Calculate OI
OI = (FiO2 × MAP × 100) / PaO2
cmH2O — read from the ventilator
% — fills in the OSI (saturation-based) readout
Inverse: what settings reach a target OI?
Fix two variables and solve for the third at a chosen target OI.
OI vs the other oxygenation metrics
| Metric | Formula | What it tells you |
|---|---|---|
| Oxygenation index (OI) | (FiO2 × MAP × 100) / PaO2 | Oxygenation relative to airway pressure — the only one here that penalizes high MAP. ECMO criteria: ≥ 40. |
| P/F ratio | PaO2 / FiO2 | Quick ABG ratio; ignores MAP. Berlin ARDS: mild 200–300, moderate 100–200, severe ≤ 100 mmHg. |
| A-a gradient | PAO2 − PaO2 | Alveolar-minus-arterial difference; localizes the cause of hypoxemia (shunt vs V/Q mismatch). |
| OSI | (FiO2 × MAP × 100) / SpO2 | Non-invasive OI substitute when no ABG is available; unreliable at SpO2 ≈ 100%. |
Boundary: OI is not the A-a gradient, and it needs MAP
This tool computes the oxygenation index only: a ventilator-context metric that combines FiO2, mean airway pressure, and PaO2. It does not compute the A-a gradient (the alveolar–arterial oxygen difference, a different formula for a different question) — and its MAP input is the ventilator's mean airway pressure, not mean arterial pressure.
Frequently asked questions
Answers match the structured data below, byte for byte.
What is the oxygenation index (OI) formula?
The oxygenation index is OI = (FiO2 x MAP x 100) / PaO2. Enter FiO2 as a fraction (0.21 to 1.0), MAP (mean airway pressure) in cmH2O, and PaO2 (arterial oxygen partial pressure) in mmHg. Example: FiO2 0.60, MAP 15 cmH2O, PaO2 80 mmHg gives OI = (0.60 x 15 x 100) / 80 = 11.25. If you enter FiO2 as a whole-number percent (e.g. 60), omit the x100 step: OI = (60 x 15) / 80 = 11.25.
Why is FiO2 multiplied by 100 in the OI formula?
The factor of 100 converts the FiO2 fraction into a percent-like scale, so the resulting index lands on the familiar 0-100+ clinical scale instead of a small decimal. If you already enter FiO2 as a whole-number percent (21-100), the conversion is done for you and you must NOT multiply by 100 again — doing so would inflate the result 100-fold. This calculator has a fraction/percent toggle so the factor is applied correctly either way.
What are the normal oxygenation index values and cutoffs?
There is no healthy normal value because OI is only meaningful during mechanical ventilation. In neonatal and pediatric practice: OI below 10 indicates mild hypoxemic respiratory failure, 10-25 moderate, above 25 severe, and above 40 is the widely accepted ECMO initiation threshold (many ECMO centers use 40-45, per ELSO guidelines). An OI of 25-40 persisting 4-6 hours despite optimized ventilation is commonly treated as a trigger for ECMO evaluation. Adults are usually staged with the P/F ratio instead.
Does an OI above 40 mean ECMO is required?
OI above 40 is a widely accepted ECMO criterion in neonatal and pediatric care (it was the main entry criterion of the UK Collaborative ECMO Trial), but it is never a solo decision rule: centers also require a reversible cause, failure of optimized conventional therapy, and absence of ECMO contraindications. Use this calculator for education and trend tracking only — any ECMO decision belongs to the bedside clinical team.
What is the difference between OI, the P/F ratio, and the A-a gradient?
The OI is (FiO2 x MAP x 100) / PaO2 and is the only one of the three that accounts for mean airway pressure, so it reflects how much ventilator pressure is needed to achieve a given oxygenation. The P/F ratio is PaO2 / FiO2 (Berlin ARDS: mild 200-300, moderate 100-200, severe 100 or less mmHg) and ignores airway pressure entirely. The A-a gradient is the alveolar-minus-arterial oxygen difference (PAO2 - PaO2), which localizes the cause of hypoxemia but uses neither MAP nor FiO2 scaling. See also our A-a gradient calculator and mean airway pressure calculator.
What is the oxygen saturation index (OSI) and when is it used?
OSI = (FiO2 x MAP x 100) / SpO2. It is the non-invasive cousin of OI, using pulse-oximetry SpO2 instead of an arterial blood-gas PaO2. Clinicians use it when an arterial line or recent ABG is unavailable. Its interpretation parallels OI (higher = worse), but SpO2 plateaus at 100%, so OSI is unreliable when SpO2 is at or near 100% — a flat SpO2 of 100% hides further PaO2 rises.
Can I enter PaO2 in kPa instead of mmHg?
Yes — switch the PaO2 unit toggle to kPa and the tool converts automatically: 1 kPa = 7.50062 mmHg. For example, PaO2 10 kPa becomes 75.01 mmHg, so FiO2 0.60 with MAP 15 cmH2O gives OI = (0.60 x 15 x 100) / 75.01 = 12.00. Double-check which unit your blood-gas analyzer reports before calculating.
How do I use the inverse mode (target OI)?
Enter a target OI plus the two variables you want to hold constant, and the tool solves for the third. Example: to reach a target OI of 15 with MAP 15 cmH2O and PaO2 80 mmHg, the required FiO2 is (15 x 80) / (15 x 100) = 0.80 (80%). If the solved FiO2 exceeds 1.0, the tool flags it as unachievable by FiO2 alone — you would need a lower MAP or a higher PaO2 instead.
Is OI valid for adult ARDS patients?
OI is most strongly validated in neonatal and pediatric populations and is embedded in pediatric ARDS (PARDS) criteria. Adult ICU practice centers on the P/F ratio with the Berlin ARDS definition, though studies such as OXIVA-CARDS have shown OI correlates well with P/F in ventilated adults. For adults, interpret OI alongside the P/F ratio rather than alone.
Why does the calculator reject FiO2 below 0.21 or PaO2 of 0?
FiO2 cannot be below 0.21, the oxygen fraction of room air, so lower entries are rejected as input errors. PaO2 of 0 would mean division by zero, which is mathematically undefined. A common mistake is entering 60 (percent) while the fraction toggle is selected — the tool catches this and tells you to switch the toggle rather than silently miscomputing.
What OI trend should I watch over time?
A single OI is a snapshot; the trend is what matters clinically. A rising OI means oxygenation is worsening relative to ventilator support, and crossing 25 (severe) or approaching 40 (ECMO range) despite optimized settings is when teams escalate. Recalculate OI after every significant ventilator change so the index reflects the new MAP and FiO2.
Is this oxygenation index calculator medical advice?
No. This tool is for education and training only. OI is an intensive-care metric used in ventilated patients, and real ECMO and ventilation decisions involve many factors this calculator cannot see. Never base a clinical decision on this result alone — always consult the treating clinician or respiratory team.