Pao2 Fio2 Ratio Calculator
Calculate the P/F ratio to classify ARDS severity (mild, moderate, severe). Enter values for instant results with step-by-step formulas.
Formula
P/F Ratio = PaO2 (mmHg) / FiO2 (decimal)
The PaO2/FiO2 ratio divides the arterial oxygen tension by the fraction of inspired oxygen. Berlin Definition ARDS severity: Mild (200-300), Moderate (100-200), Severe (100 or less), all requiring PEEP of at least 5 cmH2O. The A-a gradient is calculated as PAO2 - PaO2, where PAO2 = FiO2(760 - 47) - PaCO2/0.8.
Worked Examples
Example 1: Moderate ARDS Classification
Problem: A patient on mechanical ventilation has PaO2 of 75 mmHg on FiO2 of 50% with PEEP of 8 cmH2O. Classify the ARDS severity.
Solution: PaO2: 75 mmHg\nFiO2: 50% = 0.50\nP/F ratio: 75 / 0.50 = 150\nPEEP: 8 cmH2O (meets minimum 5 cmH2O criterion)\nBerlin Classification: P/F 100-200 = Moderate ARDS\nExpected mortality: approximately 32%\nA-a gradient: PAO2 = 0.50(760-47) - 40/0.8 = 306.5 mmHg\nA-a gradient: 306.5 - 75 = 231.5 mmHg (severely elevated)
Result: P/F Ratio: 150 | Moderate ARDS | Mortality ~32% | A-a Gradient: 231.5 mmHg
Example 2: Monitoring Response to Prone Positioning
Problem: Before proning: PaO2 72 on FiO2 70%. After 4 hours prone: PaO2 145 on FiO2 60%. Calculate improvement.
Solution: Before prone:\nP/F ratio: 72 / 0.70 = 102.9 (Severe ARDS)\n\nAfter 4 hours prone:\nP/F ratio: 145 / 0.60 = 241.7 (Mild ARDS)\n\nImprovement: 241.7 - 102.9 = 138.8 points\nSeverity change: Severe to Mild ARDS\nFiO2 was also reduced from 70% to 60%\nThis response to proning is favorable
Result: P/F improved from 103 (Severe) to 242 (Mild) | +139 points | Positive prone response
Frequently Asked Questions
What is the PaO2/FiO2 ratio and what does it indicate?
The PaO2/FiO2 ratio (also called the P/F ratio) is a measure of oxygenation efficiency calculated by dividing the arterial partial pressure of oxygen (PaO2 in mmHg) by the fraction of inspired oxygen (FiO2 as a decimal). A normal P/F ratio on room air is approximately 400-500. The ratio indicates how well the lungs transfer oxygen from inspired air into the bloodstream. A declining P/F ratio suggests worsening gas exchange, which can result from alveolar collapse, pulmonary edema, pneumonia, or any condition that impairs ventilation-perfusion matching. The P/F ratio is the primary criterion for diagnosing and classifying ARDS severity.
What is a normal PaO2/FiO2 ratio?
A normal P/F ratio for a healthy person breathing room air (FiO2 of 0.21) is approximately 400-500. For example, a PaO2 of 95 mmHg divided by FiO2 of 0.21 equals a P/F ratio of 452. As supplemental oxygen is added, the P/F ratio should remain relatively stable in healthy lungs. If a patient on 50% FiO2 has a PaO2 of 200 mmHg, their P/F ratio is 400, which is still normal. A ratio below 300 while on supplemental oxygen with PEEP of at least 5 cmH2O suggests impaired gas exchange and may meet ARDS criteria if other Berlin Definition conditions are also present.
What is the A-a gradient and how does it complement the P/F ratio?
The alveolar-arterial (A-a) oxygen gradient is the difference between the calculated alveolar oxygen tension (PAO2) and the measured arterial oxygen tension (PaO2). It quantifies the efficiency of oxygen transfer across the alveolar-capillary membrane. A normal A-a gradient is approximately 5-15 mmHg in young adults and increases with age (estimated normal = 2.5 + 0.21 x age). An elevated A-a gradient with a low P/F ratio suggests intrinsic lung disease (pneumonia, ARDS, pulmonary embolism), while a normal A-a gradient with hypoxemia suggests hypoventilation or low inspired oxygen concentration as the cause.
How does PEEP affect the P/F ratio?
Positive End-Expiratory Pressure (PEEP) maintains alveolar recruitment at end-expiration, preventing atelectasis and improving ventilation-perfusion matching. Increasing PEEP can significantly improve PaO2 and therefore the P/F ratio by recruiting collapsed alveoli and increasing the lung volume available for gas exchange. The Berlin ARDS Definition requires a minimum PEEP of 5 cmH2O for P/F ratio measurements to standardize the classification. Some patients may have a P/F ratio below 200 at PEEP of 5 but above 200 at PEEP of 10, which would change their ARDS classification. This is why PEEP level must always be documented alongside the P/F ratio.
Can the P/F ratio be calculated from SpO2 instead of PaO2?
Yes, a modified ratio called the SpO2/FiO2 ratio (S/F ratio) can be used as a noninvasive alternative when arterial blood gas data is unavailable. Research has shown that an S/F ratio of 315 correlates approximately with a P/F ratio of 300, and an S/F ratio of 235 correlates with a P/F ratio of 200. However, the S/F ratio becomes unreliable when SpO2 exceeds 96% because the oxyhemoglobin dissociation curve flattens at high saturations, making SpO2 insensitive to PaO2 changes. The S/F ratio is validated for SpO2 values between 80% and 96% and is increasingly used in resource-limited settings and for screening purposes.
How quickly can the P/F ratio change in critical illness?
The P/F ratio can change rapidly in critical illness, sometimes within hours. In acute processes like pulmonary embolism, aspiration, or transfusion-related acute lung injury (TRALI), the P/F ratio can drop precipitously from normal to severe ARDS levels within minutes to hours. Conversely, with appropriate treatment (antibiotics for pneumonia, diuresis for pulmonary edema, lung recruitment maneuvers), the P/F ratio can improve significantly within hours to days. Serial P/F ratio measurements are essential for tracking disease trajectory and treatment response. A persistently worsening P/F ratio despite escalating support indicates treatment failure and may warrant changes in management strategy.