V/Q mismatch

Question 1

Describe V/Q. (2)

Answer 1

Alveolar ventilation / perfusion rate = 1

Question 2

Describe the causes of V/Q mismatch. (6)

Answer 2

Reduced ventilation in parts of the lung: p(CO2) rises, p(O2) falls, and a new steady state (diffusion state) is set up. Eg asthma, COPD, RDS, Pneumonia.
Reduced perfusion in parts of the lung: obstruction to blood vessel (eg PE) so blood is redirected to other parts of the circulation, which increases perfusion here, meaning some bits need increased ventilation and others need reduced.

Question 3

Describe the concept of mixed blood and how you work out the p(O2) and p(CO2) of it. (5)

Answer 3

The blood from a mismatched bit would have less O2 and more CO2 in it.
Normal blood would have normal both.
When we mix them, we get a sort of muddle ground, worked out on the graphs by dividing the gap on the y axis (content) and extrapolating to x.

Question 4

Explain why hyperventilation would not help a patient who’s mixed blood had too little p(O2) in it. (3)

Answer 4

Because the mismatched bit would not increase, and the normal bit can not increase due to the saturation of Hb normally.
Would just lower normal p(CO2) which would lower mixed p(CO2).