Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric FIO2.
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.
[4] 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 PaO2/FIO2 ratio less than or equal to 250 mmHg is one of the minor criteria for severe community acquired pneumonia (i.e., possible indication for inpatient treatment).
A PaO2/FIO2 ratio less than or equal to 333 mmHg is one of the variables in the SMART-COP risk score for intensive respiratory or vasopressor support in community-acquired pneumonia.