Effect of bronchodilation on expiratory flow limitation and resting lung mechanics in COPD

Bronchodilator drugs produce variable improvements in forced expiratory volume in 1 s (FEV1), but larger changes in end-expiratory lung volume (EELV) in chronic obstructive pulmonary disease (COPD), which were suggested to be related to the presence of expiratory flow limitation (EFL) at rest. We tested this concept in 42 COPD patients (FEV1 42.3±13.8% predicted) during spontaneous breathing before and after 5 mg nebulised salbutamol. EFL was detected by within-breath changes in respiratory system reactance measured by a multifrequency forced oscillation method, while changes in EELV were assessed by inspiratory capacity (IC). Bronchodilation (BD) increased IC (from 1.8±0.5 to 2.1±0.6 L, p<0.001) and reduced inspiration resistance (R̄insp) at 5 Hz (from 5.1±1.6 to 4.2±1.5 cmH2O·s·L−1, p<0.001). R̄insp identified BD responders with a discriminative power of 80.1%. In total, 20 patients were flow-limited before BD. They showed worse spirometry and higher residual volume, but significant improvements in IC were seen in all patients irrespective of flow limitation. Changes in R̄insp were confined to flow-limited patients, as were reactance changes. BD reduced the degree of heterogeneity in the respiratory system, a change best seen with inspiratory values. BD has complex effects on lung mechanics in COPD, and EFL affects both this and the response of some respiratory variables to treatment. However, changes in EELV are consistently seen, irrespective of the presence of flow limitation at rest.

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