2021A Question 8
Describe the V/Q ratio at the apex and base of the upright lung. How does this affect regional gas exchange?
Examiner Report
35.9% of candidates achieved a pass in this question.
This question called firstly for a description and quantification of ventilation and perfusion as it is spread across the upright lung. Better answers attempted the two topics separately and then proceeded to discuss how ventilation and perfusion are matched. A correctly drawn diagram often aided the discussion.
The second part of the question called for a discussion on how the V/Q spread affects gas exchange. Examples of gas tensions in lung units across different parts of the lung should have been included, as well as identification on how and why O2 and CO2 are affected differently. Discussion of why well oxygenated alveoli do not compensate for the poorly oxygenated gained extra credit.
There were two very repeatable reasons why some candidates did not score a pass mark:
- Completely failing to discuss gas exchange
- Devoting the answer wholly, or largely to a discussion of West’s zones of the lung.
West’s zones is a model to conceptualise what may be occurring in terms of the relationship between vascular and alveolar pressures in different parts of the lung. It has relevance but should not be considered the whole story or even the basis of a discussion on V/Q mismatching.
Model Answer
Structure:
- Graph
- Factors
- Ventilation
- Perfusion
- Impact on CO2
- Impact on O2
Graphs
Discussion
| Factor | Description |
|---|---|
| Ventilation | - Gravity: - Apical traction → ↑ Volume → ↓ Compliance → ↓ V - Basal compression → ↓ Volume → ↑ Compliance → ↑ V |
| Perfusion | - Gravity → West’s zones: 1. PA > Pa > Pv: No flow (apex) 2. Pa > PA > Pv: Flow in systole (from 3cm above RV to apex) 3. Pa > Pv > PA: Flow throughout (from base to 3cm above RV) - Hypoxic pulmonary vasoconstriction: - ↓ Ventilation → ↓ PO2 in individual alveolus → Vasoconstriction - Improves local V/Q matching |
| Carbon dioxide | - CO2 flux fairly evenly distributed - Mild gradient in ventilation from base to apex - Effective compensation for ↓ V/Q regions - Hyperventilation → ↓↓ CO2 in well-matched regions - High CO2 solubility → PaCO2/CaCO2 curve is steep → V/Q inequality rarely causes hypercapnoea |
| Oxygen | - O2 flux almost all near the base - Steep gradient in perfusion from base to apex - Ineffective compensation for ↑ V/Q regions - Hyperventilation → ↔↑ O2 in well-matched regions - Low O2 solubility + finite Hb binding sites → OHDC upper part flat → V/Q inequality often causes hypoxaemia |