Association between impaired diffusion capacity and small airway dysfunction: a cross-sectional study.

Background: Small airway dysfunction (SAD) and impaired diffusion capacity of the lungs for carbon monoxide (D _LCO) are positively associated with a worse prognosis. Individuals with both dysfunctions have been identified in clinical practice and it is unknown whether they have worse health status or need management. We conducted this study to explore the association between SAD and impaired D _LCO, and the difference between the groups with two dysfunctions, with either one dysfunction and with no dysfunction.

Methods: This study involved subjects partly from those who had returned for the third-year follow-up (up to December 2022) of the Early Chronic Obstructive Pulmonary Disease study and those who newly participated. We assessed diffusion capacity, questionnaire, exacerbations, spirometry, impulse oscillometry (IOS) and computed tomography (CT). Impaired D _LCO was defined as D _LCO <80% predicted. Spirometry-defined SAD was defined using the percent predicted values of maximal mid-expiratory flow, and forced expiratory flow at 50% and 75% of forced vital capacity, at least two of these three values being <65% predicted after the use of a bronchodilator. IOS-defined SAD was defined when the difference in resistance at 5 and 20 Hz was >0.07 kPa·L^-1·s. CT-defined SAD was defined when the percentage of expiratory low-attenuation areas <-856 HU comprised ≥15% of the total lung volume. Covariate analyses and logistic regression were performed to assess the association between impaired D _LCO and SAD.

Results: This study involved 581 subjects. The occurrence of both spirometry- and CT-defined SAD was significantly higher in subjects with impaired D _LCO than normal D _LCO. Subjects with two dysfunctions were associated with worse preceding year's exacerbations than controls.

Conclusions: Impaired diffusion capacity is positively associated with SAD. Subjects with impaired diffusion capacity and SAD may have a worse health status and need additional management.