Ventilation-perfusion relationships

Question 1

What are the natural and healthy bypasses of the lungs that should exist?

Answer 1

The drainage of the bronchial veins and Thebesian veins.

Question 2

How does a right to left shunt form?

Answer 2

A ventricular septal defect will initially cause an increased blood flow from the left hand side of the heart to the right hand side of the heart as there is more muscle present on the LHS then the right so the pressure gradient move blood from left to right. Over time however there is hypertrophy of the muscle in the right ventricle so there eventually blood gets pushed from the right hand side of the heart to the LHS as there is more muscle present on the RHS now. This creates a bypass by which the blood can avoid the lungs.

Question 3

How does a R->L shunt affect pO2 and pCO2

Answer 3

There is a slight increase in the CO2 content of the blood as a result of the shunt. The corresponding pCO2 then is slightly increased.
There is a slight decrease in the O2 content of the blood but there is a large decrease in the pO2 because of the shape of the oxygen dissociation curve of O2. This means there is hypercapnia and hypoxia.

Question 4

How does a R->L shunt affect ventilation?

Answer 4

Increased pCO2 and decreased pO2 (hypercapnia and hypoxia) both increase ventilation synergistically

Question 5

How does ventilation and perfusion vary around the lung?

Answer 5

Ventilation is better higher up, and perfusion is better lower down.

Question 6

How do pulmonary capillaries react to hypoxia and why?

Answer 6

They undergo vasoconstriction, which is good because if there is localised hypoxia the bloodflow is diverted to areas with better ventilation. But if the hypoxia is global it just makes things worse. Then there is a much greater pressure required from the heart, leading to right heart failure.

Question 7

What are the mechanisms that lead to arterial hypoxia?

Answer 7

Low inspired oxygen (like at altitude)
Hypoventilation
Diffusion impairment
R-L shunt
Ventilation-perfusion mismatching (most common)

Question 8

What is the value of typical minute ventilation

Answer 8

500ml x 15 = 7500ml/min

Question 9

What is the value of typical alveolar ventilation

Answer 9

5250ml/min

roughly matched to pulmonary blood flow of 5000ml/min