Responses of diaphragm and external oblique muscles to flow-resistive loads during sleep

Although it is generally agreed that rapid respiratory compensation for externally applied inspiratory loads is impaired or absent during sleep, the individual components of the 'load-compensating reflex' may not be inhibited by sleep to the same degree. We studied the effect of inspiratory flow- resistive loading (18 cm H₂O/L/s) for two consecutive breaths on inspiratory (diaphragm) and expiratory (external oblique) muscle activity, and respiratory timing, in six awake and sleeping goats. During the first loaded breath in the awake state, peak integrated diaphragmatic electromyogram activity (EMGdi) increased 16.7 ± 3.9% (p < 0.01), peak integrated external oblique EMG activity (EMGeo) increased 21.0 ± 7.5% (p < 0.001), and electrical inspiratory time (Ti) increased 18.1 ± 2.1% (p < 0.01). In contrast, loading did not significantly change peak EMGdi or EMGeo on the first or second breaths in any sleep state. However, Ti was significantly increased during loading in all sleep states (p < 0.01) to a similar degree seen during wakefulness. Loading did not significantly alter electrical expiratory time. No significant differences were noted between the first and second loaded breaths. We conclude that the reflex increases in peak EMG of both inspiratory and expiratory muscles in response to inspiratory flow- resistive loading during the awake state are absent during all stages of sleep; however, one aspect of load compensation, prolongation of Ti, is preserved during sleep and aids in maintaining tidal volume.