Oxygenation index (OI)
Combines FiO2, mean airway pressure and PaO2 to express gas exchange severity under ventilation.
When to use
Ventilated patient where an oxygenation index including ventilatory support, not only FiO2, is wanted.
Pearls and pitfalls
- The field takes FiO2 as a percentage, for example 80 for 80% oxygen; the calculation converts it to a fraction before multiplying.
- Higher OI values are associated with mortality, longer stay and longer ventilation.
- Mean airway pressure is clinician set, so OI changes when the ventilator changes, without the lung changing.
- The index does not replace blood gas analysis or the clinical assessment of severity.
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Formula
OI = (FiO2 as a fraction × mean airway pressure in cmH2O × 100) divided by PaO2 in mmHg, with FiO2 entered as a percentage.
Evidence
Prospective study comparing OI, PaO2/FiO2 ratio and the saturation index for mortality prediction in acute respiratory distress syndrome.
References
- Naim J, Afroz N, Sultana MN, Hossain SB, Uddin ABMB. Comparative performance of oxygenation indices for mortality prediction in patients with acute respiratory distress syndrome at a tertiary care intensive care unit in Bangladesh. Cureus. 2026;18:e116997. doi:10.7759/cureus.116997
Frequently asked questions
What is Oxygenation index (OI)?
Combines FiO2, mean airway pressure and PaO2 to express gas exchange severity under ventilation. Ventilated patient where an oxygenation index including ventilatory support, not only FiO2, is wanted.
How is the result calculated?
OI = (FiO2 as a fraction × mean airway pressure in cmH2O × 100) divided by PaO2 in mmHg, with FiO2 entered as a percentage.
How should the result be interpreted?
Prospective study comparing OI, PaO2/FiO2 ratio and the saturation index for mortality prediction in acute respiratory distress syndrome.
What are the sources?
Naim J, Afroz N, Sultana MN, Hossain SB, Uddin ABMB. Comparative performance of oxygenation indices for mortality prediction in patients with acute respiratory distress syndrome at a tertiary care intensive care unit in Bangladesh. Cureus. 2026;18:e116997.