Pulmonary Diffusing Capacity: Disease (PK4)

Question 1

What is the diffusion constant?

Answer 1

A combination of diffusibility (d), area (A) and thickness (T)

Question 2

Equation for diffusion capacity using the diffusion constant

Answer 2

DL = V̇ / (P1-P2)

Question 3

Why can’t diffusion capacity of O2 be easily measured?

Answer 3

PO2 changes along pulmonary capillary, so no fixed value for P2

Question 4

What gas is used to calculate diffusion capacity instead of O2 and why?

Answer 4

• Carbon monoxide
• High affinity for haemoglobin, so P2 is effectively 0

Question 5

Normal DLO2 value

Answer 5

ca. 220 ml min-1 kPa-1

Question 6

Factors increasing pulmonary diffusing capacity

Answer 6

• Body size
• Exercise
• Lying down

Question 7

Factors reducing pulmonary diffusing capacity

Answer 7

• Factors reducing effective surface area:
  > Loss of lung tissue
  > Airway obstruction
  > Capillary obstruction
  > Ventilation/perfusion mismatch
• Factors increasing diffusion path length:
  > Thickened alveolar-capillary membrane (eg. fibrosis)
  > Accumulation of lung fluid
  > Increased intracapillary distance (eg. low haemoglobin)