L11 (pk5) - Ventilation and perfusion

Question 1

What is the average PAaO2 difference (alveolar-arterial)?

Answer 1

13kPa - 12kPa = 1kPa

Question 2

What are the two main causes of the alveolar-arterial PO2 difference?

Answer 2

1. Shunts

2. Ventilation-perfusion mismatch (V/Q)

Question 3

Why is there a ventilation-perfusion mismatch?

Answer 3

Lungs receive approx:
- 4L/min alveolar ventilation
- 5 L/min blood flow
Therefore ventilation/ perfusion ratio is 
V/Q = 4/5 
= 0.8
So, V and Q are not evenly distributed

Question 4

What will happen to the Aa PO2 difference with age?

Answer 4

The difference will inc as age inc as V/Q changes to around 2.3kPa at 60 years

Question 5

What’s the general rule relating to how much the Aa difference of PO2 changes with age?

Answer 5

(Age/30) + 0.3kPa

Question 6

How much do the shunt and V/Q mismatch contribute to the small Aa PO2 difference?

Answer 6

Contribute equally

Question 7

What is the Thebesian veins (venae cordis minimae)?

Answer 7

• Natural R-L shunt
• Numerous small valve-less venous channels that open directly into the chambers of the heart from the capillary bed in the cardiac wall
• A form of collateral circulation
• Smallest cardiac veins in the walls of all four heart chambers

Question 8

What is the bronchial circulation?

Answer 8

• Arise from the thoracic aorta
• 70% of bronchial blood flow supplies the intrapulmonary bronchi/oles
• The bronchopulmonary veins JOIN the pulmonary vein and drain into the left atrium

Question 9

What will happen if you increase the shunt fraction?

Answer 9

Increase the Aa difference –> inc deoxygenation of arterial blood (makes arterial blood more venous)
- Give rise to symptoms of breathlessness

Question 10

What is an example of a disease caused by pathological R-L shunts?

Answer 10

Pulmonary disease

Question 11

What is pulmonary disease?

Answer 11

• Airway block (foreign object/ mucus)
• Collapsed bronchi/ alveoli
• Prevent airflow downstream of block
• If blood flows to an area with no ventilation –> a R-L shunt created –> v/q falls from normal value to 0 (a total shunt)

Question 12

What are cardiovascular shunts?

Answer 12

• Anatomical abnormalities
• Often in neonates (congenital defects)
• L-R shunts instead, due to the holes in the heart and patent ductus allows blood to pass from higher pressure left atrium to pass into lower pressure right atrium
• Oxy move to deoxy areas
• Load of right heart
• Consequence on pulmonary circ
• Atrial and ventricular septal defects