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Division of Pulmonary and Critical Care Medicine and Bellevue Hospital Chest Service, Department of Medicine, New York University School of Medicine, New York, New York 10016
Maintenance of
eucapnia during sleep in obstructive sleep apnea (OSA) requires a
balance between CO2 loading during apnea and
CO2 elimination. This study examines individual
respiratory events and relates magnitude of postevent ventilation to
CO2 load during the preceding respiratory event in 14 patients with OSA (arterial PCO2 42-56
Torr). Ventilation and expiratory CO2 and O2 fractions were measured on a breath-by-breath basis
during daytime sleep. Calculations included CO2 load during
each event (metabolic CO2 production
exhaled
CO2) and postevent ventilation in the 10 s after an
event. In 12 of 14 patients, a direct relationship existed
between postevent ventilation and CO2 load during the preceding event (P < 0.05); the slope of this
relationship varied across subjects. Thus the postevent ventilation is
tightly linked to CO2 loading during each respiratory event
and may be an important mechanism that defends against development of
acute hypercapnia in OSA. An inverse relationship was noted between
this postevent ventilatory response slope and the chronic awake
arterial PCO2 (r = 0.90, P < 0.001), suggesting that this mechanism is impaired in patients with chronic hypercapnia. The link between development of
acute hypercapnia during respiratory events asleep and maintenance of
chronic awake hypercapnia in OSA remains to be further investigated.
carbon dioxide; hypercapnia; hypoventilation; sleep apnea syndromes; Pickwickian syndrome
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