Assessment of proximal and peripheral airway dysfunction by computed tomography and respiratory impedance in asthma and COPD patients with fixed airflow obstruction

OBJECTIVE: To ascertain: (i) if elderly patients with fixed airflow obstruction (FAO) due to asthma and chronic obstructive pulmonary disease (COPD) have distinct airway morphologic and physiologic changes; (ii) the correlation between the morphology of proximal/peripheral airways and respiratory impedance. METHODS: Twenty-five asthma cases with FAO and 22 COPD patients were enrolled. High-resolution computed tomography was used to measure the wall area (WA) and lumen area (LA) of the proximal airway at the apical segmental bronchus of the right upper lobe (RB1) adjusted by body surface area (BSA) and bronchial wall thickening (BWT(r)) of the peripheral airways and extent of expiratory air trapping (AT(exp)). Respiratory impedance included resistance at 5 Hz (R(5)) and 20 Hz (R(20)) and resonant frequency (Fres). Total lung capacity (TLC) and residual volume (RV) were measured. RESULTS: Asthma patients had smaller RB1-LA/BSA than COPD patients (10.5 ± 3.4 vs. 13.3 ± 5.0 mm(2)/m(2), P = 0.037). R(5)(5.5 ± 2.0 vs. 3.4 ± 1.0 cmH(2)O/L/s, P = 0.02) and R(20)(4.2 ± 1.7 vs. 2.6 ± 0.7 cmH(2)O/L/s, P = 0.001) were higher in asthma cases. AT(exp) and BWT(r) were similar in both groups. Regression analysis in asthma showed that forced expiratory volume in one second (FEV(1)) and Fres were associated with RB1-WA/BSA (R(2)= 0.34, P = 0.005) and BWT(r) (0.5, 0.012), whereas RV/TLC was associated with AT(exp) (0.38, 0.001). CONCLUSIONS: Asthma patients with FAO had a smaller LA and higher resistance of the proximal airways than COPD patients. FEV(1) and respiratory impedance correlated with airway morphology.