Fractional Inspired Oxygen (FiO2) is the fraction or percentage of oxygen present in the gas mixture that a person breathes. Patients with difficulty in breathing benefit from air enriched with oxygen, which means that the latter's FiO2 is higher than atmospheric normal. Natural air contains 20.9% oxygen, which equates to an FiO2 of 0.209. Oxygen-enriched air has an FiO2 greater than 0.21; up to 1.00 or 100% oxygen. However, FiO2 is usually kept below 0.5 even with mechanical ventilation, to avoid the toxic effects of high concentration oxygen.
If a patient is fitted with a nasal cannula or a simple mask, each litre/min of oxygen adds approximately 4 percentage points to FiO2 for the first 3 liters and only 3 percentage points for each additional liter (for example, a patient with a nasal cannula receiving 4L/min of oxygen flow would have an FiO2 equal to 21% + (3 x 4%) + (1 x 3%) = 36%).
FiO2 is often used in medicine to represent the percentage of oxygen participating in gas exchange. If the barometric pressure changes, the FiO2 can remain constant while the partial pressure of oxygen changes with the change in barometric pressure.
This ratio is an index of the severity of hypoxia during disorders of the ventilation/perfusion ratio at the pulmonary level giving rise to a shunt effect.
The normal value is 95mmHg / 21% = 452.
A value below 300 reflects severe impairment of gas exchange within the lungs.
A value of less than 200 reflects a very severe alteration in gas exchange, such as in the case of Acute Respiratory Distress Syndrome.