PF Ratio Calculation
The PaO₂/FiO₂ ratio is 325 mmHg (325 ÷ 1.0 = 325).
Calculation Method
The PaO₂/FiO₂ ratio (P/F ratio) is calculated by dividing the arterial partial pressure of oxygen (PaO₂) by the fraction of inspired oxygen (FiO₂) 1, 2, 1:
- PaO₂ = 325 mmHg
- FiO₂ = 100% = 1.0 (expressed as a decimal)
- P/F ratio = 325 ÷ 1.0 = 325 mmHg
Clinical Interpretation
This P/F ratio of 325 mmHg indicates mild hypoxemia but does NOT meet criteria for ARDS. According to the Berlin Definition, ARDS requires a PaO₂/FiO₂ ratio ≤300 mmHg 1. Your patient falls just above this threshold.
ARDS Classification Framework (for reference):
- Mild ARDS: 200 < PaO₂/FiO₂ ≤ 300 mmHg
- Moderate ARDS: 100 < PaO₂/FiO₂ ≤ 200 mmHg
- Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg 1, 3
Critical Caveats
The P/F ratio varies significantly with FiO₂ level, which is a major limitation of this metric 4. When measured at FiO₂ 100%, the ratio may underestimate the true severity of gas exchange abnormality compared to measurements at lower FiO₂ levels. Studies demonstrate that 30% of patients change disease classification when FiO₂ is altered 4.
Always report the FiO₂ level when documenting P/F ratios to allow meaningful comparison over time and between patients 4. The P/F ratio measured at FiO₂ 1.0 cannot be directly compared to one measured at FiO₂ 0.4, even in the same patient.
Additional Considerations:
- The Berlin Definition requires minimum PEEP of 5 cmH₂O for ARDS diagnosis 1
- Consider incorporating PEEP into severity assessment (PaO₂/FiO₂×PEEP ratio) for more physiologically meaningful prognostication 5
- SpO₂/FiO₂ ratio of 235 correlates with P/F ratio of 200 and can substitute when arterial blood gas is unavailable 6