Understanding the PaO2/FiO2 Ratio
The PaO2/FiO2 ratio (P/F ratio) is a measurement that evaluates the efficiency of gas exchange in the lungs by dividing the partial pressure of oxygen in arterial blood (PaO2) by the fraction of inspired oxygen (FiO2).
What the P/F Ratio Measures
The P/F ratio provides a standardized assessment of lung function by measuring how effectively oxygen moves from the alveoli into the bloodstream. This ratio:
- Indicates the severity of pulmonary shunt (blood passing through the lungs without being oxygenated)
- Helps classify the severity of respiratory failure, particularly Acute Respiratory Distress Syndrome (ARDS)
- Provides a way to monitor changes in lung function over time
Normal Values and Clinical Significance
A normal P/F ratio in healthy individuals is typically:
- 400-500 mmHg when breathing room air (FiO2 = 0.21)
According to the American Thoracic Society guidelines, ARDS severity is classified based on P/F ratio 1:
| ARDS Severity | P/F Ratio |
|---|---|
| Mild | 201-300 mmHg |
| Moderate | 101-200 mmHg |
| Severe | ≤100 mmHg |
Clinical Applications
The P/F ratio is particularly useful for:
- Diagnosing ARDS: A P/F ratio ≤300 mmHg is one of the criteria for ARDS diagnosis
- Stratifying disease severity: Helps determine appropriate ventilation strategies and interventions
- Monitoring response to treatment: Changes in P/F ratio can indicate improvement or worsening of respiratory function
- Predicting outcomes: Lower P/F ratios, especially when measured 24 hours after ARDS onset, correlate with higher mortality 2
Calculation Example
The formula is simple:
P/F ratio = PaO2 (mmHg) ÷ FiO2 (decimal)For example:
- If PaO2 = 80 mmHg and FiO2 = 0.4 (40% oxygen)
- P/F ratio = 80 ÷ 0.4 = 200 mmHg (indicating moderate ARDS)
Important Considerations
Standardized ventilator settings: P/F ratios should ideally be calculated after 24 hours of standardized ventilator settings for more accurate assessment of lung injury severity 3
Altitude effects: At higher altitudes, P/F ratios will be lower for the same degree of lung injury due to lower atmospheric pressure 4
Alternative measurement: When arterial blood gas analysis is unavailable, the SpO2/FiO2 ratio can be used as a substitute (SpO2 ≤97%). An S/F ratio of 235 corresponds to a P/F ratio of 200, while an S/F ratio of 315 corresponds to a P/F ratio of 300 5
FiO2 influence: The same shunt fraction can produce different P/F ratios at different FiO2 levels, which may affect ARDS classification 6
The P/F ratio is a valuable tool for clinicians to assess respiratory function, guide treatment decisions, and monitor patient progress in respiratory failure.