Oxygenation & Gas Exchange
A–a Gradient Explained: Alveolar-Arterial Oxygen Difference
· ~7 min read
The A–a gradient (alveolar-arterial oxygen difference) is one of the most powerful tools for evaluating hypoxemia. It helps you determine whether low oxygen levels are due to hypoventilation or a true gas exchange problem like V/Q mismatch or shunt.
For a broader oxygenation overview, see our P/F Ratio guide, or learn full interpretation in the ABG Interpretation Guide.
A–a Gradient Formula
A–a Gradient = PAO₂ − PaO₂
Where PAO₂ (alveolar oxygen) is calculated using the alveolar gas equation:
PAO₂ = FiO₂ × (Patm − PH₂O) − (PaCO₂ / R)
Normal A–a Gradient
A normal A–a gradient increases with age:
Normal ≈ (Age / 4) + 4
Interpretation
Normal A–a gradient
Suggests hypoxemia is due to hypoventilation or low FiO₂ (e.g., high altitude).
Elevated A–a gradient
Indicates a problem with gas exchange:
- V/Q mismatch (most common)
- Shunt (e.g., ARDS, pneumonia)
- Diffusion impairment
Clinical Use
- Differentiate hypoventilation vs gas exchange failure
- Evaluate unexplained hypoxemia
- Assess ARDS and lung injury
- Guide escalation of respiratory support
Pair this with the P/F ratio for a complete oxygenation assessment.
A–a Gradient vs P/F Ratio
| Feature | A–a Gradient | P/F Ratio |
|---|---|---|
| Primary use | Cause of hypoxemia | Severity of hypoxemia |
| Best for | Diagnosis | ARDS classification |