Postevent ventilation as a function of CO₂ load during respiratory events in obstructive sleep apnea

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.