What is the meaning of the P/F (PaO2/FiO2) ratio?

P/F Ratio: Definition and Clinical Meaning

The P/F ratio (PaO2/FiO2 ratio) is calculated by dividing the partial pressure of arterial oxygen (PaO2) by the fraction of inspired oxygen (FiO2), providing a standardized measure of oxygenation efficiency that is critical for assessing hypoxemia severity and guiding respiratory management. 1

Core Definition

• The P/F ratio represents how efficiently the lungs transfer oxygen from inspired air into arterial blood 1
• It is calculated as: PaO2 (in mmHg) ÷ FiO2 (as a decimal) 2
• For example, if PaO2 is 80 mmHg on 40% oxygen (FiO2 0.40), the P/F ratio = 80 ÷ 0.40 = 200 mmHg 1
• Normal P/F ratio is approximately 400-500 mmHg, indicating healthy gas exchange 1

Clinical Significance and Severity Classification

The Berlin definition uses P/F ratio thresholds to classify acute respiratory distress syndrome (ARDS) severity: 1

• Mild ARDS: P/F ratio 201-300 mmHg 1
• Moderate ARDS: P/F ratio 101-200 mmHg 1
• Severe ARDS: P/F ratio ≤100 mmHg 1

Values below 300 mmHg indicate acute lung injury requiring escalating interventions from supplemental oxygen to mechanical ventilation with lung-protective strategies 1. A low P/F ratio correlates with increased mortality, need for mechanical ventilation, and longer ICU stays 1.

Prognostic Value

• The change in P/F ratio following initial treatment of ARDS can discriminate between survivors and nonsurvivors 2
• Patients who demonstrate improvement in P/F ratio within the first 24 hours have better outcomes than those who do not 2
• However, the P/F ratio alone has limited predictive accuracy for mortality (AUROC 0.60-0.64) and should not be used as the sole prognostic tool 3
• Reclassifying P/F ratio 24 hours after initial management provides better prognostic information than the initial value 1

Important Limitations and Pitfalls

Do not rely solely on P/F ratio for clinical decisions; it must be integrated with clinical assessment and other parameters: 1

• The P/F ratio varies substantially with FiO2, particularly when shunt fraction (QS/QT) is 0.1-0.3 and FiO2 is greater than 0.4 4
• Hemoglobin concentration and arterial-venous oxygen content difference have large effects that can confound interpretation 4
• Barometric pressure has substantial effects on the ratio 4
• For the same P/F ratio, a patient on higher positive end-expiratory pressure (PEEP) may have more severe lung injury than one on lower PEEP 5
• The P/F ratio may be affected by factors other than lung pathology, including cardiac output and hemoglobin concentration 1

Measurement Considerations

• The P/F ratio is most reliable when measured at FiO2 ≥0.5 and PEEP ≥10 cmH2O at 24 hours after ARDS onset 6
• Consider arterial blood gas analysis rather than pulse oximetry alone for accurate P/F ratio calculation in severe cases 1
• Indices of oxygenation, such as the P/F ratio, are unreliable predictors of weaning outcome except in patients with marked hypoxemia (PaO2 <55 mmHg at FiO2 ≥0.40) 2

Clinical Application in Respiratory Management

For severe hypoxemia (P/F ratio <150 mmHg), preoxygenation with non-invasive positive pressure ventilation (NIPPV) is suggested before intubation: 2

• In patients with P/F ratio <200 mmHg, desaturation rates during intubation are 35% with high-flow nasal oxygen versus 24% with NIPPV 2
• Patients typically have at least moderate acute respiratory failure when baseline P/F ratio is <200 mmHg 2
• Post-treatment P/F ratio values are generally higher with NIPPV compared to high-flow nasal oxygen in post-extubation management 2