This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Measuring how well the lungs oxygenate
The PaO2/FiO2 ratio, or P/F ratio, compares the oxygen pressure in arterial blood with the oxygen concentration the patient breathes. Because it takes the inspired oxygen into account, it lets doctors compare oxygenation across different settings, from room air to a ventilator, and it is central to the diagnosis of acute respiratory distress syndrome (ARDS).
The formula
- P/F ratio = PaO2 (mmHg) ÷ FiO2 (as a fraction)
- Estimated FiO2 on nasal cannula ≈ 21% + 4% per liter per minute
- Oxygenation index = FiO2 × mean airway pressure × 100 ÷ PaO2
A healthy person breathing room air has a PaO2 of about 90 to 100 mmHg, so a P/F ratio of about 430 to 480.
Berlin definition of ARDS
ARDS requires an onset within a week of a known insult, bilateral opacities on imaging not fully explained by fluid overload or collapse, and a positive end-expiratory pressure (PEEP) of at least 5 cmH2O. Severity is then set by the P/F ratio:
| P/F ratio (mmHg) | Severity | Mortality in the Berlin study |
|---|---|---|
| 200 – 300 | Mild | about 27% |
| 100 – 200 | Moderate | about 32% |
| ≤ 100 | Severe | about 45% |
Example
A patient with a PaO2 of 75 mmHg on 40% oxygen:
- P/F ratio: 75 ÷ 0.40 = 188 mmHg, or 25 kPa
- That is in the moderate ARDS range if the other Berlin criteria are met
Practical points
- The ratio only makes sense with a known FiO2. On nasal cannula or a simple mask, the true FiO2 is uncertain, so treat the result as an estimate.
- The ratio changes with PEEP and ventilator settings, which is why Berlin requires a minimum PEEP.
- When no blood gas is available, the SpO2/FiO2 ratio gives an approximation: an S/F ratio of about 315 corresponds to a P/F of 300, as long as SpO2 is 97% or lower.
- In children, the oxygenation index is preferred because it accounts for the pressure needed to achieve oxygenation; values of 16 or more define severe pediatric ARDS.
- At altitude, PaO2 is lower, so P/F thresholds are sometimes adjusted by multiplying by barometric pressure ÷ 760.