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Am. J. Respir. Crit. Care Med., Volume 162, Number 3, September 2000, 1004-1008

Hypercapnia Can Induce Arousal from Sleep in the Absence of Altered Respiratory Mechanoreception

NAJIB T. AYAS, ROBERT BROWN, and STEVEN A. SHEA

Pulmonary and Critical Care Medicine Section, Medical Service, and Spinal Cord Injury Service, Brockton/West Roxbury VA Medical Center, Brockton, Massachusetts; New England Sinai Hospital and Rehabilitation Center, Stoughton, Massachusetts; Sleep Disorders Program, Brigham and Women's Hospital, Boston, Massachusetts; and Harvard Medical School, Boston, Massachusetts

Possible mechanisms of arousal from respiratory stimuli include changes in PO2, PCO2, central respiratory drive, or respiratory mechanoreceptor activity. We sought to determine whether hypercapnia alone could induce arousal from sleep in four subjects with high (>=  C3) neurologically complete spinal cord injuries while on constant positive pressure mechanical ventilation (hence, respiratory mechanoreceptor activity remained constant). Subjects were chronically hypocapnic (mean baseline PETCO2 = 21 mm Hg; range, 13-30 mm Hg). On the first night, the baseline rate of spontaneous awakenings was determined by polysomnography. On night two, FICO2 was increased rapidly in stable NREM sleep. Awakenings occurred in 19 of 19 trials within 5 min, with each subject waking and complaining of shortness of breath (mean time to arousal, 115 s; range, 26-264 s). It is unlikely that these were spontaneous, as the times to awakening during hypercapnia were much higher than during baseline conditions (p < 0.05). During rapidly induced hypercapnia, PETCO2 overestimates the PCO2 at the central chemoreceptors. To determine more precisely the PETCO2 arousal threshold, PETCO2 was increased slowly (approximately 2 mm Hg/min); arousal occurred at a mean PETCO2 of 37 mm Hg (range, 23-45 mm Hg; mean change from baseline, 15.8 mm Hg, range, 10-20 mm Hg). Hence, both rapid and slow increases in PETCO2 can induce arousal in humans in the absence of changes in respiratory mechanoreceptor activity.




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