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Fraction of inspired oxygen (FiO2) is the fraction or percentage of oxygen in the space being measured. Often used in medicine the FiO2 is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, the FiO2 may remain constant while the partial pressure of oxygen changes with the change in barometric pressure.
PAO2, PEO2, and PiO2 are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and VD/VT is the ratio of physiologic dead space over tidal volume.
The ratio between partial pressure of oxygen in arterial blood (PaO2) and FiO2 is used as an indicator of hypoxemia per the American-European Consensus Conference on lung injury. A high FiO2 has been shown to alter the ratio of PaO2/FiO2.
The ratio of partial pressure arterial oxygen and fraction of inspired oxygen, sometimes called the Carrico index, is a comparison between the oxygen level in the blood and the oxygen concentration that is breathed. This helps to determine the degree of any problems with how the lungs transfer oxygen to the blood. A sample of arterial blood is collected for this test. A PaO2/FiO2 ratio less than or equal to 200 is necessary for the diagnosis of Acute Respiratory Distress Syndrome. 
The alveolar air equation is the following formula, used to calculate the partial pressure of alveolar gas:
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