Airway Remodeling in Asthma: Evaluation in 5 Consecutive Bronchial Generations by Using High-Resolution Computed Tomography

BACKGROUND

Airway remodeling is a characteristic structural change that occurs extensively in the airways of patients with asthma. The change can be evaluated by measuring airway dimensions by using high-resolution computed tomography. This study aimed to explore the variation trends of airway dimensions from the second- to sixth-generation bronchi in subjects with asthma.

METHODS

Sixty subjects with asthma and 40 healthy controls underwent high-resolution computed tomography. The right upper lobe apical segmental bronchus, right lower lobe posterior basal segmental bronchus, left upper lobe apicoposterior segmental bronchus, and the left lower lobe posterior basal segmental bronchus were identified on computed tomography images. The luminal area, wall area, and wall area percentage of each bronchus were measured from the second (lobar) to the sixth generation. The variation trends of these indices (luminal area, wall area, and wall area percentage) were then compared between the subjects with asthma and the healthy controls.

RESULTS

From the proximal to the distal airway, the luminal area and wall area decreased gradually, whereas the wall area percentage increased gradually in all the participants; the difference between the participants with asthma and the healthy controls in the luminal area, wall area, and wall area percentage increased gradually from the second (lobar) to the sixth generation; there was a significant difference between the participants with asthma and the healthy controls in the right lower lobe posterior basal segmental bronchus, left upper lobe apicoposterior segmental bronchus, and left lower lobe posterior basal segmental bronchus wall area percentage (P = .001, .01 and .001, respectively) but not in the right upper lobe apical segmental bronchus wall area percentage (P = .050).

CONCLUSIONS

Airway remodeling in asthma was more prominent in distal airways and at lower lobe bronchi.

Computed Tomographic Airway Morphology in Chronic Obstructive Pulmonary Disease. Remodeling or Innate Anatomy?

Computed tomographic measures of central airway morphology have been used in clinical, epidemiologic, and genetic investigation as an inference of the presence and severity of small-airway disease in smokers. Although several association studies have brought us to believe that these computed tomographic measures reflect airway remodeling, a careful review of such data and more recent evidence may reveal underappreciated complexity to these measures and limitations that prompt us to question that belief. This Perspective offers a review of seminal papers and alternative explanations of their data in the light of more recent evidence. The relationships between airway morphology and lung function are observed in subjects who never smoked, implying that native airway structure indeed contributes to lung function; computed tomographic measures of central airways such as wall area, lumen area, and total bronchial area are smaller in smokers with chronic obstructive pulmonary disease versus those without chronic obstructive pulmonary disease; and the airways are smaller as disease severity increases. The observations suggest that (1) native airway morphology likely contributes to the relationships between computed tomographic measures of airways and lung function; and (2) the presence of smaller airways in those with chronic obstructive pulmonary disease versus those without chronic obstructive pulmonary disease as well as their decrease with disease severity suggests that smokers with chronic obstructive pulmonary disease may simply have smaller airways to begin with, which put them at greater risk for the development of smoking-related disease.

Computed tomographic measures of airway morphology in smokers and never-smoking normals

Bronchial wall area percent (WA% = 100 × wall area/total bronchial cross sectional area) is a standard computed tomographic (CT) measure of central airway morphology utilized in smokers with chronic obstructive pulmonary disease (COPD). Although it provides significant clinical correlations, the range of reported WA% is narrow. This suggests limited macroscopic change in response to smoking or that remodeling proportionally affects the airway wall and lumen dimensions such that their ratio is preserved. The objective of this study is to assess central airway wall area (WA), lumen area (Ai), and total bronchial area (Ao) from CT scans of 5,179 smokers and 92 never smoking normal subjects. In smokers, WA, Ai, and Ao were positively correlated with forced expiratory volume in 1 s (FEV1) expressed as a percent of predicted (FEV1%), and the WA% was negatively correlated with FEV1% (P < 0.0001 for all comparisons). Importantly, smokers with lower FEV1% tended to have airways of smaller cross-sectional area with lower WA. The increases in the WA% across GOLD stages of chronic obstructive pulmonary disease (COPD) can therefore not be due to increases in WA. The data suggest two possible origins for the WA% increases: 1) central airway remodeling resulting in overall reductions in airway caliber in excess of the decreased WA or 2) those with COPD had smaller native airways before they began smoking. In both cases, these observations provide an explanation for the limited range of values of WA% across stages of COPD.