Pulmonology

A-a Gradient

Alveolar-arterial oxygen gradient (kPa).

Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.

When to use

Use to assess whether hypoxaemia is due to a problem with gas exchange (raised gradient) vs hypoventilation (normal gradient).

Why use

It distinguishes the mechanism of hypoxaemia.

Critical actions

This uses kPa at sea level (atmospheric − water vapour ≈ 95 kPa, RQ 0.8). The normal gradient rises with age (roughly age/10 + 0.5 kPa). A raised gradient indicates impaired gas exchange.

Evidence & references

Alveolar gas equation; A-a gradient = PAO₂ − PaO₂.

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