Differential Inspiratory Muscle Pressure Contributions to Breathing during Dynamic Hyperinflation

During dynamic hyperinflation, the ventilatory pump is facing increased demand because it must overcome the intrinsic positive end-expiratory pressure (PEEPi) and decreased capacity since it must operate at a dynamically increased end-expiratory lung volume (EELV). The aim of this study was to evaluate the relative pressure contribution by the diaphragm and inspiratory rib cage muscles (RCMs) during dynamic hyperinflation. In six healthy subjects, dynamic hyperinflation was induced by limiting expiratory flow. The global inspiratory muscle pressure (delta Pmus,i) and transdiaphragmatic pressure (delta Pdi) were partitioned into the portion used to overcome PEEPi and the portion used to inflate the respiratory system. The delta Pdi/delta Pmus,i ratio was used to estimate the pressure contribution of RCMs relative to that of the diaphragm. Our results suggest that (1) with increasing severity of dynamic hyperinflation, there is a significant increase in the inspiratory pressure contribution of RCMs relative to that of the diaphragm for inflating the respiratory system; (2) during dynamic hyperinflation, especially at high EELV, the major pressure contribution of the diaphragm is to overcome the PEEPi-imposed inspiratory threshold load, whereas the inspiratory pressure needed for the subsequent task of inflating the respiratory system is largely contributed by RCMs. This arrangement is consistent with the change in mechanical advantages of RCMs and the diaphragm during the development of dynamic hyperinflation.