In the medical industry, FiO2 is an acronym that stands for fraction of inhaled oxygen. This is a measure of the concentration of oxygen a person breathes in, according to WebMD. It is only useful for people who are receiving oxygen therapy from a tank or artificial ventilator.
Doctors treating a patient on a ventilator do not just measure the fraction of inhaled oxygen, explains WebMD. They also take other measurements relating to the gases in a patient's blood. These other measurements include the oxygen content; oxygen saturation; pH, which is a measure of acidity; and the partial pressures of oxygen and carbon dioxide.