V/Q mismatch: shunt, dead space and the oxygen test
Shunt, dead space and low V/Q all mismatch ventilation and perfusion, yet only shunt barely responds to oxygen. The oxygen test sorts the three.
Distinguish shunt from dead-space physiology and predict the response of each to supplemental oxygen.
The short version
- Shunt is perfused but not ventilated (V/Q 0), and it responds poorly to oxygen.
- Dead space is ventilated but not perfused (V/Q towards infinity), and it mainly impairs carbon dioxide removal.
- Low V/Q means ventilation is reduced but not absent; the hypoxaemia it causes is corrected by oxygen.
- The oxygen test sorts them: shunt barely improves, while dead space and low V/Q improve.
- Hypoventilation and diffusion limitation also respond to oxygen.
V/Q mismatch: two patients, one PaO₂, two responses
Two patients arrive breathless, each with an arterial PO₂ of 55 mmHg on room air. Both are given oxygen. In one, the PaO₂ recovers within minutes. In the other, it barely budges.
The answer lies in V/Q mismatch, and in telling shunt from dead space and from low V/Q by how each responds to oxygen. The lab shows only a low PaO₂, but the cause matters.
V and Q: two flows that must meet
Gas exchange needs two flows to meet: air arriving at the alveoli, called ventilation (V), and blood arriving at the capillaries, called perfusion (Q).[2]
The V/Q ratio compares them, one region at a time. Across a healthy lung it averages about 0.8, although some textbooks say close to 1.[1][2]
Shunt, dead space and low V/Q: block the airway, block the vessel
Block an airway completely, so the alveolus gets blood but no air: V/Q is zero. That is shunt. Block the blood vessel instead, so the alveolus gets air but no blood: V/Q heads towards infinity. That is dead space.[1][2]
In between sits the everyday problem: a low V/Q unit, where ventilation is reduced but not absent.[1]
| Pattern | V/Q | What is missing |
|---|---|---|
| Shunt | 0 | Perfused but not ventilated |
| Dead space | Towards infinity | Ventilated but not perfused |
| Low V/Q | Below normal | Ventilation reduced but not absent |
Shunt: venous blood mixes into arterial blood
Blood passes an unventilated alveolus and leaves still venous, because the gas in that alveolus is no better than mixed venous blood. That venous blood then mixes into the oxygenated blood from healthy units, and the arterial PO₂ falls.[1][2]
A small shunt is normal, a few percent of cardiac output, from bronchial and thebesian veins.[1][5] A large one comes from pneumonia, pulmonary oedema, ARDS, alveolar collapse, or an abnormal link between pulmonary artery and vein.[1][3]
Extra oxygen only reaches blood passing ventilated alveoli. The unventilated unit gets none, so its blood stays venous. Poor response to oxygen is the feature that separates shunt from the other mechanisms.[1][2]
Dead space and the pulmonary embolus
Take a pulmonary embolus blocking a branch of the pulmonary artery. The alveoli beyond it are still ventilated, but receive no blood. That air is wasted breath, and the main cost is less efficient carbon dioxide removal.[1][2]
The effect on oxygen is smaller and falls on the rest of the lung: diverted blood crowds into other units, which can become low V/Q. So when dead space leaves a patient hypoxaemic, the main cause is low V/Q in the rest of the lung, and oxygen helps.[1][2]
The same goes for low V/Q units, as in asthma and COPD: hypoxaemia that oxygen corrects.[1][4]
The oxygen test sorts the causes
| Mechanism | Response to oxygen |
|---|---|
| Shunt | Barely improves |
| Dead space (hypoxaemia, if any) | Improves |
| Low V/Q | Improves |
| Hypoventilation | Improves |
| Diffusion limitation | Improves |
Two other causes of low oxygen, hypoventilation and diffusion limitation, also respond to oxygen.[1]
Worked example: asthma versus a collapsed lung
Back to the two patients, each with an arterial PO₂ of 55 mmHg on room air.
