Role of small airways in asthma: Investigation using high-resolution computed tomography

BackgroundSmall airways may have an important role in asthma but are more difficult to assess pathologically than central airways. Computed tomographic indices of lung density are assumed to reflect air trapping and may be a useful noninvasive measure of small airways disease, but their pathophysiological relevance remains undetermined.ObjectiveTo evaluate lung density on high-resolution computed tomography and examine its correlations with clinical and physiologic variables in 29 patients with stable asthma.MethodsBoth lungs were scanned at full-inspiratory and full-expiratory phases to quantify percentage of lung field occupied by low attenuation area (LAA%; < −960 Hounsfield units) and mean lung density. Asthma severity, pulmonary function, methacholine airway sensitivity and reactivity, and sputum eosinophil counts were evaluated.ResultsThe mean lung density increased and LAA% decreased in all patients at expiratory phase compared with inspiratory phase. The inspiratory density indices and expiratory mean lung density correlated only with FEV1/forced vital capacity (FVC). Expiratory LAA% correlated more strongly than other variables with FEV1/FVC and with indices of peripheral airflow obstruction. Expiratory/inspiratory ratios of LAA% and mean lung density correlated, the former more strongly, with disease severity, residual volume/total lung capacity, and airway sensitivity, as well as with indices of global (FEV1 and FEV1/FVC) and peripheral airflow obstruction.ConclusionExpiratory/inspiratory high-resolution computed tomography is useful for assessing small airways disease in asthma. Small airways involvement is associated with airflow obstruction, airway hypersensitivity, and more severe disease.Clinical implicationsSmall airways are an important therapeutic target in asthma.