Differences in the Quantitative HRCT Characteristics of Patients with Asthma, COPD and Asthma–COPD Overlap and Their Relationships with Pulmonary Function

Purpose: We compared pulmonary function indices and quantitative CT parameters of airway remodeling, air trapping, and emphysema in asthmatic patients and patients with COPD and asthma–COPD overlap (ACO) and explored their relationships with airflow limitation.
Patients and Methods: Patients with asthma (n=48), COPD (n=52), and ACO (n=30) and controls (n=54) who completed pulmonary function tests and HRCT scans were retrospectively enrolled in our study. Quantitative CT analysis software was used to assess emphysema (LAA%), airway wall dimensions (wall area (WA), luminal area (LA), and wall area percentage (WA%)), and air trapping ((relative volume change of − 860 HU to − 950 HU (RVC_{− 860 to-950}) and the expiration-to-inspiration ratio of the mean lung density (MLD_E/I))). Differences in pulmonary function and HRCT parameters were compared among the groups. Spearman correlation analysis and regression analysis were utilized to explore structure‒function relationships.
Results: The LAA% in COPD and ACO patients was significantly greater than that in asthmatic patients and controls. The WA% and WA in COPD and ACO patients were greater than those in controls, whereas the WA% and LA between asthmatic patients and controls reached statistical significance. The RVC_{− 860 to − 950} levels decreased in the following order: ACO, COPD, and asthma. RVC_{− 860 to − 950} independently predicted FEV1% in asthmatic patients; LAA% and MLD_E/I in COPD patients; and LAA%, WA% and RVC_{− 860 to − 950} in ACO patients.
Conclusion: Comparable emphysema was observed in patients with COPD and ACO but not in asthmatic patients. All patients exhibited proximal airway remodeling. The bronchi were thickened outward in COPD and ACO patients but are thickened inward in asthmatic patients. Furthermore, air trapping in ACO patients was the most severe among all the groups. Indirect lung densitometry measurements might be more predictive of the degree of airflow limitation than direct airway measurements in obstructive airway diseases.

Keywords: asthma, COPD, asthma-COPD overlap, quantitative HRCT, structure-function relationship