Initial airway caliber
Consider three subjects whose airways have the same outer airway diameter, but the airway wall thickness internal to the airway smooth muscle increases from person 1 to person 3. At baseline, such differences in airway wall thickness will mostly go undetected by pulmonary function tests. Their airways are challenged, e.g. with histamine or methacholine. We now compare the effects for the same smooth muscle shortening. We depict an identical decrease in airway circumference. For the same smooth muscle shortening the airway lumen decreases more if the initial airway has a thicker wall internal to the airway smooth muscle. Thus a smaller initial airway calibre leads to a more pronounced increase in airflow limitation upon airway challenge. This may lead to the misconception that a more pronounced airflow limitation in response to bronchial challenge signifies greater smooth muscle shortening. From geometrical considerations this conclusion is unwarranted.
This means that patients with narrow airways (mucus, edema, infiltration, cell hyperplasia and cell hypertrophy) may appear to be abnormally reactive to stimuli, when in fact (part of) this may be explained by their small initial airway calibre.