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Alveolar-arterial (A-a) oxygen gradient

Acid-base & electrolytes

Is the hypoxaemia from the lung (raised gradient: V/Q mismatch, shunt, diffusion) or from hypoventilation alone (normal gradient)?

What you enter

  • FiO₂ (fraction (0.21–1.0))
  • PaO₂ (mmHg or kPa)
  • PaCO₂ (mmHg or kPa)
  • Age (years)
  • Barometric pressure (mmHg)
FORMULAPAO₂ = FiO₂ × (Patm − 47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂; expected ≈ age/4 + 4 (on room air)
SourceWest JB. Respiratory Physiology: The Essentials (gas exchange); standard alveolar gas equationReviewedFormula and thresholds checked against the source on 2026-08-22. If the source updates, this line will say so.