Abnormal pulmonary gas exchange

Question 1

What is normal PaO2 and PaCO2 ?

Answer 1

Normal PaO2- 10.5 - 13.5 kPa

Normal PaCO2- 4.8-6 kPa

Question 2

What is type I and type II respiratory failure ?

Answer 2

Type I- PaO2 <8kPa (PaCO2 normal or low)

Type II PaCO2 >6.5kPa (PaO2 usually low)

Question 3

What are the 4 abnormal states associated with Abnormal pulmonary gas exchange ?

Answer 3

Alveolar hypoventilation
Shunt
Ventilation / Perfusion Imbalance
Diffusion Impairment

Question 4

What is the result of alveolar hypoventilation on PACO2 and PAO2 ?

Answer 4

Hypoventilation increases PACO2 and thus increases PaCO2
Increases in PACO2 decreases PAO2, which cause PaO2 to fall
Fall in PaO2 due to hypoventilation is corrected by raising FIO2
(FIO2= the fraction of inspired air which is oxygen )

Question 5

How can you correct a fall in PaO2 due to hypoventilation ?

Answer 5

Raising the FIO2

Question 6

Why is normal V/Q ratio 0.8 ?

Answer 6

Normally breathing 4 l/min

CO is 5 l/min so normal V/Q is 4/5 or 0.8

Question 7

What is the commonest cause of hypoxaemia clinically ?

Answer 7

Low V/Q

Question 8

How does low V/Q sometimes arise in alveoli ?

Answer 8

Due to local alveolar hypoventilation due to some, focal disease

Question 9

How can you treat hypoxaemia due to low V/Q ?

Answer 9

Increase in FIO2

Question 10

Will Interstitial lung disease affect CO2 levels ?

Answer 10

Not usually because CO2 diffuses 20 times faster than O2

Question 11

How does diffusion impairment affect diffusing speed of oxygen ?

Answer 11

Diffusion impairment means it takes LONGER for blood and alveolar air to equilibrate, particularly for oxygen

Question 12

Why does diffusion impairment result in a fall in Pa O2 during exercise ?

Answer 12

Capillary transit time decreases during exercise, less time for equilibration

Question 13

Why don’t we have 100% haemoglobin saturation ?

Answer 13

Because we have a 2-4% shunt

Question 14

When do we see a pathological shunt ?

Answer 14

Alveolar ventilation malformations, congenital heart disease and pulmonary disease

Question 15

Does large shunt respond well to increase in FIO2 ?

Answer 15

No, blood leaving normal sections of the lung is already 98% saturated

Question 16

Why is there hypoxaemia in pneumonia ?

Answer 16

Ventilation perfusion mismatch:
    - bronchitis/ bronchopneumonia 
Shunt:
    - severe bronchopneumonia 
    - lobar pattern with large areas of consolidation

Question 17

Why is the hypoxaemia in COPD ?

Answer 17

V/Q mismatch- airway obstruction +/- bronchopneumonia
Diffusion impairement- loss of alveolar surface area in emphysema
Alveolar hypoventilation- reduced respiratory drive
Shunt- only during acute exacerbation if pneumonia us extensive enough