Abstract
BackgroundDecreased airway lumen size and increased lung volume are major structural changes in chronic obstructive pulmonary disease (COPD). However, even though the outer wall of the airways is connected with lung parenchyma and the mechanical properties of the parenchyma affect the behaviour of the airways, little is known about the interactions between airway and lung sizes on lung function and symptoms. The present study examined these effects by establishing a novel computed tomography (CT) index, namely, airway volume percent (AWV%), which was defined as a percentage ratio of the airway tree to lung volume.MethodsInspiratory chest CT, pulmonary function, and COPD Assessment Tests (CAT) were analysed in 147 stable males with COPD. The whole airway tree was automatically segmented, and the percentage ratio of the airway tree volume in the right upper and middle-lower lobes to right lung volume was calculated as the AWV% for right lung. Low attenuation volume % (LAV%), total airway count (TAC), luminal area (Ai), and wall area percent (WA%) were also measured.ResultsAWV% decreased as the Global Initiative for Chronic Obstructive Lung Disease (GOLD) spirometric grade increased (p < 0.0001). AWV% was lower in symptomatic (CAT score ≥ 10) subjects than in non-symptomatic subjects (p = 0.036). AWV% was more closely correlated with forced expiratory volume in 1 s (FEV1) and ratio of residual volume to total lung capacity (RV/TLC) than Ai, Ai to lung volume ratio, and volume of either the lung or the airway tree. Multivariate analyses showed that lower AWV% was associated with lower FEV1 and higher RV/TLC, independent of LAV%, WA%, and TAC.ConclusionsA disproportionally small airway tree with a relatively large lung could lead to airflow obstruction and gas trapping in COPD. AWV% is an easily measured CT biomarker that may elucidate the clinical impacts of the airway-lung interaction in COPD.
Highlights
Decreased airway lumen size and increased lung volume are major structural changes in chronic obstructive pulmonary disease (COPD)
These smaller lumens of the airways reduce the number of airways visible on computed tomography (CT), and this impaired visibility is currently assessed as total airway count (TAC) [16], which has been shown to be associated with forced expiratory volume in 1 s (FEV1), dyspnoea, exercise tolerance and future lung function decline [16, 17]
There was no difference in height, the airway volume in the right lung (AWV) was smaller (9 vs 18 ml) and right lung volume was larger (2998 vs 2519 ml) in case B than Case A, which made a substantial difference in the AWV% between the two cases
Summary
Decreased airway lumen size and increased lung volume are major structural changes in chronic obstructive pulmonary disease (COPD). A small airway lumen area in COPD is an important CT finding that is closely correlated with airflow obstruction, as assessed by forced expiratory volume in 1 s (FEV1) [14, 15] and gas trapping, as assessed by residual volume (RV), or its ratio to total lung capacity (TLC) [RV/TLC] [7]. These smaller lumens of the airways reduce the number of airways visible on CT, and this impaired visibility is currently assessed as total airway count (TAC) [16], which has been shown to be associated with FEV1, dyspnoea, exercise tolerance and future lung function decline [16, 17]
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