What is the PaO2 (partial pressure of oxygen)/FiO2 (fraction of inspired oxygen) ratio with a PaO2 of 85 on 70% FiO2?

P/F Ratio Calculation for PO2 85 on 70% FiO2

The P/F ratio for a PaO2 of 85 mmHg on 70% FiO2 is 121.4, which indicates moderate acute respiratory distress syndrome (ARDS) according to the Berlin definition. 1

Calculation Method

• The P/F ratio is calculated by dividing the partial pressure of arterial oxygen (PaO2) by the fraction of inspired oxygen (FiO2) 1
• For this patient: P/F ratio = 85 mmHg ÷ 0.70 = 121.4 mmHg 1

Clinical Significance of This P/F Ratio

• A P/F ratio of 121.4 falls within the moderate ARDS category (101-200 mmHg) according to the Berlin definition 1
• This level of hypoxemia indicates significant impairment in gas exchange that requires careful clinical attention 2
• Patients with P/F ratios in this range have higher mortality risk, longer ICU stays, and increased likelihood of requiring mechanical ventilation 1

Management Implications Based on P/F Ratio

• Consider high-flow nasal oxygen or non-invasive ventilation if no contraindications exist 1
• If using non-invasive ventilation, monitor closely for signs of failure requiring intubation 1
• If intubation is required, implement lung-protective ventilation strategies with low tidal volumes (4-8 mL/kg predicted body weight) 1
• Consider higher PEEP strategy based on the severity of hypoxemia 1

Important Considerations When Interpreting P/F Ratio

• The P/F ratio varies with changes in FiO2, which can affect disease classification and management decisions 3
• Studies have shown that up to 30% of patients may change ARDS classification when measured at different FiO2 levels 3
• The P/F ratio should ideally be calculated under standardized ventilator settings for more accurate assessment of lung injury severity 4
• Higher FiO2 levels (>0.7) can significantly increase the P/F ratio, potentially leading to underestimation of disease severity 5

Pitfalls to Avoid

• Do not rely solely on P/F ratio for clinical decisions; integrate with clinical assessment and other parameters 1
• Remember that P/F ratio may be affected by factors other than lung pathology, such as cardiac output and hemoglobin concentration 1
• The relationship between PaO2 and SaO2 follows a non-linear pattern (oxygen-hemoglobin dissociation curve), meaning that changes in PaO2 do not correlate linearly with changes in oxygen saturation 6
• When calculating P/F ratio for ARDS classification, some studies suggest using a standardized FiO2 of 1.0 for the most accurate classification 7