- Patient 1, severe asthma attack: on 40% oxygen, arterial PO₂ rises 55 → 95. Oxygen corrects it, so low V/Q.
- Patient 2, a whole lung collapsed by a mucus plug in its main bronchus: on 100% oxygen, arterial PO₂ reaches only 78. Barely improved, so shunt.
Practice question
Original · USMLE styleA 31-year-old woman is brought to the emergency department with 4 days of fever, productive cough and breathlessness. Respiratory rate is 28/min. A chest radiograph shows dense consolidation of both lower lobes. On room air, arterial blood gas shows PaO₂ 49 mmHg and PaCO₂ 30 mmHg. After 20 minutes of breathing 100% oxygen through a tight-fitting mask, PaO₂ is 68 mmHg.
Common questions
What is the difference between shunt and dead space?
Shunt is perfused but not ventilated (V/Q 0). Dead space is ventilated but not perfused (V/Q towards infinity).
Why does shunt respond poorly to oxygen?
Extra oxygen only reaches blood passing ventilated alveoli. An unventilated unit gets none, so its blood stays venous.
Does dead space cause hypoxaemia?
Its main cost is less efficient carbon dioxide removal. Hypoxaemia, if present, comes mainly from low V/Q in the rest of the lung, where diverted blood crowds in, and oxygen helps.
What is a normal V/Q ratio?
Across a healthy lung it averages about 0.8, although some textbooks say close to 1.
What is the oxygen test?
Give extra oxygen and watch the arterial PO₂. Shunt barely improves; dead space and low V/Q improve.
Transcript
Read the full narration, by chapter689 words
0:00The puzzle: two patients, same PaO2, different response to oxygen
Two patients arrive breathless, each with an arterial PaO2 of 55 on room air. We give both of them oxygen. One is fixed within minutes. The other barely budges. Why? Today: tell shunt from dead space and from low V over Q, and predict how each responds to oxygen.
0:20V and Q: two flows that must meet
Gas exchange needs two flows to meet: air arriving at the alveoli, called ventilation or V, and blood arriving at the capillaries, called perfusion or Q. The V over Q ratio compares them, one region at a time. Across a healthy lung it averages about 0.8, although some textbooks say close to one.
0:42Shunt, dead space and low V/Q: block the airway, block the vessel
Block an airway completely, so the alveolus gets blood but no air. V over Q is zero. That is shunt. Block the blood vessel instead, so the alveolus gets air but no blood. V over Q heads towards infinity. That is dead space. Shunt is perfused but not ventilated. Dead space is ventilated but not perfused. In between sits the everyday problem: a low V over Q unit, where ventilation is reduced but not absent.
1:17Shunt: venous blood mixes into arterial blood
Now back to shunt. Blood passes an unventilated alveolus and leaves still venous, because the gas in that alveolus is no better than mixed venous blood. That venous blood mixes into the oxygenated blood from healthy units, and the arterial PaO2 falls. A small shunt is normal, a few percent of cardiac output, from bronchial and thebesian veins. A large one comes from pneumonia, pulmonary oedema, ARDS, alveolar collapse, or an abnormal link between pulmonary artery and vein.
1:51Why shunt responds poorly to oxygen
Here is the exam point. Shunt responds poorly to oxygen. Extra oxygen only reaches blood passing ventilated alveoli; the unventilated unit gets none, so its blood stays venous.
2:05Dead space and the pulmonary embolus
Take a pulmonary embolus blocking a branch of the pulmonary artery. The alveoli beyond it are still ventilated, but receive no blood. That air is wasted breath, and the main cost is less efficient carbon dioxide removal. The effect on oxygen is smaller and falls on the rest of the lung: diverted blood crowds into other units, which can become low V over Q. So when dead space leaves a patient hypoxaemic, the cause is low V over Q in the rest of the lung, and oxygen helps. The same goes for low V over Q units, as in asthma and COPD: hypoxaemia that oxygen corrects.
2:47The oxygen test sorts the causes
So the oxygen test sorts them. Shunt barely improves. Dead space and low V over Q improve. Two other causes of low oxygen, hypoventilation and diffusion limitation, also respond to oxygen.
