Expiratory muscle pressure and breathing mechanics in chronic obstructive pulmonary disease

Expiratory muscle recruitment is common in stable chronic obstructive pulmo nary disease (COPD) patients. Due to airway obstruction, there is little re ason to believe that active expiration in COPD would be mechanically effect ive in lowering operating lung volume. The physiological significance of ex piratory muscle recruitment in COPD, therefore, remains unknown, The purpos e of this study was to assess, in COPD patients breathing at rest, the effe ct of expiratory muscle contraction on force generating ability of the diap hragm, The force generating ability of the diaphragm was evaluated from its pressu re swing (Pdi) far a given diaphragm electrical activity (Edi), where Edi w as normalized as % of its maximal value (Pdi/Edi/Edi,max). Phasic expirator y muscle contraction was measured as the total expiratory rise in gastric p ressure (Pga,exp.rise). Nineteen seated patients with moderate to severe COPD, participated in the study and 10 exhibited phasic rise in Pga during expiration with a mean Pga ,exp.rise of 1.91+/-0.89 cmH(2)O, The patients were thus divided into passi ve expiration (PE) and active expiration (AE) groups. There was no signific ant difference in various lung function and breathing pattern parameters be tween the two groups. Pdi/Edi/Edi,max was 0.63+/-0.07 and 0.54+/-0.07 cmH(2 )O/% in PE and AE groups, respectively, and was not significantly different between each other. Compared with PE group, AE group not only recruited ex piratory muscles, but also preferentially recruited inspiratory rib cage mu scles and derecruited the diaphragm, The results do not support a significant improvement of the force-generatin g ability of the diaphragm by phasic contraction of expiratory muscles at r est in chronic obstructive pulmonary disease patients.