What is the PaO2/FiO2 (Pulse Oxygen/Fraction of Inspired Oxygen) ratio and how is it calculated in patients with Acute Respiratory Distress Syndrome (ARDS)?

What is the P/F Ratio and How to Calculate It

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 how efficiently your lungs transfer oxygen into the blood. 1

Simple Calculation Steps

Step 1: Obtain PaO2 Value

• Get an arterial blood gas (ABG) measurement to determine the PaO2 (partial pressure of oxygen in arterial blood) 1
• This value is measured in millimeters of mercury (mmHg)
• Example: PaO2 = 80 mmHg

Step 2: Determine FiO2 Value

• Identify the fraction of inspired oxygen the patient is receiving 1
• Room air = 0.21 (21% oxygen)
• Oxygen via nasal cannula at 2L/min ≈ 0.28
• Oxygen via face mask at 10L/min ≈ 0.50
• Mechanical ventilation: read directly from ventilator settings
• Example: FiO2 = 0.40 (40% oxygen)

Step 3: Divide PaO2 by FiO2

• Use the formula: P/F Ratio = PaO2 ÷ FiO2 1
• Example: 80 mmHg ÷ 0.40 = 200 mmHg
• The result is expressed in mmHg

Clinical Interpretation

Normal Values

• A healthy person breathing room air (FiO2 = 0.21) with PaO2 = 100 mmHg has a P/F ratio of approximately 476 mmHg 1
• Normal P/F ratio is typically >400 mmHg

ARDS Severity Classification

• Mild ARDS: P/F ratio 201-300 mmHg (measured with PEEP ≥5 cmH2O) 1, 2
• Moderate ARDS: P/F ratio 101-200 mmHg (measured with PEEP ≥5 cmH2O) 1, 2
• Severe ARDS: P/F ratio ≤100 mmHg (measured with PEEP ≥5 cmH2O) 1, 2

Critical Measurement Considerations

Standardized Ventilatory Settings

• For accurate ARDS classification, measure P/F ratio at 24 hours after ARDS onset with PEEP ≥10 cmH2O and FiO2 ≥0.5, as this provides superior prognostic information compared to baseline measurements 3, 4
• The 24-hour reclassification under standardized settings greatly improves risk stratification, with mortality rates of 17% for mild, 40.9% for moderate, and 58.1% for severe ARDS 3

Common Pitfalls to Avoid

• Do not calculate P/F ratio using pulse oximetry estimates alone in severe cases—arterial blood gas analysis is required for accurate assessment 1
• Baseline P/F ratio at ARDS onset has poor predictive accuracy; approximately 61.3% of patients initially classified as severe ARDS are reclassified to less severe categories when reassessed at 24 hours under standardized settings 4
• The P/F ratio can be affected by factors other than lung pathology, including cardiac output, hemoglobin concentration, and applied PEEP levels 1, 5

Clinical Decision Thresholds

P/F Ratio <150 mmHg

• Consider prone positioning for ≥12 hours daily, as this threshold identifies patients most likely to benefit from this mortality-reducing intervention 6, 2
• Evaluate for neuromuscular blocking agents if signs of injurious respiratory effort persist despite optimized ventilator settings 6, 2
• Monitor for right ventricular dysfunction, which is more common at this threshold 6, 1

P/F Ratio <100 mmHg

• Implement prone positioning as a strong recommendation for >12 hours per day 6, 2
• Initiate ECMO evaluation when P/F ratio remains <100 mmHg despite lung-protective ventilation, prone positioning, and neuromuscular blockade 6, 2

P/F Ratio >200 mmHg

• Consider weaning from mechanical ventilation when P/F ratio exceeds 200 mmHg and PEEP is <10 cmH2O 2