Physiology Gas Exchange

Question 1

What is the formula indicating a gas exchange problem is present?

Answer 1

A-a gradient
Should be less than 10mmHg
If increased, a gas exchange problem is present - don’t get appropriate loading of oxygen into arterial blood

Question 2

What are possible causes for a gas exchange problem?

Answer 2

Impaired diffusion
Shunt
V/Q mismatch

Question 3

What are the causes of impaired diffusion?

Answer 3

Thickening of the diffusion barrier
Decrease in the surface area
Exercise
Breathing low oxygen mix

Question 4

Describe why significant impairment of the diffusion of oxygen is needed before there is a reduction of PaO2 at rest.

Answer 4

Each erythrocyte spends 0.74s in the pulmonary capillaries.
Equilibration between alveolar gas and capillary blood occurs within 0.25s.
Equilibration occurs even when transit time is reduced.

Question 5

How do we know if there is a problem with oxygen diffusion?

Answer 5

Problems with exercise.

Question 6

Describe the equilibrium between alveolar and capillary PaCO2.

Answer 6

Takes 0.25s
Carbon dioxide diffusion is rarely impaired.
More diffusible than oxygen

Question 7

Which diffusion would first have issues?

Answer 7

Diffisuion of oxygen

Question 8

Describe the issue surrounding shunting.

Answer 8

Some blood goes not get oxygenated - lowers the PaO2.

Question 9

What is a shunt?

Answer 9

Blood that re-enters the arterial system without going through the ventilated areas of the lung

Question 10

What can be causes of a shunt?

Answer 10

Anatomical - e.g. cardiac defect, bronchial artery blood, myocardial blood via Thebesian veins
Pathological - e.g. pneumonia - alveoli full of exudate (pus) so not ventilated. Blood still perfuses these alveoli, but not oxygenated.

Question 11

State the important factors to note about a shunt causing hypoxaemia.

Answer 11

Shunted blood is essentially venous blood which dilutes the oxygenated arterial blood and reduces the PaO2.
A-a gradient is increased by the shunt.
Shunt responds poorly to additional oxygen.
Shunt size can be calculated from the shunt equation.

Question 12

Does impaired diffusion respond well to increased oxygen?

Does a shunt respond well to increased oxygen? Y

Answer 12

Yes - an oxygen diffusion problem does.

No - a shunt does not.

Question 13

What determines the PO2 of the gas in each alveolus?

Answer 13

Ratio of ventilation and perfusion to that alveolus.
Anytime ventilation and perfusion aren’t matched, gas exchange will be altered leading to an abnormal A-a gradient.
= Wasted ventilation/perfusion

Question 14

When might non-uniform distribution of alveolar ventilation and perfusion occur?
ALVEOLAR VENTILATION ISSUES

Answer 14

Uneven resistance to airflow

Non-uniform compliance in different parts of the lung

Question 15

What are the causes of uneven resistance to airflow?

Answer 15

Airway collapse e.g. emphysema, asthma, bronchitis, compression by tumours or oedemas

Question 16

What are the causes of uneven compliance?

Answer 16

Fibrosis, regional variation in surfactant production, pulmonary congestion or oedema, emphysema, atelectasis (collapse of lung tissue), pneumothorax or compression by tumours or cysts

Question 17

Why might non-uniform distribution of alveoli ventilation and perfusion occur?
PERFUSION

Answer 17

Embolization or thrombosis 
Compression of the pulmonary vessels by high alveolar pressures 
Tumours
Exudates
Pneumothorax 
Destruction/occlusion of pulmonary vessels by various diseases 
Pulmonary vascular hypotension 
Collapse or overexpansion of alveoli

Question 18

Explain why V/Q mismatch occurs in normal lungs.

Answer 18

Both ventilation and perfusion increase from the lung apex to the base.
Change in perfusion is greater than the change in ventilation.
A distribution of V/Q ratios exist even in a normal lung

Question 19

Describe the perfusion status of alveoli at different parts of the lung.

Answer 19

Alveoli at apex are underperfused/overventilated

Alveoli at base are underventilated/overperfused.

Question 20

What causes V/Q mismatch.

Answer 20

Any disease that alters ventilation or perfusion (arterial blood not fully oxygenated)

Question 21

Is a V/Q mismatch responsive to supplementary oxygen?

Answer 21

Yes
Enriching air in low volumes
Alveoli can oxygenate blood - do not get wasted perfusion

Question 22

List the factors that cause arterial hypoxaemia.

Answer 22

Reduced PB or FIO2
Hypoventilation
Impaired diffusion 
Shunt 
V/Q mismatch