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Calculates the Oxygen Index to assess severity of hypoxemic respiratory failure in mechanically ventilated patients.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
A higher Oxygen Index indicates more severe oxygenation failure and is commonly used in neonatal and pediatric intensive care.
Specialty
PulmonologyCategory
PulmonologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-07
Formula
Keywords
| Classification | Range |
|---|---|
Mild | <5 |
Moderate | 5–15 |
Severe | 16–25 |
Very Severe | >25 |
Calculates the Oxygen Index (OI), combining FiO₂, mean airway pressure, and PaO₂ to gauge the severity of oxygenation failure in mechanically ventilated patients.
Oxygen index = (FiO2 x mean airway pressure x 100) / PaO2. FiO2 is entered as a fraction of inspired oxygen, mean airway pressure in cmH2O, and PaO2 in mmHg; higher values indicate more severe oxygenation failure.
Fraction of inspired oxygen as a decimal (for example 0.21 for room air and 1.0 for 100% oxygen), multiplied by mean airway pressure and 100.
Common error: Entering FiO2 as a percentage (for example 60) instead of a fraction (0.6) grossly overstates the index.
Mean airway pressure in cmH2O, representing the average pressure delivered by the ventilator.
Arterial partial pressure of oxygen in mmHg, used as the denominator.
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.
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.
A ventilated patient with ARDS has FiO₂ 0.7, a mean airway pressure of 18 cmH₂O, and a PaO₂ of 70 mmHg.
Inputs:
Expected Result: OI = (0.7 × 18 × 100) / 70 ≈ 18 — severe oxygenation impairment.
The Oxygen Index is intended for mechanically ventilated patients, because the formula multiplies FiO2 by mean airway pressure, which must be read directly from the ventilator. It should not be used in patients breathing without ventilatory support or when mean airway pressure has to be estimated. The calculation also requires an arterial PaO2 from an arterial blood gas, since oxygen saturation surrogates are not accepted, and results are interpreted with the calculator's severity bands.
The value depends on the current ventilator settings and on when the arterial blood gas was drawn, so one reading describes a single moment rather than the overall course, and serial measurements are more informative than an isolated number. The calculator's bands, below 5, 5 to 15, 16 to 25, and above 25, are pragmatic tool conventions because no single universally standardized classification exists for the Oxygen Index.
The tool assigns a normal status to values below 5 as part of its banding scheme, even though its own interpretation text describes that range as mild oxygenation impairment. The discrepancy reflects how the categories are drawn in this calculator and does not mean oxygenation is fully normal. Since the Oxygen Index has no universally standardized classification, the banding is a pragmatic guide that should be read with ventilator settings, the arterial blood gas, and the patient's clinical trajectory.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The Oxygen Index is one component of oxygenation assessment in ventilated patients and must be interpreted with the full clinical context.