Asthma diagnosis and treatment
Role of small airways in asthma: Investigation using high-resolution computed tomography

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Background

Small 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.

Objective

To evaluate lung density on high-resolution computed tomography and examine its correlations with clinical and physiologic variables in 29 patients with stable asthma.

Methods

Both 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.

Results

The 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.

Conclusion

Expiratory/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 implications

Small airways are an important therapeutic target in asthma.

Section snippets

Subjects

We studied 29 patients with asthma diagnosed according to the American Thoracic Society criteria.24 At study entry, all patients were clinically stable and had not had disease exacerbations or respiratory infections for at least 1 month. Severity of asthma was mild persistent (step 2) in 10 patients, moderate persistent (step 3) in 13, and severe persistent (step 4) in 6.25 The quantitative index of asthma severity was defined according to this step classification (2, 3, or 4) of Global

Characteristics of patients with asthma

The characteristics of the subjects, including CT indices, are presented in Table I. On expiratory CT scans, LAA% decreased and MLD increased in all patients compared with the values on inspiratory scans (P < .0001 for both LAA% and MLD). All patients had received inhaled corticosteroids at daily doses equivalent to 800 μg (400-1600 μg) chlorofluorocarbon-beclomethasone dipropionate.

Relations between HRCT indices and clinical indices

Table II shows the coefficients of correlations between HRCT indices and clinical indices. All LAA% and MLD

Discussion

In the current study, we assessed LAA% and MLD on inspiratory and expiratory HRCT scans and calculated the expiratory/inspiratory ratios of these variables in patients with stable asthma. To our knowledge, this is the first study to examine the relations of lung density on CT with airway inflammation and airway responsiveness. Inspiratory LAA% and MLD weakly correlated only with FEV1/FVC. Expiratory LAA% and MLD more strongly correlated with FEV1/FVC, and the former also correlated with indices

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    Disclosure of potential conflict of interest: The authors have declared that they have no conflict of interest.

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