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A-a Gradient Calculator
Pulmonology / Respiratory
The alveolar-arterial (A-a) oxygen gradient measures the difference between alveolar and arterial oxygen tension. It helps differentiate causes of hypoxemia and assess gas exchange efficiency.
Formula
PAO₂ = FiO₂ × (P_atm − P_H₂O) − PaCO₂ / 0.8
A-a Gradient = PAO₂ − PaO₂
Expected Normal = Age/4 + 4 (on room air)
A-a Gradient = PAO₂ − PaO₂
Expected Normal = Age/4 + 4 (on room air)
Reference
| A-a Gradient | Interpretation | Common Causes |
|---|---|---|
| ≤ Age/4+4 | Normal | Hypoventilation only |
| Slightly elevated | Mild elevation | Early V/Q mismatch |
| 20–40 mmHg | Moderate elevation | V/Q mismatch, PE |
| >40 mmHg | Severe elevation | Shunt, severe V/Q mismatch |
Clinical Notes
Normal A-a gradient with elevated PaCO₂: Pure hypoventilation — consider CNS depression, neuromuscular disease, or obesity hypoventilation syndrome.
Elevated A-a gradient: Indicates V/Q mismatch, intrapulmonary shunt, diffusion impairment, or pulmonary embolism. The A-a gradient increases with age and with higher FiO₂.
Elevated A-a gradient: Indicates V/Q mismatch, intrapulmonary shunt, diffusion impairment, or pulmonary embolism. The A-a gradient increases with age and with higher FiO₂.
For educational use only. Not a substitute for clinical judgment.
