How do you perform the partial pressure of oxygen (Pao2) to fraction of inspired oxygen (Fio2) test?

How to Perform the PaO2/FiO2 Ratio Test

The PaO2/FiO2 ratio is calculated by dividing the partial pressure of arterial oxygen (PaO2) measured from an arterial blood gas by the fraction of inspired oxygen (FiO2) the patient is receiving, with both measurements taken simultaneously. 1

Step-by-Step Procedure

1. Document the Current FiO2

• Record the exact FiO2 the patient is receiving at the time of blood gas sampling 1
• For patients on supplemental oxygen via Venturi mask, note the percentage (e.g., 28%, 35%, 40%) 1
• For nasal cannula, estimate FiO2 (approximately 24% at 1 L/min, 28% at 2 L/min, though this is less precise) 1
• For mechanically ventilated patients, record the FiO2 setting on the ventilator 1
• For room air, FiO2 = 0.21 (21%) 1

2. Allow Adequate Equilibration Time

• Wait 5-10 minutes after any change in FiO2 before obtaining the arterial blood gas 2, 3
• In patients with COPD, allow up to 15 minutes for equilibration, as their t90% (time to reach 90% of final PaO2) averages 7.1 minutes compared to 4.4 minutes in non-COPD patients 3
• After a 0.2 increase in FiO2, 90% of patients reach equilibrium within 15 minutes 3
• If the patient's clinical status deteriorates, measure blood gases immediately without waiting 1

3. Obtain Arterial Blood Gas Sample

• Perform arterial puncture (typically radial artery) or draw from existing arterial line 1
• Ensure proper technique with heparinized syringe to prevent clotting 1
• Process sample immediately or place on ice if delay anticipated 1

4. Calculate the Ratio

• Divide PaO2 (in mmHg) by FiO2 (expressed as a decimal) 1
• Example: If PaO2 = 80 mmHg and FiO2 = 0.40 (40%), then PaO2/FiO2 = 80/0.40 = 200 mmHg 1
• For high altitude locations (>1000 m above sea level), apply correction: PaO2/FiO2 × [760/atmospheric pressure in mmHg] 1

Critical Timing Considerations

When to Recheck Blood Gases

• Within 60 minutes of starting oxygen therapy 1
• Within 60 minutes of any change in inspired oxygen concentration 1
• Within 1 hour (or sooner if conscious level deteriorates) after any increase in FiO2 4
• Immediately if clinical deterioration occurs 1

Important Caveats About FiO2 Selection

The FiO2 Dependency Problem

• The PaO2/FiO2 ratio is NOT constant across different FiO2 levels—it decreases as FiO2 increases, even with the same underlying shunt fraction 5, 6
• This variation can cause 30% of patients to change disease classification (e.g., from mild to moderate ARDS) simply due to the FiO2 at which the ratio was measured 5
• Research suggests that FiO2 of 1.0 provides the most accurate classification of ARDS severity when compared to shunt-based calculations 6
• However, in clinical practice, the ratio is typically measured at whatever FiO2 the patient requires 1

Standardization Requirements

• Always document the FiO2 at which the PaO2/FiO2 ratio was measured when reporting results or comparing serial measurements 5
• The ratio is most stable and clinically useful at FiO2 levels >0.3 and PaO2 levels <100 mmHg 7
• Avoid comparing PaO2/FiO2 ratios obtained at different FiO2 levels without acknowledging this limitation 5

Clinical Interpretation

ARDS Severity Classification (Berlin Definition)

• Mild ARDS: PaO2/FiO2 200-300 mmHg 1
• Moderate ARDS: PaO2/FiO2 100-200 mmHg 1
• Severe ARDS: PaO2/FiO2 <100 mmHg 1
• These thresholds apply with PEEP ≥5 cm H2O 1

COVID-19 Specific Criteria

• Severe COVID-19 in adults: PaO2/FiO2 ≤300 mmHg 1
• This criterion is used alongside respiratory rate ≥30 breaths/min and oxygen saturation ≤93% on room air 1

Pediatric Considerations

• Children requiring FiO2 ≥0.50 to maintain SpO2 >92% should be admitted to ICU or monitored unit 8
• Use PEEP-to-FiO2 grids for pediatric ARDS management 8

Common Pitfalls to Avoid

• Never compare PaO2/FiO2 ratios obtained at different FiO2 levels without accounting for the inherent variation 5
• Do not obtain blood gas before adequate equilibration time has elapsed after FiO2 changes 2, 3
• Avoid using estimated FiO2 from nasal cannula when precise measurements are needed for ARDS classification 1
• In COPD patients, do not give FiO2 >28% via Venturi mask until arterial blood gases are known, as this can precipitate hypercapnic respiratory failure 1, 4
• Never leave patients on unnecessarily high FiO2 "to be safe"—titrate down to target saturations to avoid hyperoxia-related harm 4, 8