Association between Functional Small Airway Disease and FEV1 Decline in Chronic Obstructive Pulmonary Disease.

Background: The small conducting airways are the major site of airflow obstruction in COPD and may precede emphysema development. We hypothesized a novel CT biomarker of small airways disease predicts FEV1 decline. Methods: We analyzed 1,508 current and former smokers from COPDGene with linear regression to assess predictors of change in FEV1 (ml/year) over 5 years. Separate models for non-obstructed and obstructed subjects were generated using baseline clinical and physiologic predictors in addition to two novel CT metrics created by Parametric Response Mapping (PRM), a technique pairing inspiratory and expiratory CT images to define emphysema (PRMemph) and functional small airways disease (PRMfSAD), a measure of non-emphysematous air trapping. Results: Mean (SD) rate of FEV1 decline in ml/year for GOLD 0-4 was as follows: 41.8 (47.7), 53.8 (57.1), 45.6 (61.1), 31.6 (43.6), and 5.1 (35.8) respectively (trend test for grades 1-4, p<0.001). In multivariable linear regression, for non-obstructed participants, PRMfSAD but not PRMemph was associated with FEV1 decline, p<0.001. In GOLD 1-4 participants, both functional small airways disease (PRMfSAD) and emphysema (PRMemph) were associated with FEV1 decline (p<0.001 and p=0.001, respectively). Based on the model, the proportional contribution of the two CT metrics to FEV1 decline, relative to each other, was 87% vs. 13% and 68% vs. 32% for PRMfSAD and PRMemph in GOLD 1/2 and 3/4, respectively.

Conclusions: Both CT assessed functional small airways disease and emphysema are associated with FEV1 decline, but the association with functional small airways disease has greatest importance in mild-to-moderate stage COPD where the rate of FEV1 decline is the greatest.