Background: We investigated whether a relationship between small airways dysfunction and bronchodilator responsiveness occurs in COPD patients. Methods: We studied 100 (20 F; mean age 68 yr ± 10) COPD patients (FEV₁: 55% pred ± 21; FEV₁/FVC: 53% ± 10) by impulse oscillometry system (IOS). Resistance at 5 and at 20 Hz (R5 and R20, in kPa s l⁻¹) and the fall in resistance from 5 to 20 Hz (R5-R20) were used as indices of total, proximal and peripheral airway resistance; reactance at 5 Hz (X5, in kPa s l⁻¹) was also measured. Significant response to bronchodilator (salbutamol 400 μg) was expressed as absolute (≥ 0.2 L) and percent (≥ 12%) change over pre-bronchodilator value of FEV₁ (Flow Responders, FR) and FVC (Volume Responders, VR). Results: 80 out of 100 patients had R5-R20 > 0.03 kPa sl^-1 (>upper normal limit) and, compared to patients with ≤ 0.030 kPa s l⁻¹, showed a poorer health status, lower values of FEV₁, FVC, FEV₁/FVC, X5 and higher values of RV/TLC and R20 (p<0.05 for all comparisons). Compared to the 69 no responders and the 8 FR, the 16 VR had significantly higher R5 and R5-R20 values (p<0.05), lower X5 values (p<0.05) and greater airflow obstruction and lung hyperinflation. Conclusions: This study shows that peripheral airway resistance, is increased in the vast majority of COPD patients. These patients showed an increased respiratory reactance, a worse spirometry, a more severe lung hyperinflation and a poorer health status. Small airway dysfunction was also associated to the bronchodilator responsiveness in terms of FVC, but not of FEV₁.

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