ToolsCalculators
A–a oxygen gradient
Alveolar–arterial oxygen difference to characterise hypoxaemia.
A–a oxygen gradient
Age
years
FiO₂
%
PaCO₂
mmHg
PaO₂
mmHg
Complete the fields above to see the result.
When to use
- Working up hypoxaemia: a normal gradient points to hypoventilation or low FiO₂; a raised gradient points to V/Q mismatch, shunt or a diffusion defect.
Formula
PAO₂ = FiO₂ × (Patm − PH₂O) − PaCO₂ / R = FiO₂ × (760 − 47) − PaCO₂ / 0.8 at sea level. A–a gradient = PAO₂ − PaO₂. Expected ≈ age/4 + 4.