Hypoxaemia and V/Q

Question 1

What are the causes of hypoxaemia in a patient breathing room air?

Answer 1

Shunt

V/Q mismatch

Hypoventilation

Diffusion limitation (e.g. altitude, exercise, pulmonary fibrosis)

(3 of 4 to pass)

Question 2

How does the ventilation/perfusion ratio change in different regions of the lung?

Answer 2

V/Q is lowest at the base of the lung, and highest at the apex of the lung. At the apex, the alveolar PO2 is as high as 130mmHg, and at the base, as low as 90mmHg. At the apex, PCO2 is as low as 30mmHg, and at the base, as high as 42mmHg.

Question 3

What is the effect of V/Q inequality on arterial PO2 and arterial PCO2?

Answer 3

Reduced PO2, but a relatively normal PCO2. This is due to the difference in the shapes of their dissociation curves: PO2 is non-linear, but PCO2 is linear. More CO2 can be eliminated simply by increasing ventilation.