Hypercapnia and ventilatory periodicity in obstructive sleep apnea syndrome

Prevention of acute hypercapnia during obstructive events in obstructive sleep apnea requires a balance between carbon dioxide (CO₂) loading during the event and CO₂ unloading in the interevent period. Earlier studies have demonstrated that acute CO₂ retention may occur despite high interevent ventilation when the interevent duration is short relative to the duration of the preceding event. The present study examines the relationship between apnea and interapnea durations and relates this assessment of ventilatory periodicity to the degree of chronic hypercapnia in subjects with severe sleep apnea. A total of 18 subjects with sleep apnea (> 40 apnea/hour; chronic awake PaCO₂ 36-62 mm Hg) and without underlying lung disease underwent polysomnography. For each event, apnea duration, interapnea duration, and apnea/interapnea duration ratio were determined. No relationship was observed between chronic PaCO₂ and mean apnea or interapnea duration (p > 0.1). However, PaCO₂ was directly related to apnea/interapnea duration ratio (r = 0.48; p < 0.05) such that with increasing chronic hypercapnia the interapnea duration shortens relative to the apnea duration. The present study suggests that control of the interapnea ventilatory duration relative to the duration of the preceding apnea, is an important component of the integrated ventilatory response to CO₂ loading during apnea and may contribute toward the development and/or maintenance of chronic hypercapnia in obstructive sleep apnea/hypopnea syndrome.