How is the paO2 (partial pressure of oxygen in arterial blood)/FiO2 (fraction of inspired oxygen) ratio calculated for an Acute Respiratory Distress Syndrome (ARDS) patient?

Calculating the PaO2/FiO2 Ratio in ARDS

The PaO2/FiO2 ratio is calculated by dividing the partial pressure of arterial oxygen (PaO2 in mmHg) by the fraction of inspired oxygen (FiO2 expressed as a decimal), yielding a value in mmHg that classifies ARDS severity: mild (201-300), moderate (101-200), or severe (≤100). 1

Basic Calculation Formula

PaO2/FiO2 = PaO2 (mmHg) / FiO2 (decimal)

• Obtain PaO2 from arterial blood gas analysis 1
• Express FiO2 as a decimal (e.g., 40% oxygen = 0.40,100% oxygen = 1.0) 1
• For example: if PaO2 = 80 mmHg and FiO2 = 0.40, then PaO2/FiO2 = 80/0.40 = 200 mmHg 1

Timing and Standardization for Accurate Assessment

The most clinically relevant PaO2/FiO2 ratio should be measured 24 hours after ARDS onset using standardized ventilator settings (PEEP ≥10 cmH2O and FiO2 ≥0.5), as this provides superior mortality prediction compared to initial measurements. 2, 3

• The initial PaO2/FiO2 at ARDS diagnosis (Day 0) has limited prognostic accuracy 3
• Measuring at 24 hours (Day 1) with standardized settings significantly improves risk stratification, with area under the curve for mortality prediction of 0.64 versus 0.58 for Day 0 measurements 2, 3
• Up to 56% of patients change ARDS severity classification between Day 0 and Day 1, and 5% no longer meet ARDS criteria 3

ARDS Severity Classification

Based on the PaO2/FiO2 ratio measured under standardized conditions:

• Mild ARDS: 201-300 mmHg (mortality ~17%) 1, 2
• Moderate ARDS: 101-200 mmHg (mortality ~41%) 1, 2
• Severe ARDS: ≤100 mmHg (mortality ~58%) 1, 2

These thresholds are used for ECMO consideration, with severe ARDS (PaO2/FiO2 <150 within 7 days of onset, or <70 for ≥3 hours, or <100 for ≥6 hours) meeting criteria for venovenous ECMO evaluation 1

Critical Pitfalls and Limitations

The PaO2/FiO2 ratio is significantly influenced by PEEP level and FiO2, which can lead to misclassification of ARDS severity if not standardized. 4, 2

• For the same PaO2/FiO2 value, patients on higher PEEP have more severe lung injury than those on lower PEEP 4
• The ratio decreases as FiO2 increases even with the same shunt fraction, potentially overestimating severity 5
• When PEEP >5 cmH2O, incorporating PEEP into a modified P/FP ratio [(PaO2×10)/(FiO2×PEEP)] improves mortality prediction (AUC 0.710 vs 0.659) 4
• Using FiO2 of 1.0 for calculation provides the best match with shunt-based ARDS classification in COVID-19 pneumonia 5

The PaO2/FiO2 ratio alone has modest predictive accuracy for mortality (AUROC 0.60-0.64) and should not be used as the sole prognostic tool. 6

• At a cutoff of 100 mmHg: sensitivity 44.8%, specificity 70.6% 6
• At a cutoff of 200 mmHg: sensitivity 83.9%, specificity 26.1% 6
• Additional factors including age, number of organ failures, and positive fluid balance within 5 days significantly impact prognosis 3

Practical Application Algorithm

1. Obtain arterial blood gas while patient is on mechanical ventilation 1
2. Record concurrent ventilator settings: FiO2 and PEEP 2
3. Calculate initial ratio at ARDS diagnosis for classification 1
4. Remeasure at 24 hours with PEEP ≥10 cmH2O and FiO2 ≥0.5 for prognostic assessment 2
5. Reclassify severity based on Day 1 measurements to guide treatment intensity 2, 3
6. Consider P/FP ratio [(PaO2×10)/(FiO2×PEEP)] when PEEP >5 cmH2O for improved mortality prediction 4