What is the PaO2/FiO2 Ratio?
The PaO2/FiO2 ratio (also called the P/F ratio) is a calculated measure of how efficiently your lungs transfer oxygen from inspired air into arterial blood, computed by dividing the partial pressure of arterial oxygen (PaO2 in mmHg) by the fraction of inspired oxygen (FiO2 as a decimal), with normal values ranging from 400-500 mmHg. 1
Calculation Method
- Obtain PaO2 from an arterial blood gas measurement (expressed in mmHg) 2
- Express FiO2 as a decimal fraction (for example, 40% oxygen = 0.40, room air = 0.21,100% oxygen = 1.0) 2, 3
- Apply the formula: PaO2 (mmHg) ÷ FiO2 (decimal) = P/F ratio (mmHg) 1
Practical Example
If a patient has a PaO2 of 80 mmHg while receiving 40% oxygen (FiO2 0.40), the calculation is: 80 ÷ 0.40 = 200 mmHg 1, 2
Clinical Significance and ARDS Classification
The P/F ratio is the cornerstone of the Berlin definition for classifying acute respiratory distress syndrome (ARDS) severity, which directly correlates with mortality risk: 1, 2
- Mild ARDS: P/F ratio 201-300 mmHg (mortality approximately 17%) 2
- Moderate ARDS: P/F ratio 101-200 mmHg (mortality approximately 41%) 2
- Severe ARDS: P/F ratio ≤100 mmHg (mortality approximately 58%) 2
Any P/F ratio below 300 mmHg indicates acute lung injury requiring escalating interventions from supplemental oxygen to mechanical ventilation with lung-protective strategies. 1
Prognostic Value and Serial Monitoring
The change in P/F ratio following initial treatment discriminates between survivors and nonsurvivors better than a single measurement, with patients demonstrating improvement within the first 24 hours having significantly better outcomes. 1
- Reclassifying the P/F ratio at 24 hours after initial management provides superior prognostic information compared to the baseline value alone 1
- For severe hypoxemia with P/F ratio <150 mmHg, preoxygenation with non-invasive positive pressure ventilation is recommended before intubation 1
- Severe ARDS meeting criteria of P/F <150 within 7 days of onset, or <70 for ≥3 hours, or <100 for ≥6 hours warrants evaluation for venovenous ECMO 2
Important Clinical Pitfalls and Limitations
The P/F ratio is significantly affected by the FiO2 at which it is measured—at the same shunt fraction, the P/F ratio decreases as FiO2 increases, particularly problematic with shunt fractions of 10-30% when FiO2 exceeds 0.4. 4
- Hemoglobin concentration and arterial-venous oxygen content difference have large confounding effects on P/F ratio interpretation 4
- In patients with physiologic shunt exceeding 30% of cardiac output, incremental increases in FiO2 produce diminishing improvements in arterial oxygenation, making P/F ratio more stable and reliable 3, 4
- Barometric pressure has a substantial effect on P/F ratio calculations; PCO2, base excess, and respiratory quotient have smaller but measurable effects 4
- Assessment under standardized ventilator settings at 24 hours after ARDS onset provides more accurate severity classification than baseline measurements, as 61.3% of patients initially classified as severe ARDS were reclassified to lower severity categories when reassessed under standardized conditions 5
Alternative Non-Invasive Assessment
The SpO2/FiO2 (S/F) ratio can substitute for the P/F ratio when arterial blood gas sampling is not feasible, with an S/F ratio of 235 corresponding to a P/F ratio of 200 (ARDS threshold) and an S/F ratio of 315 corresponding to a P/F ratio of 300 (ALI threshold), achieving 85% sensitivity and 85% specificity for ARDS diagnosis. 6