Question 271

Question:

Scanning of inhaled, radioactively labeled aerosols has been used to study the distribution of pulmonary ventilation. However, data obtained using aerosols with a particle diameter of 20 micrometers do not accurately reflect the distribution of alveolar ventilation in the lungs. Which of the following is the best explanation for this finding?

Correct Answer: D.  Most particles this size are deposited in the conducting airways

The test is attempting to measure the alveolar ventilation, or the amount of gas reaching the alveoli that is capable of gas exchange. The 20 micrometer particles are not arriving as they are too large to pass through the 10+ levels of conducting airways.

One can reach this answer through elimination of the others:
A – macrophage ingestion takes a long time, and would not affect a quick scan such as this. There is no evidence that nonuniform ingestion, if it exists, can explain the problem with this aerosol.

B – If the particles adhered to the walls, the more distal parts of the lung would be less ventilated by the particles, which would exacerbate the problem with the aerosol.

C – This would happen uniformly throughout the lung, which would not affect the measurement of alveolar ventilation.

E – The mucociliary system is exceedingly slow – it is an “elevator.” This alone cannot explain the difficulty with the aerosol.