Effect of lung inflation on pulmonary resistance during NREM sleep

Previous investigations have demonstrated an inverse relationship between lung volume and airway resistance in awake humans. We wished to examine this relationship in the absence of conscious influences. We therefore studied eight healthy subjects who slept in a tank respirator. Hyperinflation was induced by continuous negative tank pressure while the subjects breathed spontaneously. Ventilation, pulmonary resistance (total pulmonary resistance, seven subjects; upper airway resistance, one subject), diaphragm and genioglossus electromyograms (EMGs), and sleep state were measured. During control NREM sleep, group mean maximal pulmonary resistance was 42.5 cm H2O/L/s (range, 17.4 to 106.4 cm H2O/L/s). During steady-state hyperinflation (mean increase in lung volume = 0.53 L), pulmonary resistance decreased 40% (range, -3 to -90%). Ventilation, sleep state, and end-tidal CO2 were unchanged. Inspiratory muscle EMG was increased in two of two subjects during hyperinflation. Genioglossus EMG was characterized by phasic and tonic activity during the control period in two of two subjects. Both components were decreased during steady-state hyperinflation. When lung volume was returned to baseline, pulmonary resistance and genioglossus EMG increased to baseline levels. We conclude that alteration in lung volume within the tidal volume range significantly alters pulmonary resistance during NREM sleep. This influence occurs independent of chemical stimuli or genioglossal muscle activity, and may be related to traction on neck structures caused by descent of mediastinal structures.