FiO2

FiO₂, in the field of medicine, is the fraction of inspired oxygen in a gas mixture.

The FiO₂ is expressed as a number from 0 (0%) to 1 (100%). The FiO₂ of normal room air is 0.21 (21%).

A patient’s FiO₂ may be varied through the use of different Venturi masks, in combination with varying oxygen flow rates. In addition, most mechanical ventilators have controls for adjusting FiO₂. An increased FiO₂ is necessary in managing adequate oxygenation in patients who are critically ill due to causes such as major surgery, acute lung injury, sepsis, pneumonia, congestive heart failure, or other cardiopulmonary disease.

Another common misconception is that the FiO₂ changes with elevation. It remains at 0.21 at all altitudes within the atmosphere. What changes is the barometric pressure of air. At altitude, therefore, the partial pressure of oxygen delivered by that 21% of oxygen is lower. The partial pressure is the driving force to oxygenate the blood and therefore a lower partial pressure makes it that much harder to get O₂ delivered to the tissues that require it, resulting in hypoxia.

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