Physiology chapters
Ventilation-perfusion matching and causes of hypoxaemia
How ventilation and blood flow differ from the top to the bottom of the upright lung, what the V/Q ratio means, the effect of dead space and shunt, and the five causes of low arterial oxygen with the A-a gradient.
- The ideal V/Q ratio is about 0.8 (alveolar ventilation about 4 L/min, blood flow about 5 L/min).
- In the upright lung, both ventilation and blood flow are greater at the base, but blood flow rises more, so V/Q is high at the apex (about 3) and low at the base (about 0.6).
- Hypoxaemia has five causes: low inspired O2, hypoventilation, diffusion defect, V/Q mismatch and shunt; only shunt does not correct with 100% oxygen.
On this page
Overview
For gas exchange to work well, each alveolus needs the right amount of air (ventilation, V) and the right amount of blood (perfusion, Q).
The ventilation-perfusion ratio (V/Q) compares the two. When they are mismatched, blood leaves the lungs poorly oxygenated: the commonest cause of hypoxaemia in lung disease.
The normal V/Q ratio
For the whole lung at rest:
- Alveolar ventilation about 4 L/min.
- Pulmonary blood flow about 5 L/min.
- Overall V/Q about 0.8.
Regional differences in the upright lung
Gravity affects both blood and air.
Blood flow
The pulmonary circulation is a low-pressure system. In an upright person, pressure in the pulmonary arteries is lowest at the apex and highest at the base, so blood flow is much greater at the base.
Physiologists describe three zones:
- Zone 1: no flow at any time (alveolar pressure higher than capillary pressure). Not present in normal lungs; appears in haemorrhage or positive-pressure ventilation.
- Zone 2: flow only during systole. Normal apices.
- Zone 3: continuous flow. Normal bases.
Ventilation
The alveoli at the apex are already more expanded at rest (more negative pleural pressure there), so they expand less with each breath. Ventilation is also greater at the base, but less so than blood flow.
The result
| Region | Ventilation | Perfusion | V/Q | Alveolar PO2 | Alveolar PCO2 |
|---|---|---|---|---|---|
| Apex | Low | Very low | High (about 3) | Higher | Lower |
| Base | Higher | Much higher | Low (about 0.6) | Lower | Higher |
The apex has the highest V/Q and the highest alveolar PO2. This is one reason tuberculosis (which favours high O2) settles in the apices.
Both ventilation and perfusion are greater at the base. The ratio is lower at the base because perfusion increases more.
The two extremes
| Situation | V/Q | Meaning | Example |
|---|---|---|---|
| Dead space | Infinity (∞) | Ventilated but not perfused; air is wasted | Pulmonary embolism |
| Shunt | Zero (0) | Perfused but not ventilated; blood passes unoxygenated | Collapsed lung, pneumonia, mucus plug, right-to-left heart defect |
When V/Q is high, the alveolar gas approaches inspired air (PO2 high, PCO2 low). When V/Q is low, alveolar gas approaches venous blood (PO2 low, PCO2 high).
Hypoxic pulmonary vasoconstriction
When an area of lung is poorly ventilated, its low alveolar PO2 makes the small pulmonary arteries there constrict. Blood is diverted to better-ventilated areas, improving V/Q matching.
This is the opposite of the systemic circulation, where hypoxia causes vasodilation.
Hypoxia constricts pulmonary vessels but dilates systemic vessels. Widespread hypoxia (altitude, COPD) can therefore cause pulmonary hypertension.
In the fetus, hypoxic vasoconstriction keeps lung blood flow low; it relaxes at the first breath.
The alveolar-arterial (A-a) gradient
The A-a gradient is the difference between alveolar PO2 (calculated from the alveolar gas equation) and arterial PO2 (measured).
- Normal: small, about 10–15 mmHg or less in young adults; it increases with age.
- A raised A-a gradient means a problem with gas exchange in the lungs.
The alveolar gas equation (simplified): PAO2 = PIO2 − (PaCO2 ÷ 0.8). Breathing room air at sea level, PIO2 is about 150 mmHg.
Causes of hypoxaemia
| Cause | A-a gradient | Corrects with 100% O2? | Examples |
|---|---|---|---|
| Low inspired O2 | Normal | Yes | High altitude |
| Hypoventilation | Normal (PCO2 high) | Yes | Opioid overdose, neuromuscular weakness, chest wall disease |
| Diffusion impairment | Raised | Yes | Pulmonary fibrosis (mainly on exercise) |
| V/Q mismatch (commonest) | Raised | Yes | COPD, asthma, pneumonia, pulmonary embolism |
| Right-to-left shunt | Raised | No (or very little) | Collapsed lung, ARDS, cyanotic congenital heart disease |
Hypoventilation and low inspired O2 cause hypoxaemia with a normal A-a gradient. Shunt is the only cause that does not improve much with 100% oxygen, because the shunted blood never meets the oxygen.
Types of hypoxia (tissue level)
- Hypoxic hypoxia: low arterial PO2 (any cause above).
- Anaemic hypoxia: low haemoglobin or CO poisoning; PO2 normal, content low.
- Stagnant (circulatory) hypoxia: low blood flow, as in shock or heart failure.
- Histotoxic hypoxia: cells cannot use oxygen, as in cyanide poisoning (blocks cytochrome oxidase); venous O2 is high.
Clinical relevance
Pulmonary embolism
How it happens: a clot blocks a pulmonary artery.
What you see: areas of high V/Q (dead space). Hypoxaemia arises because blood is diverted to other areas, overloading them and lowering their V/Q. Breathlessness and rapid breathing usually lower the PCO2.
Pneumonia and lung collapse
How it happens: alveoli fill with fluid or collapse but are still perfused.
What you see: shunt and low V/Q areas; hypoxaemia that responds only partly to oxygen.
Common questions: the normal V/Q ratio, apex vs base for ventilation, perfusion, V/Q, PO2 and PCO2, dead space vs shunt with examples, hypoxic pulmonary vasoconstriction, the A-a gradient, the five causes of hypoxaemia, and which do not respond to oxygen.
Quick revision
- Normal V/Q about 0.8.
- Apex: high V/Q (about 3), high PO2, low PCO2. Base: low V/Q (about 0.6).
- Dead space: V/Q infinity (embolism). Shunt: V/Q zero.
- Hypoxic pulmonary vasoconstriction diverts blood from poorly ventilated alveoli.
- Normal A-a gradient: hypoventilation and low inspired O2.
- Shunt does not correct with 100% oxygen.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
Where in the lung is the ventilation-perfusion ratio highest?
At the apex of the upright lung.
What is the V/Q ratio in pulmonary embolism?
High, approaching infinity in the affected area, because it is ventilated but not perfused (dead space).
Which causes of hypoxaemia have a normal A-a gradient?
Hypoventilation and a low inspired oxygen, such as at high altitude.
Why does a shunt not correct with oxygen?
The shunted blood does not pass ventilated alveoli, so extra oxygen in the alveoli cannot reach it.