🫁 A–a Oxygen Gradient Calculator
Calculate the alveolar–arterial oxygen gradient to characterise hypoxaemia.
A–a Oxygen Gradient Calculator
PaO₂ (arterial) (mmHg)
PaCO₂ (mmHg)
FiO₂ (fraction)
Age (for expected) (years)
When to use
Distinguish causes of hypoxaemia (shunt/V-Q mismatch vs hypoventilation).
How it works
PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 (sea level); A–a = PAO₂ − PaO₂. Expected ≈ age/4 + 4.
Key points
- Widened gradient: V/Q mismatch, shunt or diffusion defect.
- Normal gradient with hypoxaemia: hypoventilation or low inspired O₂.
- Expected value rises with age and FiO₂.
References
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.