A-a oxygen gradient

Distinguishes hypoxemia from hypoventilation (normal A-a) vs V/Q mismatch · shunt · diffusion limitation (elevated A-a). Normal on room air ≈ age/4 + 4 mmHg.

PaO₂ (mmHg)PaCO₂ (mmHg)FiO₂ (decimal — 0.21 room air)Patient age (yr)

A-a gradient

9.7 mmHg

Expected for age 40: ~14.0 mmHg

PAO₂ = 0.21 × (760 − 47) − (40 / 0.8) = 99.7 mmHg
A-a = 99.7 − 90 = 9.7 mmHg
Normal (room air) ≈ age/4 + 4