3:02Back to our two patients
Back to our two patients. The first has a severe asthma attack. On 40% oxygen, her arterial PaO2 rises to 95. The second has a whole lung collapsed by a mucus plug in its main bronchus. On 100% oxygen his arterial PaO2 reaches only 78. The first is low V over Q. The second is shunt.
3:28Your turn: consolidated lower lobes and a PaO2 of 68
Your turn. A 31-year-old woman has a fever, a cough, and dense consolidation of both lower lobes on her chest film. On room air, her arterial PaO2 is 49 and her PaCO2 is 30. After 20 minutes of 100% oxygen, her arterial PaO2 is only 68. Which best explains her hypoxaemia? A, alveolar hypoventilation. B, alveolar dead space. C, intrapulmonary shunt. D, low V over Q. E, diffusion limitation. Pause here and decide.
4:10The options, one by one
A is out: hypoventilation would raise her PaCO2, but hers is low, and oxygen would correct it. B is out: dead space mainly impairs carbon dioxide removal, and any hypoxaemia from it improves with oxygen. D is out: oxygen corrects low V over Q hypoxaemia, so a rise this small is not expected. E is out: diffusion limitation also responds
4:37The answer: consolidation means shunt
well to oxygen. So the answer is C. Consolidated lobes are perfused but not ventilated, and that blood cannot take up the extra oxygen, so she barely improves.
4:48Pocket-card recap and the next lesson
Three things to keep. One: shunt is perfused but not ventilated, and it responds poorly to oxygen. Two: dead space is ventilated but not perfused, and mainly impairs carbon dioxide removal. Three: low V over Q hypoxaemia is corrected by oxygen, so the oxygen test sorts them. Next lesson: the alveolar-arterial gradient.
Sources
Numbers in the text, e.g. [1], point here- Mechanisms of hypoxemia. Lung India 2017 (PMC5234199). pmc.ncbi.nlm.nih.gov/articles/PMC5234199/
- Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches (PMC8274320). pmc.ncbi.nlm.nih.gov/articles/PMC8274320/
- Preoxygenation when standard approaches fail (PMC13073621). pmc.ncbi.nlm.nih.gov/articles/PMC13073621/
- Supplementary oxygen efficacy for chronic pulmonary disorders and exertion desaturation (PMC11626617). pmc.ncbi.nlm.nih.gov/articles/PMC11626617/
- Mechanisms of platypnea-orthodeoxia syndrome 2022 (PMC9005182), normal small shunt. pmc.ncbi.nlm.nih.gov/articles/PMC9005182/
- Ventilation/perfusion ratio and Alveolar-arterial gradient. Wikipedia (secondary sources, used for cross-checking only).
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (cropped, recoloured, outlined): airway and vessels, fluid-filled alveoli, pulmonary embolus, inflamed bronchus, lungs. Oxygen tank icon: Health Icons (healthicons.org), CC0 1.0. Original diagrams drawn in code: ABG slips, blockage marks, mixing schematic, FiO2 dial, number line, oxygen-test table, pocket card. Patient values in the worked example are illustrative. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-RESP-004.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Lung volumes and capacities, explained5:26
- EP 02Obstructive vs restrictive lung disease: FEV1/FVC and loops5:23
- EP 03Lung compliance, Laplace and surfactant: why alveoli stay open5:23
- EP 04V/Q mismatch: shunt, dead space and the oxygen test5:18
- EP 05A-a gradient: the alveolar gas equation and the causes of hypoxaemia5:39
- EP 06Oxygen-hemoglobin dissociation curve: P50, shifts and content5:19
- EP 07CO₂ transport: bicarbonate, the chloride shift, Bohr vs Haldane5:29
- EP 08Central vs peripheral chemoreceptors and oxygen-induced hypercapnia5:27
- EP 09Hypoxic pulmonary vasoconstriction and cor pulmonale5:41
- EP 10CO, cyanide and methaemoglobin: why SpO₂ and PaO₂ fool you5:34