Oxygen Index

Calculates the Oxygen Index to assess severity of hypoxemic respiratory failure in mechanically ventilated patients.

Pulmonology PulmonologyEvidence-BasedUpdated 2026-07

Clinical Notes

Formula

OI = (FiO₂ × Mean Airway Pressure × 100) ÷ PaO₂

Clinical Notes

A higher Oxygen Index indicates more severe oxygenation failure and is commonly used in neonatal and pediatric intensive care.

References

AAP Neonatal Guidelines.
ATS Mechanical Ventilation Guidance.

Calculator Information

Specialty

Pulmonology

📋 Reference Ranges

Reference ranges for this calculator
Classification	Range
Mild	<5
Moderate	5–15
Severe	16–25
Very Severe	>25

What This Calculator Measures

Calculates the Oxygen Index (OI), combining FiO₂, mean airway pressure, and PaO₂ to gauge the severity of oxygenation failure in mechanically ventilated patients.

How to Use

Use only in mechanically ventilated patients.
Enter FiO₂ as a fraction, the mean airway pressure (MAP) measured directly from the ventilator, and PaO₂ from a concurrent arterial blood gas.
Interpret the result against the calculator's severity bands.

Interpretation Guide

The Oxygen Index reflects the amount of oxygen and ventilatory pressure required to achieve a given PaO₂. In this calculator, OI <5 is reported as mild oxygenation impairment, 5–15 moderate, 16–25 severe, and >25 very severe. Note that values <5 are assigned a 'normal' status by the calculator even though the interpretation describes mild impairment; this reflects the tool's banding and does not mean oxygenation is fully normal.

Clinical Significance

The Oxygen Index integrates the intensity of oxygen and pressure support with the resulting arterial oxygenation, making it a widely used severity metric for hypoxemic respiratory failure in critical care, including pediatric and neonatal intensive care.

When to Use

Severity assessment of oxygenation failure in mechanically ventilated patients
Monitoring the intensity of respiratory support in critical care, including pediatric and neonatal settings
Serial tracking of response to ventilatory management

When Not to Use

Non-ventilated patients
Without a directly measured mean airway pressure
In isolation to guide withdrawal of respiratory support

Limitations

The severity bands used by this calculator are pragmatic thresholds; the Oxygen Index does not have a single universally standardized classification.
Results depend on ventilator settings and the timing of PaO₂, so serial measurements are more informative than a single value.
Mean airway pressure must be measured from the ventilator; estimated values will bias the result.
Requires an arterial blood gas; SpO₂-based surrogates are not used by this calculator.

Worked Example

A ventilated patient with ARDS has FiO₂ 0.7, a mean airway pressure of 18 cmH₂O, and a PaO₂ of 70 mmHg.

Inputs:

fio2: 0.7

map: 18

pao2: 70

OI = (0.7 × 18 × 100) / 70 ≈ 18 — severe oxygenation impairment.

Evidence

Source: Pediatric Acute Lung Injury Consensus Conference (PALICC)
Reference: Pediatric Acute Lung Injury Consensus Conference Group. Pediatric Acute Respiratory Distress Syndrome: Consensus Recommendations From the Pediatric Acute Lung Injury Consensus Conference. Pediatr Crit Care Med. 2015;16(5):428-439.
Reviewed By: MedCalcHub Clinical Team
Version: 2015
Updated: 2026-08

Pediatric Acute Lung Injury Consensus Conference Group. Pediatr Crit Care Med. 2015;16(5):428-439. doi:10.1097/PCC.0000000000000350.

References

Pediatric Acute Lung Injury Consensus Conference Group. Pediatric Acute Respiratory Distress Syndrome: Consensus Recommendations From the Pediatric Acute Lung Injury Consensus Conference. Pediatr Crit Care Med. 2015;16(5):428-439. (Consensus Recommendation)

Disclaimer: This calculator is intended for educational and clinical decision support. The Oxygen Index is one component of oxygenation assessment in ventilated patients and must be interpreted with the full clinical context.