Anaes · Applied cardiovascular & respiratory physiology
Ventilation, perfusion & dead space
Also known as Ventilation-perfusion ratio · V/Q mismatch · Shunt · Dead space · Hypoxaemia · Alveolar-arterial gradient
Gas exchange requires that ventilation and perfusion are matched, and most clinical hypoxaemia is a disorder of that matching. The framework rests on five exam-critical ideas: the ventilation-perfusion (V/Q) ratio is normally about 0.8 to 1.0, and both ventilation and perfusion increase from apex to base, but perfusion more, so the apex is high V/Q and the base low V/Q; ventilation-perfusion mismatch spans a spectrum from shunt (V/Q of zero, perfusion without ventilation) through normal to dead space (V/Q very high, ventilation without perfusion); true shunt is hypoxaemia that is NOT corrected by 100 percent oxygen, whereas low-V/Q mismatch IS corrected by oxygen — the test that distinguishes them; dead space is ventilated lung that does not exchange gas, raised by pulmonary embolism; and the five causes of hypoxaemia are low inspired oxygen, hypoventilation, diffusion impairment, shunt, and ventilation-perfusion mismatch — separated by the alveolar-arterial oxygen gradient. Built on the acute-hypoxaemic-respiratory-failure imaging review (Coppola 2026), the Eisenmenger-syndrome review (the classic pulmonary shunt, Baino 2026), the patent-foramen-ovale platypnea study (right-to-left shunt hypoxaemia, Ramidi 2026), the pulmonary-embolism perfusion-imaging study (Evans 2026), the portopulmonary-gas-exchange study (Lacoste-Palasset 2026), and the partial-pressure-of-oxygen review (Hoecker 2026). V/Q ratios, West-zone conditions, shunt/Bohr equations and gradient sorting are textbook teaching carried with explicit honesty notes; retrieved abstracts own only the narrowed imaging/shunt/embolism/gas-exchange points cited inline.)
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Red flags
- True shunt (perfusion with no ventilation, V/Q of zero) causes hypoxaemia that is NOT corrected by 100 percent oxygen — this distinguishes shunt from low-V/Q mismatch, which DOES respond to oxygen.
- The alveolar-arterial oxygen gradient separates the hypoxaemias: hypoventilation and low inspired oxygen give a NORMAL gradient, while shunt, mismatch and diffusion impairment give a RAISED gradient.
- Both ventilation and perfusion rise from lung apex to base, but perfusion more, so the apex is high V/Q (dead-space-like) and the base low V/Q (shunt-like) — the basis of the West zones and of basal atelectasis.
- Pulmonary embolism is the classic high-V/Q (dead-space) state — ventilated but unperfused lung raises physiological dead space and, with reflex bronchoconstriction and redistribution, causes hypoxaemia.
- Hypoxic pulmonary vasoconstriction is the mechanism that defends V/Q matching: it diverts perfusion away from poorly ventilated (hypoxic) alveoli, and it is impaired by volatile anaesthetics and a low systemic vascular resistance.
Meet the patient
A supine, anaesthetised patient desaturates to 90 percent on an oxygen fraction of 0.6 with a confirmed endotracheal tube. The registrar turns up the flowmeter; the saturation barely moves. This is the moment to stop chasing the dial and think in V/Q terms — the tube is right, so this is most likely compression atelectasis behaving as shunt, and the fix is recruitment, not oxygen.[1]
Hold the question that frames the whole topic: is the desaturation a problem the flowmeter can fix, or a shunt it cannot? The answer is the oxygen test, and it divides every cause of hypoxaemia into two camps.[1][6]
References6ShowHide
- [1]Coppola S, et al. Lung Imaging in Acute Hypoxemic Respiratory Failure: From Physics to Bedside Applications J Clin Med, 2026.PMID 42279206
- [2]Baino B, et al. Eisenmenger Syndrome: The Pulmonology Perspective Chest, 2026.PMID 42341977
- [3]Ramidi SR, et al. Refractory Hypoxemia Secondary to Patent Foramen Ovale Presenting With Platypnea-Orthodeoxia Syndrome Cureus, 2026.PMID 42333279
- [4]Evans B, et al. Quantitative perfusion imaging from non-contrast micro-ct for pulmonary embolism evaluation in preclinical models Phys Med Biol, 2026.PMID 42335950
- [5]Lacoste-Palasset T, et al. Impact of Pulmonary Arterial Hypertension Therapies on Gas Exchange in Portopulmonary Hypertension Chest, 2026.PMID 42066896
- [6]Hoecker RN. Partial Pressure of Oxygen StatPearls, 2026.PMID 29630271