Alveolar gas equation, diffusion and the A-a gradient
Calculating alveolar PO2 and the A-a gradient to separate hypoventilation from lung disease, Fick's law and why oxygen transfer is normally perfusion-limited, and how DLCO and KCO patterns point to the cause of impaired gas transfer.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- The alveolar gas equation predicts alveolar PO2 from inspired oxygen and PaCO2. The A-a gradient is PAO2 minus measured PaO2.
- A normal A-a gradient with hypoxaemia means hypoventilation or low inspired PO2. A raised gradient means a lung problem: V/Q mismatch, shunt or diffusion limitation.
- Oxygen transfer at rest is perfusion-limited, not diffusion-limited. Diffusion problems surface on exercise or at altitude.
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Sources
- West's Respiratory Physiology: The Essentials (11th ed.)Wolters Kluwer (West and Luks, 2021), 2021
- 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lungEuropean Respiratory Journal (Graham et al.), 2017
- ERS/ATS technical standard on interpretive strategies for routine lung function testsEuropean Respiratory Journal (Stanojevic et al.), 2022
- A New Global Definition of Acute Respiratory Distress SyndromeAmerican Journal of Respiratory and Critical Care Medicine (Matthay et al.), 2024
- Last updated
- 14 November 2026
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