Oxygenation · PaO₂:FiO₂ and A-a gradient
Quantify oxygenation efficiency and identify the physiologic cause of hypoxemia. P:F ratio classifies severity from normal to very severe (Berlin/Wilkins cutoffs at 300, 200, 100). A-a gradient with PaCO₂ context discriminates hypoventilation (normal A-a + high PaCO₂), V/Q mismatch (high A-a, oxygen-responsive), and true shunt (high A-a, oxygen-refractory). Altitude-aware via custom barometric pressure.
The P:F ratio quantifies oxygenation severity on a continuous scale comparable across patients receiving different FiO₂. The A-a gradient, when combined with PaCO₂, identifies the physiologic cause of any hypoxemia: hypoventilation (normal A-a + high PaCO₂), V/Q mismatch (high A-a, oxygen-responsive), or true shunt (high A-a, oxygen-refractory). Use both, and interpret in the context of the patient's clinical status, not the numbers alone.
Enter PaO₂, FiO₂, and PaCO₂ to compute the P:F ratio and A-a gradient.
Formulas
PaO₂:FiO₂ ratio
FiO₂ as a decimal (0.21 to 1.0). A healthy patient on room air with PaO₂ ≈ 95 mmHg has a P:F of 95 / 0.21 ≈ 450. Berlin-adapted cutoffs (per Wilkins et al 2007 for vet use):
| P:F ratio | Classification |
|---|---|
| > 400 | Normal oxygenation |
| 300–400 | Mild oxygenation impairment |
| 200–300 | Moderate (ALI threshold) |
| 100–200 | Severe (ARDS equivalent) |
| < 100 | Very severe |
Alveolar gas equation and A-a gradient
Default constants: Patm = 760 mmHg (sea level), PH₂O = 47 mmHg (water vapor pressure at 37 °C), R = 0.8 (respiratory quotient on a mixed diet). At sea level with default R:
Interpreting the A-a gradient
A normal A-a gradient on room air is below 15 mmHg in young healthy patients; it rises modestly with age. On supplemental oxygen the alveolar PO₂ rises faster than the patient can equilibrate, so the expected A-a rises with FiO₂. A rough rule of thumb is A-a < (FiO₂ × 100) − 10 mmHg; the qualitative interpretation (high vs normal) matters more than the precise cutoff.
| A-a gradient | PaCO₂ | Interpretation |
|---|---|---|
| Normal | High | Hypoventilation (anesthetic depth, opioids, neuromuscular, pleural) |
| Elevated | Normal or low | V/Q mismatch or shunt. Differentiate by O₂ response. |
| Elevated, O₂-responsive | Normal | V/Q mismatch (pneumonia, atelectasis, edema, asthma) |
| Elevated, O₂-refractory | Normal | True shunt (consolidation, severe edema, intracardiac R-to-L) |
Altitude effect
Barometric pressure falls roughly 120 mmHg per 1500 m of elevation. The maximal achievable PaO₂ on room air drops proportionally: at sea level ~100 mmHg; in Denver (1600 m) ~75 mmHg; on the high plateau of Mexico City (2240 m) ~65 mmHg. The calculator accepts a custom Patm to handle this; default 760 is correct for Ridgefield CT and most US coastal cities.
Source citations
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2020. Foundational physiology for the alveolar gas equation and A-a gradient.
- Lumb AB, Jones GM. Lumb and Jones' Veterinary Anesthesia and Analgesia. 6th ed. Wiley-Blackwell; 2024. Ch. 22 (Respiratory Monitoring).
- Silverstein DC, Hopper K, eds. Small Animal Critical Care Medicine. 4th ed. Elsevier; 2023. Ch. 23 (Oxygenation and Ventilation Monitoring).
- ARDS Definition Task Force. Acute Respiratory Distress Syndrome: the Berlin Definition. JAMA. 2012; 307(23):2526–2533. doi:10.1001/jama.2012.5669. Source of the P:F cutoffs.
- Wilkins PA, Otto CM, Baumgardner JE, et al. Acute lung injury and acute respiratory distress syndromes in veterinary medicine: consensus definitions. J Vet Emerg Crit Care. 2007;17(4):333–339. Veterinary ALI/ARDS adaptation of Berlin.