Pao2 Fio2 Ratio Calculator

Dividing arterial oxygen tension by the inspired oxygen fraction gives a single number that stays roughly constant in a healthy lung as supplemental oxygen is changed, which is what makes it useful for tracking a patient over hours and days. A person breathing room air with a PaO2 of 95 mmHg has a ratio near 450; a patient with a PaO2 of 200 mmHg on 50 percent oxygen has a ratio of 400 and is still essentially normal. The ratio falls when shunt or ventilation-perfusion mismatch prevents inspired oxygen from reaching capillary blood, and it does so in a way that is far less arithmetic-heavy than the alveolar-arterial difference, requiring no arterial carbon dioxide value and no barometric correction at the bedside. The Berlin criteria of 2012 turned it into a formal severity classification: 201 to 300 is mild, 101 to 200 moderate, and 100 or below severe acute respiratory distress syndrome, with reported mortality in the original cohorts of about 27, 32, and 45 percent respectively. Crucially, the measurement is only valid at a positive end-expiratory pressure of at least 5 cmH2O, because recruitment alone can move a patient a whole severity band; the pressure at which the gas was drawn must always be recorded alongside the ratio. Diagnosis also requires onset within a week of a known insult, bilateral opacities on imaging, and respiratory failure not explained by cardiac failure or volume overload. The ratio is unreliable when the inspired fraction is only estimated from a low-flow device, at altitude where the barometric pressure is reduced, and in shunt physiology where it varies with the inspired fraction itself.

The syndrome was described in 1967 by David Ashbaugh, D. Boyd Bigelow, Thomas Petty, and Bernard Levine in a Lancet report of twelve patients with refractory hypoxaemia, reduced lung compliance, and diffuse infiltrates, which they called adult respiratory distress syndrome for its resemblance to the neonatal disease. For the next quarter century there was no agreed definition, and reported incidence and mortality varied wildly between centres because each group used its own criteria. The American-European Consensus Conference imposed order in 1994, defining acute lung injury as a PaO2/FiO2 ratio of 300 or less and ARDS as 200 or less, with bilateral infiltrates and a pulmonary artery wedge pressure of 18 mmHg or under to exclude cardiogenic oedema. That definition drove a decade of trials but had known weaknesses: it did not specify ventilator settings, so the ratio was not comparable between patients, and wedge pressure measurement fell out of favour. The ARDS Definition Task Force convened in Berlin and published the replacement in JAMA in 2012, dropping the acute lung injury category, requiring a minimum PEEP of 5 cmH2O, adding a one-week timeframe, and grading severity into the three bands used here. A further global definition proposed in 2023 extended the concept to patients on high-flow nasal oxygen and permitted an SpO2/FiO2 ratio of 315 or less when arterial blood gas sampling is unavailable, aimed largely at resource-limited settings.