Effect of sleep on respiratory muscle activity during mechanical ventilation.

The purpose of this study was to determine whether consciousness was critical for the expression of neuromechanical inhibition of breathing during mechanical ventilation. This same mechanical ventilation model also was used to evaluate the relative importance of sleep state in causing CO2 retention during sleep. Positive pressure ventilation was used to suppress respiratory muscle activity; CO2 was then added until a reappearance of inspiratory effort, which defined the recruitment threshold (PCO2RT). Keeping the mechanics of the respiratory system constant through the use of passive mechanical ventilation allowed us to measure the output of the respiratory controller, independent of these parameters. Eight normal subjects were mechanically hyperventilated with a nasal mask during wakefulness and sleep with matched flow rates, frequencies, and tidal volumes. When inspiratory muscle activity was undetectable and end-tidal PCO2 (PETCO2) fell below 30 mm Hg, inspired CO2 was added in stepped increments until inspiration reoccurred. The sleeping state increased both eupneic PETCO2 (42 +/- 4 versus 38 +/- 3 mm Hg) and PCO2RT (48 +/- 3 versus 46 +/- 2 mm Hg) compared with that during wakefulness. Neuromechanical inhibition of inspiratory muscle activity during mechanical ventilation was present during both wakefulness and sleep, as evidenced by the mean difference between PCO2RT and eupneic PETCO2 of 8 and 6 mm Hg, respectively. Recruitment thresholds during wakefulness and sleep were compared to evaluate the effect of sleep on respiratory motor output independent of changes in load, i.e., respiratory mechanics held constant.(ABSTRACT TRUNCATED AT 250 WORDS)