Oxygenation Index (OI) Calculator
The Oxygenation Index (OI) combines mean airway pressure, inspired oxygen fraction, and arterial oxygen tension into a single number that quantifies the severity of respiratory failure in ventilated patients — particularly useful for ECMO candidacy assessment.
cmH₂O
mmHg
Moderate impairment
- 1
Numerator (MAP × FiO₂ × 100)
20 × 0.6 × 100 = 1,200 - 2
P/F ratio (PaO₂ ÷ FiO₂)
80 ÷ 0.6 = 133The P/F ratio does not incorporate mean airway pressure. - 3
Oxygenation Index
1,200 ÷ 80 = 15
How does this calculator work?
OI = (MAP × FiO₂ × 100) ÷ PaO₂. Enter the ventilator MAP (cmH₂O), the FiO₂ as a decimal (0.21–1.0), and the arterial PaO₂ (mmHg). OI < 5 is normal; OI > 40 signals severe impairment often prompting ECMO evaluation.
Formula
How this is calculated
The Oxygenation Index integrates both the level of ventilatory support and the resulting gas exchange into one metric. Mean airway pressure (MAP, in cmH₂O) represents the mechanical burden of ventilation; FiO₂ (as a decimal from 0.21 to 1.0) represents the inspired oxygen concentration; and PaO₂ (in mmHg from an arterial blood gas) represents the achieved arterial oxygenation. A higher OI means worse oxygenation despite more aggressive support.
Interpretation thresholds used here reflect published consensus guidelines: OI < 5 is considered normal; 5–15 mild impairment; 15–25 moderate; 25–40 severe (roughly equivalent to moderate or severe ARDS on the Berlin Definition); OI > 40 is often used as an ECMO threshold in paediatric and adult centres, though institutional protocols vary. The P/F ratio (PaO₂ ÷ FiO₂) is also reported as a complementary index that does not include MAP.
This calculator is a clinical reference tool. OI thresholds and ECMO criteria differ between centres, patient populations (neonatal, paediatric, adult), and clinical contexts. Always interpret results within the full clinical picture and local institutional guidelines.
Frequently asked questions
The P/F ratio (PaO₂ ÷ FiO₂) measures gas exchange efficiency but ignores ventilatory support. The OI adds mean airway pressure to the numerator, so it penalises patients who require high MAP to achieve acceptable oxygenation, making it a more complete measure of respiratory failure severity.
Many centres use OI > 40 (or sometimes > 25 for 6 hours) as a threshold for ECMO referral in severe ARDS or neonatal respiratory failure. Exact criteria vary by institution and patient age group — consult your centre's protocol.
A related metric, the Oxygen Saturation Index (OSI), substitutes SpO₂ for PaO₂ to avoid arterial blood gas sampling. This calculator uses the classic OI definition requiring PaO₂ from an ABG. For OSI, use SpO₂ (%) as the denominator (OSI = MAP × FiO₂ × 100 / SpO₂).
Also known as
TG we-Calculate Editorial Team. (2026). Oxygenation Index (OI) Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/oxygenation-index-calculator
TG we-Calculate Editorial Team. "Oxygenation Index (OI) Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/oxygenation-index-calculator.
TG we-Calculate Editorial Team, "Oxygenation Index (OI) Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/oxygenation-index-calculator
@misc{wecalculate_oxygenation_index_calculator, title = {Oxygenation Index (OI) Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/oxygenation-index-calculator}}, year = {2026}, note = {TG we-Calculate} }
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