Enter the fraction of inspired oxygen, mean airway pressure, and arterial oxygen tension into the calculator to determine the Oxygenation Index.
Educational use only; not medical advice. Oxygenation Index interpretation and ventilator/oxygen management should be performed by qualified clinicians using local protocols and the full clinical context.
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Oxygenation Index Formula
The oxygenation index (OI) is a clinical measure used to describe how much oxygen and ventilatory support is required to produce a given arterial oxygen level. It is most useful in patients receiving respiratory support because it combines three important inputs: FiO2, mean airway pressure (Paw), and arterial oxygen tension (PaO2).
OI = \frac{FiO_2 \times Paw \times 100}{PaO_2}For a manual calculation, use FiO2 as a fraction, Paw in cmH2O, and PaO2 in mmHg. If your values are entered as a percent or in kPa, convert them first or let the calculator perform the conversion.
FiO_2\;(\text{fraction}) = \frac{FiO_2\;(\%)}{100}| Input | Meaning | Typical manual-calculation unit |
|---|---|---|
| FiO2 | Fraction of inspired oxygen | Fraction from 0 to 1 |
| Paw | Mean airway pressure used during ventilation | cmH2O |
| PaO2 | Arterial oxygen tension from an arterial blood gas | mmHg |
| OI | Oxygenation Index | Index value |
Important: some references use the abbreviation MAP for this formula, but here it means mean airway pressure, not mean arterial pressure.
How to Calculate the Oxygenation Index
- Determine the patient’s FiO2. If it is written as a percent, convert it to a fraction.
- Record the mean airway pressure (Paw) from the ventilator settings.
- Obtain PaO2 from an arterial blood gas measurement.
- Insert the values into the formula above.
- Interpret the result in clinical context and, when possible, compare it with prior values to evaluate the trend.
Example
If FiO2 is 60%, Paw is 12 cmH2O, and PaO2 is 80 mmHg, first convert FiO2 to 0.60 and then calculate:
OI = \frac{0.60 \times 12 \times 100}{80} = 9An oxygenation index of 9 means the patient requires a moderate combination of inspired oxygen and airway pressure to achieve a PaO2 of 80 mmHg. In general, a higher OI reflects worse oxygenation efficiency and/or greater respiratory support requirements.
How to Interpret the Result
- Lower OI values generally indicate better oxygen transfer with less support.
- Higher OI values generally indicate worsening oxygenation or the need for more aggressive ventilatory support.
- Trend matters more than a single isolated number; a rising OI is usually more concerning than a stable or improving value.
- Population-specific thresholds vary by age group, diagnosis, and local protocol, so OI should not be interpreted without clinical context.
- OI is not a diagnosis by itself; it is one data point alongside examination findings, imaging, gas exchange, and ventilator settings.
Why OI Is Often More Useful Than a Simple Oxygenation Ratio
The oxygenation index is helpful because it includes mean airway pressure. Two patients can have the same FiO2 and PaO2, but if one patient requires a much higher airway pressure to maintain that PaO2, that patient is generally more difficult to oxygenate. OI captures that difference, while simpler oxygenation comparisons do not.
Common Input Mistakes
- Entering 60 instead of 0.60 when the calculator expects FiO2 as a fraction.
- Using SpO2 from pulse oximetry instead of PaO2 from an arterial blood gas.
- Confusing mean airway pressure with mean arterial pressure.
- Mixing kPa, mmHg, and cmH2O during hand calculations.
- Comparing values taken at very different times or under very different ventilator settings without noting the change in support.
Quick FiO2 Conversion Reference
| FiO2 (%) | FiO2 (fraction) |
|---|---|
| 21% | 0.21 |
| 30% | 0.30 |
| 40% | 0.40 |
| 50% | 0.50 |
| 60% | 0.60 |
| 80% | 0.80 |
| 100% | 1.00 |
Pressure Conversion Notes for Manual Calculation
If you are not using the calculator’s built-in unit handling, convert values to the standard manual-calculation units before using the formula.
| Conversion | Approximate value |
|---|---|
| 1 kPa to mmHg | 7.50 mmHg |
| 1 kPa to cmH2O | 10.2 cmH2O |
When This Calculator Is Useful
- Reviewing oxygenation support requirements in mechanically ventilated patients.
- Tracking whether oxygenation is improving, worsening, or unchanged over time.
- Comparing severity of respiratory impairment when airway pressure is an important part of the picture.
- Learning how FiO2, Paw, and PaO2 interact in respiratory care calculations.
This calculator is best used for education, estimation, and trend review. Clinical interpretation and treatment decisions should always be made by qualified healthcare professionals using the full patient context.
