Oxygenation Index Calculator (OI & OSI)
Oxygenation index (FiO₂ × mean airway pressure ÷ PaO₂) and oxygen saturation index for ventilated patients — paediatric ARDS severity, ECMO thresholds and neonatal criteria.
How it works
OI = FiO₂ (%) × mean airway pressure (cmH₂O) ÷ PaO₂ (mmHg); OSI = FiO₂ × MAP ÷ SpO₂
The P/F ratio ignores how hard the ventilator is working to achieve a given PaO₂. The oxygenation index fixes that by multiplying the inspired oxygen by the mean airway pressure, so a patient needing 60% oxygen and a mean pressure of 18 cmH₂O scores far worse than one on the same FiO₂ at 8 cmH₂O. It is the standard severity measure in neonatal and paediatric intensive care — the PALICC definition of paediatric ARDS is graded by OI, and neonatal ECMO criteria have long cited an OI above 40. The oxygen saturation index substitutes pulse oximetry for arterial blood gas, using SpO₂ values of 97% or less so the sigmoid saturation curve is not on its flat part. Higher is worse; healthy ventilated lungs run an OI below 4.
Worked example
FiO₂ 60%, mean airway pressure 18, PaO₂ 65: OI = 60 × 18 / 65 = 16.6 — severe paediatric ARDS; OSI at SpO₂ 92% = 11.7.
Frequently asked questions
Why not just use P/F in children?
Children are often managed with high mean airway pressures on HFOV; P/F does not capture that support, so PALICC chose OI/OSI for ventilated children and P/F only for non-invasive support.
Where do I find mean airway pressure?
Modern ventilators display it (Paw mean). On HFOV it is the set mean pressure; on conventional ventilation it depends on PEEP, peak pressure and inspiratory time.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.