Respiratory muscle function during CO2 rebreathing with inspiratory flow-resistive loading.

We investigated the respiratory muscle contribution to inspiratory load compensation by measuring diaphragmatic and intercostal electromyograms (EMGdi and EMGic), transdiaphragmatic pressure (Pdi), and thoracoabdominal motion during CO2 rebreathing with and without 15 cmH2O X l-1 X s inspiratory flow resistance (IRL) in normal sitting volunteers. During IRL compared with control, Pdi measured during airflow and during airway occlusion increased for a given change in CO2 partial pressure and EMGdi, and there was a greater decrease in abdominal (AB) end expiratory anteroposterior dimensions with increased expiratory gastric pressure (Pga), this leading to an inspiratory decline in Pga with outward AB movement, indicating a passive component to the descent of the abdomen-diaphragm. The response of EMGic to IRL was similar to that of EMGdi, though rib cage (RC)-Pga plots did infer intercostal muscle contribution. We conclude that during CO2 rebreathing with IRL there is improved diaphragmatic neuromuscular coupling, the prolongation of inspiration promoting a force-velocity advantage, and increased AB action serving to optimize diaphragm length and configuration, as well as to provide its own passive inspiratory action. Intercostal action provides increased assistance also. Therefore, compensation for inspiratory resistive loads results from the combined and integrated effort of all respiratory muscle groups.