Air trapping without emphysema as a marker of small airway dysfunction in severe asthma: A quantitative high-resolution computed tomography study

RATIONALE Small airway dysfunction (SAD) contributes to airflow limitation in severe asthma. Functional fraction of air trapping (fAT) on high-resolution computed tomography (HRCT) may serve as an imaging marker of SAD, but the contribution of its components to airflow limitation and longitudinal changes in small airway function remains unclear.OBJECTIVE The objective of this study was to evaluate whether fAT is associated with reduced FEF25–75% and to identify which fAT components are related to longitudinal FEF25–75% change.METHODS HRCT-derived airway wall area percentage (WA%), emphysema, fAT and fAT components were retrospectively assessed in 149 patients with asthma stratified by FEV1% predicted: ≥100% (n = 39), 80–99% (n = 55), 60–79% (n = 39) and <60% (n = 16). fAT components included fAT without emphysema (fAT^wtEmp), fAT with emphysema (fAT^wEmp) and emphysema without fAT (Emp^wtfAT). Longitudinal FEF25–75% change was assessed in 10 patients.MAIN RESULTS WA% increased with declining FEV1 and was greatest at the segmental level. Among 138 patients with parenchymal imaging data, fAT^wtEmp was the predominant component (median 6.09%), compared with Emp^wtfAT (1.06%) and fAT^wEmp (0.07%). Total fAT, fAT^wtEmp, and fAT^wEmp were highest in patients with FEV1 < 60%, whereas emphysema and Emp^wtfAT did not differ across groups. FEF25–75% correlated inversely with fAT^wtEmp and fAT^wEmp. Baseline fAT^wtEmp was associated with longitudinal FEF25–75% change.CONCLUSIONS fAT^wtEmp was associated with small airway dysfunction and subsequent changes in FEF25–75%. Prospective studies with repeated HRCT are needed to determine whether this imaging abnormality is reversible.