A-a gradient calculator

A-a O₂ Gradient

Alveolar-arterial (A-a) O₂ Gradient

Parameter	Value
Age (years)
FiO₂ (%)
PaCO₂ (mmHg)
PaO₂ (mmHg)
A-a Gradient	mmHg
Expected A-a	mmHg (age-adjusted normal)

Interpretation

Normal A-a gradient ≈ (Age/4) + 4 on room air Elevated A-a gradient suggests: V/Q mismatch, shunt, or diffusion impairment Normal A-a gradient + hypoxia suggests: hypoventilation or low FiO₂

References

Formula: A-a = [FiO₂ × (Patm – PH2O)] – (PaCO₂/0.8) – PaO₂ Kanber GJ, et al. The alveolar-arterial oxygen gradient in young and elderly men during air and oxygen breathing. Am Rev Respir Dis. 1968;97(3):376-381. PMID 5637791.