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

¹ TBM Lab, Dipartimento di Bioingegneria, Politecnico di Milano University, Milano, Italy
² University Hospital Aintree, University Department of Medicine, Liverpool, UK

^* To whom correspondence should be addressed. E-mail: raffaele.dellaca{at}polimi.it.

	Abstract

Bronchodilator (BD) drugs produce variable improvements in FEV₁ but larger changes in end-expiratory lung volume (EELV) in 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%pred) during spontaneous breathing before and after 5mg nebulised salbutamol. EFL was detected by within-breath changes in respiratory system reactance measured by a multi-frequency forced oscillation method while changes in EELV were assessed by inspiratory capacity (IC). BD increased IC (from 1.8±0.5L to 2.1±0.6L, p<0.001) and reduced inspiratory resistance (Rinsp) at 5Hz (from 5.1±1.6 to 4.2±1.5cmH₂O*s·L^-1, p<0.001). Rinsp identified BD responders with a discriminative power of 80.1%. Twenty patients were flow-limited before BD. They showed worse spirometry and higher RV, but significant improvements in IC were seen in all patients irrespective of flow-limitation. Changes in Rinsp were confined to flow-limited patients as were reactance changes. BD reduced the degree of heterogeneity in respiratory system, a change best seen with inspiratory values. BD have 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.