Pulmonary gas exchange

Question 1

What is normal PaO2?

Answer 1

10.5-13.5 kPa

Question 2

What is normal PaCO2?

Answer 2

4.8 – 6.0 kPa

Question 3

What defines type 1 respiratory failure?

Answer 3

PaO2 <8 kPa (PaCO2 normal or low

Question 4

what defines Type 2 respiratory failure?

Answer 4

PaCO2 >6.5 kPa (PaO2 usually low)

Question 5

What are the four abnormal states associated with hypoxaemia?

Answer 5

1. V/Q mismatch
2. Diffusion impairment
3. Alveolar hypoventilation
4. Shunt

Question 6

What is normal V/Q?

Answer 6

normal V/Q is 4/5 or 0.8.

Question 7

What should you do if your patient has hypoxaemia due to low V/Q?

Answer 7

Put them on oxygen

Question 8

What does the degree of diffusion and the efficiency of diffusion depend upon?

Answer 8

thickness and surface areas of membrane and gas pressure across it

Question 9

What can cause alveolar hypoventilation?

Answer 9

an upper airway or tracheal obstruction, mechanical problems (such as chest wall damage, functional breathing problems (such as muscle paralysis or diaphragmatic damage) or neurological problems with breathing

Question 10

What happens to PACO2, PaCO2, PAO2 and PaO2 in alveolar hypoventilation?

Answer 10

Hypoventilation increases PACO2, and thus increases PaCO2

An increase in PACO2 decreases PAO2, which causes PaO2 to fall

Question 11

How is alveolar hypoventilation managed?

Answer 11

Give oxygen

Question 12

What is shunt?

Answer 12

Shunt is when blood passes from the Right to Left side of the Heart WITHOUT interacting with ventilated alveoli

Question 13

How long does the equilibrium of gas across the blood air barrier usually take?

Answer 13

0.25 seconds

Question 14

How long is the capillary transit time (the speed of blood moving past an alveoli)?

Answer 14

0.75 seconds

Question 15

Explain how pneumonia causes hypoxaemia?

Answer 15

Pneumonia causes ventilation/perfusion mismatch due to bronchitis and bronchopneumonia which will respond well to increases in FIO2
Severe pneumonia can cause shunt due to a lobar pattern of consolidation

Question 16

Explain how COPD causes pneumonia

Answer 16

• Airflow limitation and obstruction results in a V/Q mismatch
• Emphysema results in a loss of alveolar surface area which causes a diffusion impairment
• The reduced respiratory drive observed in COPD patients results in alveolar hypoventilation
• Infective exacerbations of COPD (e.g. pneumonia) can cause shunt