Q5
LONG ANSWER
10M
★ High-yield
Define the ventilation–perfusion ratio. Describe its regional variation in the upright lung and discuss the consequences of V/Q mismatch.
Definition
V/Q ratio = ratio of alveolar ventilation to pulmonary capillary blood flow.Normal: V = 4.2 L/min, Q = 5.0 L/min → V/Q = 0.8
Regional Variation (upright Lung)
- Both ventilation and perfusion ↑ from apex to base (gravity)
- But perfusion increases much more steeply than ventilation
- Therefore V/Q falls from apex to base
| Region | Ventilation | Perfusion | V/Q | Alveolar gas |
|---|---|---|---|---|
| Apex | Low | Very low | 3.3 | ↑ PO2 (132 mmHg), ↓ PCO2 |
| Middle | Moderate | Moderate | 0.8 | Ideal |
| Base | High | Very high | 0.6 | ↓ PO2 (89 mmHg), ↑ PCO2 |
Reasons
- Apical alveoli already more expanded at end-expiration → lie on flatter part of compliance curve → expand less
- Apical perfusion low — pulmonary artery pressure barely reaches apex (West's zones)
West’s Zones of the Lung
| Zone | Relationship | Blood flow |
|---|---|---|
| Zone 1 (apex) | PA > Pa > Pv | No flow — alveolar dead space (not present normally) |
| Zone 2 (middle) | Pa > PA > Pv | Intermittent flow |
| Zone 3 (base) | Pa > Pv > PA | Continuous flow |
The Two Extremes
| V/Q = 0 | V/Q = ∞ | |
|---|---|---|
| Name | Shunt | Alveolar dead space |
| Defect | Perfused but not ventilated | Ventilated but not perfused |
| Causes | Airway obstruction, consolidation, collapse, ARDS | Pulmonary embolism, ↓ cardiac output |
| End-capillary gas | Same as mixed venous blood | Same as inspired air |
| Responds to O2? | NO | Yes |
Effects of V/Q Mismatch
- Commonest cause of hypoxaemia in clinical practice
- Causes hypoxaemia but usually not hypercapnia
Why no hypercapnia?
| O2 curve | CO2 curve |
|---|---|
| Sigmoid, flat at top | Steep, almost linear |
| Well-ventilated alveoli cannot raise saturation above 97.5% to compensate | ↑ ventilation of normal alveoli readily washes out extra CO2 |
| → PaO2 falls | → PaCO2 normal or low |
Compensatory Mechanisms
- Hypoxic pulmonary vasoconstriction → diverts blood away from poorly ventilated regions
- Local bronchoconstriction in response to ↓ alveolar PCO2 → diverts air away from poorly perfused regions
Alveolar Gas Equation and A–a Gradient
PAO2 = PIO2 − (PaCO2 / R)where PIO2 = 150 mmHg at sea level and R (RQ) = 0.8
- Normal PAO2 = 150 − (40/0.8) = 100 mmHg
- A–a gradient = PAO2 − PaO2 = normally 5–10 mmHg (↑ with age)
| A–a gradient | Interpretation | Examples |
|---|---|---|
| Normal | Hypoxaemia is not of pulmonary origin | Hypoventilation, high altitude |
| Raised | Gas exchange defect in the lung | V/Q mismatch, shunt, diffusion defect |
V/Q in Exercise
- ↑ cardiac output → recruitment and distension of apical capillaries
- → perfusion becomes more uniform
- → V/Q matching improves and the A–a gradient narrows
Causes of Hypoxaemia — Summary
| Cause | A–a gradient | Corrects with O2? | Example |
|---|---|---|---|
| Hypoventilation | Normal | Yes | Opioid overdose |
| High altitude | Normal | Yes | Mountaineering |
| V/Q mismatch | ↑ | Yes | COPD, asthma |
| Shunt | ↑ | NO | ARDS, consolidation |
| Diffusion defect | ↑ | Yes | Fibrosis, pulmonary oedema |
Applied Aspects
- Bedside test — hypoxaemia of V/Q mismatch corrects with O2; true shunt does not, since shunted blood never contacts alveolar gas
- Pulmonary embolism → ↑ alveolar dead space
- Lobar pneumonia, ARDS, atelectasis → shunt
- Pulmonary tuberculosis → apical predilection, attributed to the high PO2 at the apex favouring this strict aerobe
All questions in Respiratory System
- Q1Describe the mechanics of breathing. Discuss the muscles involved and the pressure changes occurring during one normal respiratory cycle. Add a note on lung compliance. ★
- Q2Describe the transport of oxygen in the blood. Draw and explain the oxygen–haemoglobin dissociation curve and enumerate the factors that shift it. ★
- Q3Describe the nervous and chemical regulation of respiration. Add a note on the Hering–Breuer reflex. ★
- Q4Describe the transport of carbon dioxide in the blood. Explain the chloride shift and the Haldane effect. ★
- Q5Define the ventilation–perfusion ratio. Describe its regional variation in the upright lung and discuss the consequences of V/Q mismatch. ★
- SN1Write a short note on pulmonary surfactant. ★
- SN2Define lung volumes and capacities. Give their normal values and state which of them cannot be measured by spirometry. ★
- SN3Classify hypoxia. Give the causes and effects of each type. ★
- SN4Write a short note on cyanosis.
- SN5Write a short note on Cheyne–Stokes breathing.
- SN6Define dead space. Describe its types and explain its significance in alveolar ventilation. ★
- SN7Describe the physiological changes that occur during acclimatisation to high altitude. ★