[Role of expiratory muscles in COPD patients].

Expiratory muscle recruitment is common in stable COPD patients. However, physiological significance of expiratory muscle recruitment in COPD remains unclear. The purpose of this study was to assess the effect of expiratory muscle contraction on force generating ability of the diaphragm in COPD patients breathing at rest. The force generating ability of the diaphragm was evaluated from its pressure swing (Pdi) for a given diaphragm electrical activity (Edi), where Edi was normalized as % of its maximal value [Pdi/(Edi/Edimax)]. Phasic expiratory muscle contraction was measured as the total expiratory rise in gastric pressure (Pgaexp.rise). Nineteen patients with severe COPD, participated in the study but only 10 exhibited phasic rise in Pga during expiration with a mean Pgaexp.rise of 1.91 +/- 0.89 cmH2O. Patients were divided into passive expiration (PE) and active expiration (AE) groups. There was no significant difference in lung function and breathing pattern parameters between the two groups. Pdi/(Edi/Edimax) was 0.63 +/- 0.07 and 0.54 +/- 0.07 cm cmH2O/% in PE and AE groups, respectively, and was not significantly different between each other. Compared with PE group, AE group not only recruited expiratory muscles, but also preferentially recruited inspiratory rib cage muscles and derecruited the diaphragm. In conclusion phasic contraction of expiratory muscles at rest in COPD patients do not improve the force-generating ability of the diaphragm.