What is the Oxygenation Index formula?

OI = (FiO2 (%) × Mean Airway Pressure (cmH2O)) / PaO2 (mmHg). A higher value means worse oxygenation relative to the ventilatory support required.

What OI value is considered severe?

In adults, OI > 25 generally indicates severe respiratory failure. In neonates, OI ≥ 40 is a common ECMO entry criterion. Values 5–25 suggest moderate impairment; below 5 is typically normal.

How is OI different from PaO2/FiO2 ratio?

The P/F ratio ignores ventilator support (mean airway pressure), making it less sensitive to changes in PEEP or pressure-control settings. OI accounts for airway pressure, so it better reflects total respiratory support burden.

Is FiO2 entered as a decimal or percent?

This calculator accepts FiO2 as a percentage (e.g., enter 60 for 60% oxygen). Some published formulas use the decimal form (0.60); the result is identical either way.

Can I use SpO2 instead of PaO2?

The classic OI formula requires an arterial PaO2 from a blood gas. An OSI (oxygen saturation index) substitutes SpO2 and is used when ABG is unavailable, but that is a different calculation not covered here.

Does this tool store patient data?

No. All calculations run in your browser. No values are sent to any server, and nothing is stored after you close or refresh the page.

Oxygenation Index Calculator

Calculate the Oxygenation Index (OI) from FiO2, mean airway pressure, and PaO2 to grade hypoxemic respiratory failure severity.

Enter Ventilator & ABG Values

All three fields are required. Values must be from simultaneous measurements.

FiO₂ (fraction of inspired oxygen, %)

Mean Airway Pressure (MAP) (cmH₂O)

cmH₂O

PaO₂ (arterial O₂ partial pressure, mmHg)

mmHg

Severity Reference (Adults)

Normal / mild impairment OI < 5

Mild respiratory failure OI 5–15

Moderate respiratory failure OI 16–25

Severe respiratory failure OI > 25

Neonatal ECMO threshold is typically OI ≥ 40. Always interpret in full clinical context.

Clinical disclaimer: This tool is for educational and reference purposes only. OI values must be interpreted by qualified clinicians in conjunction with the full clinical picture. Do not make treatment decisions based solely on this calculator.

Summary

The Oxygenation Index (OI) is a clinical metric used in intensive care units to quantify the severity of hypoxemic respiratory failure. It combines the fraction of inspired oxygen (FiO2), mean airway pressure (MAP), and arterial partial pressure of oxygen (PaO2) into a single value. Higher OI values indicate more severe impairment of gas exchange and greater ventilatory support requirements.

How it works

Enter the FiO2 as a percentage (21–100%) — the fraction of oxygen delivered to the patient.
Enter the mean airway pressure (MAP) in cmH2O from the ventilator display.
Enter the arterial PaO2 in mmHg from the most recent arterial blood gas (ABG) result.
Click Calculate — the tool applies the formula: OI = (FiO2% × MAP) / PaO2.
Review the computed OI value and the corresponding severity classification.

Use cases

Grade the severity of acute respiratory distress syndrome (ARDS) at the bedside.
Track respiratory failure progression or improvement over serial ABGs.
Support ECMO candidacy decisions when OI thresholds are met.
Assess neonatal hypoxic respiratory failure in NICU settings.
Document OI values for ICU quality-improvement or research databases.
Educate medical students and residents on ventilator-management concepts.

Frequently Asked Questions

Last updated: 2026-05-28 · Reviewed by Nham Vu