Hong Life CalculatorsUnderstanding the oxygenation index calculator — concept and principle
The oxygenation index (OI) combines how much pressure and oxygen a ventilated patient needs with the oxygen level achieved. This calculator also gives the oxygen saturation index (OSI), which uses pulse oximetry instead of a blood gas, the PaO₂/FiO₂ ratio, and the paediatric ARDS severity bands of the PALICC 2015 consensus.
How it is calculated
OI = mean airway pressure (cmH₂O) × FiO₂ (%) ÷ PaO₂ (mmHg)
OSI = mean airway pressure × FiO₂ (%) ÷ SpO₂ (%), valid only when SpO₂ is 97% or less
P/F ratio = PaO₂ ÷ (FiO₂ ÷ 100)
- PaO₂ in kPa is converted with 1 kPa = 7.50062 mmHg.
- PALICC bands: OI 4 to under 8 mild, 8 to under 16 moderate, 16 or more severe; OSI 5 to under 7.5, 7.5 to under 12.3, 12.3 or more.
The principle
The P/F ratio ignores how much ventilator pressure was needed to reach the oxygen level. Multiplying by mean airway pressure adds that information, so a patient needing high pressure and oxygen for a modest PaO₂ scores high. Above about 97% saturation the oxygen dissociation curve is flat, so SpO₂ no longer tracks PaO₂, which is why OSI is valid only at 97% or less.
Worked example
Default inputs: mean airway pressure 14 cmH₂O, FiO₂ 60%, PaO₂ 70 mmHg, SpO₂ 92%.
- OI = 14 × 60 ÷ 70 = 12 → Moderate (OI 8 to under 16).
- OSI = 14 × 60 ÷ 92 = 9.1 → Moderate (OSI 7.5 to under 12.3).
- P/F = 70 ÷ 0.6 = 117 mmHg.
| PaO₂ (arterial blood gas) | Oxygenation index (OI) |
|---|---|
| 17.5 | 48 |
| 35 | 24 |
| 52.5 | 16 |
| 70 | 12 |
| 87.5 | 9.6 |
| 105 | 8 |
| 140 | 6 |
How to read the result
Higher OI and OSI mean worse oxygenation. In children, very high OI values are used in some centres as one of the triggers to consider advanced support such as ECMO. The PALICC bands apply only to invasively ventilated children who meet the other paediatric ARDS criteria (timing, new chest X-ray changes, not fully explained by heart failure).
Common mistakes
- Entering FiO₂ as a fraction (0.6) instead of a percentage (60).
- Using SpO₂ above 97% for the OSI.
- Mixing kPa and mmHg.
Frequently asked questions
What OI is severe?
In PALICC 2015, an OI of 16 or more is severe paediatric ARDS.
Why use OSI?
It avoids repeated arterial blood sampling, especially in children.
Where does the mean airway pressure come from?
The ventilator displays it; it can also be estimated from the pressure settings.
Sources: Khemani RG, Smith LS, Zimmerman JJ, Erickson S, for the PALICC Group, "Pediatric acute respiratory distress syndrome: definition, incidence, and epidemiology", Pediatr Crit Care Med 2015;16(5 Suppl 1):S23–S40