Ventilation/perfusion matching and the mechanisms of hypoxaemia
Regional V/Q differences in the upright lung, why low V/Q units cause hypoxaemia that high V/Q units can't offset while CO2 is compensated, shunt and the shunt equation, the five mechanisms of hypoxaemia and their response to oxygen, and how posture, vasodilators and oxygen itself change V/Q.
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
- Overall V/Q is about 0.8 (alveolar ventilation 4 L/min, cardiac output 5 L/min). Matching ranges from shunt (V/Q = 0) to dead space (V/Q = infinity).
- V/Q mismatch is the commonest cause of hypoxaemia: asthma, COPD, pneumonia, PE, pulmonary oedema.
- Low V/Q units lower PaO2. High V/Q units can't compensate, because haemoglobin is already saturated. CO2 behaves differently, so PaCO2 is usually normal or low until the patient tires.
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Sources
- West's Respiratory Physiology: The Essentials (11th ed.)Wolters Kluwer (West and Luks, 2021), 2021
- Murray and Nadel's Textbook of Respiratory Medicine (7th ed.)Elsevier (Broaddus et al., 2022), 2022
- Prone positioning in severe acute respiratory distress syndrome (PROSEVA)New England Journal of Medicine (Guérin et al.), 2013
- Thoracic Society of Australia and New Zealand Position Statement on Acute Oxygen Use in Adults: 'Swimming between the flags'AU/NZRespirology (Barnett et al.), 2022
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Personal emails, About story and Australian-style copy