Respiratory Emergency Medicine Anaesthesia / Critical Care Standard clinical practice

A-a Oxygen Gradient

Calculates the alveolar-arterial oxygen gradient to assess for V/Q mismatch and diffusion defects.

Score interpretation

Normal 0–15

A-a gradient ≤15 mmHg: Normal at room air (increases with age; normal ≈ (Age/4) + 4). Suggests hypoventilation if hypoxic.

→ If hypoxic with normal A-a gradient: consider hypoventilation, low FiO₂, or high altitude.

Mildly Elevated 16–29

A-a gradient 16–29 mmHg: Mildly elevated. May be normal in older patients.

→ Correlate with clinical picture. Check age-adjusted normal: (Age/4) + 4 mmHg.

Elevated 30–59

A-a gradient 30–59 mmHg: Elevated. Suggests V/Q mismatch, diffusion defect, or shunt.

→ Consider: PE, pneumonia, pulmonary oedema, ARDS, ILD. Further investigation required.

Markedly Elevated ≥ 60

A-a gradient ≥60 mmHg: Markedly elevated. Significant V/Q mismatch or intracardiac shunt.

→ Urgent assessment. Consider large PE, ARDS, severe pneumonia, intracardiac shunt.

Interpretation bands for the A-a Gradient. Apply clinical judgement and local guidance.

Stapczynski JS. Respiratory distress. In: Tintinalli JE (ed). Emergency Medicine. 8th ed. McGraw-Hill.

Curated clinical cross-links plus same-class fallbacks.

Drugs

Pathways

Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.