anesthesia workstation into the breathing circuit.
This calculator helps clinicians and students determine the exact composition of the gas mixture being delivered to the patient, specifically focusing on the Delivered Oxygen Fraction (FdO2).
How to Calculate Delivered Oxygen Concentration
Most modern anesthesia machines have electronic mixers that display the FiO2, but understanding the manual calculation is essential for board exams and equipment failure scenarios. The calculation relies on the known oxygen concentration of the source gases:
Oxygen (O2): Contains 100% Oxygen.
Medical Air: Contains approximately 21% Oxygen and 79% Nitrogen.
Nitrous Oxide (N2O): Contains 0% Oxygen.
The Formula
To find the percentage of oxygen in the final mixture, we first calculate the total volume of pure oxygen and divide it by the total flow rate:
Total Flow = O2 Flow + Air Flow + N2O Flow
Total O2 Vol = O2 Flow + (Air Flow × 0.21)
FdO2 (%) = (Total O2 Vol / Total Flow) × 100
Clinical Application: Low Flow Anesthesia
Calculating FGF is particularly important in Low Flow Anesthesia. By reducing the total FGF (often below 1 L/min), anesthesiologists preserve heat and humidity in the breathing circuit and reduce the consumption of volatile agents and atmospheric pollution. However, when using low flows, the difference between the delivered gas concentration and the gas concentration inspired by the patient may vary due to rebreathing.
Hypoxic Guard Systems
Modern machines utilize hypoxic guard systems (like the Link-25 system) which mechanically or electronically link the Nitrous Oxide and Oxygen flow controls. This ensures that the delivered mixture never drops below a safe oxygen concentration (typically 23-25%) to prevent the delivery of a hypoxic mixture to the patient.