Inhibition of respiratory activity during passive ventilation: a role for intercostal afferents?

The purpose of this study was to determine whether receptors from the rib cage are primarily responsible for inhibitory feedback of inspiratory muscle activity during mechanical ventilation. Seven quadriplegics with C5-C6 lesions were compared to 6 normals during mechanical ventilation. All subjects were mechanically hyperventilated with a nasal mask to suppress intrinsic inspiratory muscle activity. End-tidal partial pressure of carbon dioxide (PETCO2) was increased by either adding CO2 (FICO2) or decreasing tidal volume (VT) until reoccurrence of inspiratory activity, defined as the recruitment threshold (PCO2RT). The difference between PCO2RT and eupneic PETCO2 indicated the presence and magnitude of volume-related inhibition of inspiratory muscle activity during mechanical ventilation. Substantial inhibition of inspiratory activity was observed in both quadriplegics and normals. We conclude that afferent information from the rib cage is not obligatory for the mediation of volume-related inhibition of inspiratory muscle activity during mechanical ventilation.