Inspiratory muscle mechanics of patients with chronic obstructive pulmonary disease during incremental exercise

Inspiratory muscles are weak and contribute to exercise limitation in chronic obstructive pulmonary disease (COPD). Differential inspiratory pressure contributions from the diaphragm and inspiratory rib cage muscles (RCMs) during exercise in patients with COPD patients are insufficiently described. We measured, in 16 patients with COPD, the global inspiratory muscle pressure (delta Pmus) and transdiaphragmatic pressure (delta Pdi) during an incremental bicycle exercise to exhaustion. The pressures needed to overcome the elastic load were further partitioned into portions for overcoming the PEEPi-imposed inspiratory threshold load (before the beginning of inspiratory flow) and for inflating the respiratory system (between the beginning and end of inspiratory flow). The delta Pdi/delta Pmus ratio was used to quantify the pressure contribution from RCMs relative to that from the diaphragm for a given inspiratory effort. We observed that in patients with COPD during exercise (1) there is a progressive increase in total inspiratory pressure contribution from RCMs relative to that of the diaphragm, and the magnitude of this increase appears to depend on the RCMs reserves during resting breathing; (2) most of the diaphragmatic pressure contribution occurs before the beginning of inspiratory flow, to overcome the PEEPi-imposed inspiratory threshold load; (3) RCMs pressure contribution predominates during the period of inspiratory flow once PEEPi is neutralized, not only for overcoming the elastic load caused by increased tidal volume, but also for compensating for the diaphragmatic pressure contribution during this interval that was gradually lost with increasing exercise work load.