CAAT-Defined chronic bronchitis is associated with small airway dysfunction in moderate to very severe COPD: insights from a real-world cohort

Background Chronic bronchitis (CB) and small airway dysfunction (SAD) are each linked to exacerbations and disease progression in COPD, but their interrelationship remains incompletely understood. Research question Is chronic bronchitis associated with SAD in moderate-to-very severe COPD? Methods In this real-world cohort, 102 consecutively recruited patients with GOLD 2–4 COPD underwent clinical characterization, spirometry, and impulse oscillometry (IOS). CB was defined by the Chronic Airways Assessment Test (CAAT) and SAD severity was assessed using a composite score based on IOS z-scores of R5, AX, and sign-reversed X5. Results Among 102 patients, 46 were CB+ and 56 CB−. FEV 1 % predicted was similar between groups (38.7 versus 37.1; p=0.45), whereas CB+ patients had more exacerbations during the previous year (2 versus 0.5; p=0.04) and higher CAAT scores (30 versus 19.5; p<0.001). Several IOS indices indicated greater impairment in CB+, particularly reactance parameters and composite SAD severity. CB was associated with increased odds of moderate-to-severe SAD in unadjusted analysis (OR 2.64, 95% CI 1.02–7.24) and after adjustment for age, sex, current smoking, BMI, FEV 1 % predicted, and previous exacerbation history (adjusted OR 4.47, 95% CI 1.40–16.67). Within CB+, IOS correlated with the modified CAAT scores (ρ=0.39–0.51; all p<0.01) and poorer 1-minute sit-to-stand performance (ρ=−0.35 to −0.41; p=0.035 to<0.01). CB+ patients with moderate-to-severe SAD were more likely to have previous COPD exacerbations than those with no/mild SAD (RR 1.90, 95% CI 1.09–5.40). Interpretation CAAT-defined CB in moderate-to-very severe COPD is independently associated with more pronounced IOS-defined SAD. In patients with CB, greater SAD relates to higher symptom burden, poorer functional performance, and increased exacerbation risk based on historical data. These findings suggest that SAD detected by IOS is clinically relevant in patients with COPD and CB.