High-yield map
Key points before the detail
- 01
PaCO2 rises when effective alveolar ventilation falls if CO2 production is unchanged.
- 02
Shunt is the V/Q extreme of perfusion without ventilation and responds relatively poorly to oxygen alone.
- 03
Dead space is ventilation without effective perfusion.
- 04
Surfactant lowers surface tension and helps stabilise smaller alveoli; loss of compliance increases the work needed to inflate the respiratory system.
Minute and alveolar ventilation
Minute ventilation is tidal volume multiplied by respiratory rate. Not all inspired gas reaches perfused alveoli because some remains in conducting airways and other dead-space regions. Alveolar ventilation is therefore the component that actually participates in CO2 exchange. If metabolic CO2 production is stable, PaCO2 varies inversely with alveolar ventilation.
V/Q matching
| Pattern | Meaning | Examples |
|---|---|---|
| Low V/Q | Perfusion exceeds ventilation | Atelectasis, pneumonia, pulmonary oedema, mucus plugging |
| Shunt (V/Q = 0) | Blood perfuses an unventilated unit | Complete alveolar collapse or fluid-filled/consolidated units |
| High V/Q | Ventilation exceeds perfusion | Reduced regional blood flow |
| Dead space | Ventilation with no effective perfusion | Pulmonary embolic obstruction or excessive alveolar overdistension |
Mechanisms of hypoxaemia
- Low inspired oxygen tension.
- Alveolar hypoventilation.
- V/Q mismatch.
- Right-to-left shunt.
- Diffusion limitation, particularly during exertion or severe interstitial disease.
Compliance, recoil and surfactant
Compliance describes change in volume for a given change in pressure. Low compliance means a stiffer respiratory system and greater pressure/work is needed to inflate it. Elastic recoil favours expiration, while surfactant reduces alveolar surface tension and helps prevent collapse, especially in smaller alveoli.
Why postoperative lungs are vulnerable
Anaesthesia, pain, recumbency and abdominal or thoracic surgery can reduce functional residual capacity and promote dependent atelectasis. Secretions and impaired cough further worsen V/Q matching. Prevention and treatment therefore target both the cause and the physiology: adequate analgesia, mobilisation, lung expansion, secretion clearance and escalation of respiratory support when needed.
- A normal SpO2 does not exclude hypercapnia because saturation measures oxygenation, not ventilation.
- A tiring patient may become less tachypnoeic as respiratory muscles fail; trajectory matters more than a single respiratory rate.
Active recall
Close the notes and answer these
Try each question from memory before revealing the answer. These public prompts are a small preview of the integrated retrieval system inside SurgAtlas.
01What is the difference between minute ventilation and alveolar ventilation?
Minute ventilation is total tidal volume × respiratory rate; alveolar ventilation subtracts the portion ventilating dead space and therefore reflects effective gas exchange.
02What is a pulmonary shunt?
Perfusion of lung units that receive no ventilation, the V/Q = 0 extreme.
03Why can SpO2 be normal in severe hypoventilation?
Supplemental oxygen can maintain arterial saturation while alveolar ventilation is inadequate and PaCO2 rises.
Sources & editorial basis
References & editorial basis
- SurgAtlas production chapter — Applied Physiology — Respiratory Physiology. Primary source for this public lesson. Stable physiological principles have been condensed from the corresponding SurgAtlas production teaching.
- West’s Respiratory Physiology. Reference for stable ventilation, perfusion and gas-exchange physiology.
This lesson is derived from the corresponding SurgAtlas production teaching material. Where the source makes current management or guideline claims, the public lesson uses the cited contemporary guidance. It is written for education and examination preparation, not as patient-specific clinical advice.
Editorial details
Medical Doctor (MD) · MRCS Part A · Physician · Surgical Educator
SurgAtlas is an educational resource. For patient care, verify current national guidance, local antimicrobial and transfusion policies, specialty pathways and individual patient factors.
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