How this calculator works
Oxygenation index includes the mean airway pressure required to achieve arterial oxygenation. The conventional calculation combines mean airway pressure in cmH2O with PaO2 in mmHg; it is not a ratio of pressures expressed in matching units. The factor of 100 assumes FiO2 is a fraction. The page converts blood gas pressure units but keeps the airway-pressure entry explicitly in cmH2O. Clinical severity definitions and management require more than this arithmetic. The result is an estimate for information only and is not medical advice.
Formula
OI = 100 x FiO2 x MAP(cmH2O)/PaO2(mmHg). MAP = OI x PaO2/(100 x FiO2). PaO2 = 100 x FiO2 x MAP/OI. FiO2 = OI x PaO2/(100 x MAP). P/F = PaO2/FiO2.
Worked example
With FiO2 0.50, MAP 10 cmH2O and PaO2 50 mmHg, OI = 100 x 0.50 x 10/50 = 10, and P/F = 100 mmHg. Doubling PaO2 to 100 mmHg gives OI 5.
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Frequently asked questions
Why are two different pressure units used?
The conventional OI is defined using cmH2O for airway pressure and mmHg for arterial oxygen pressure. Converting both to the same unit without adjusting the formula changes the index.
Can I use oxygen saturation?
No. Replacing PaO2 with saturation creates a different index, the oxygen saturation index, with different interpretation.
Can this set ventilator pressure?
No. Inverse modes only rearrange a relationship. Ventilator changes require bedside assessment by the treating team.