Determinants of Ventilatory Instability in Obstructive Sleep Apnea: Inherent or Acquired?

STUDY OBJECTIVES: Certain respiratory control characteristics determine whether patients with collapsible upper airway develop stable or unstable breathing during sleep, thereby influencing the severity of obstructive apnea (OSA). These include arousal threshold (T(A)), response to transient hypoxia and hypercapnia (Dynamic Response) and the increase in respiratory drive required for arousal-free airway opening (T(ER)). We wished to determine whether these characteristics are inherent or are acquired during untreated OSA. DESIGN: T(A), Dynamic Response, and T(ER) were measured in patients with severe OSA before and after treatment with continuous positive airway pressure (CPAP). Changes observed after treatment were deemed to have been acquired during untreated OSA. SETTING: University-based sleep laboratory. PATIENTS: 15 patients with severe OSA. INTERVENTIONS: (1) 30-sec alterations in inspired gases during sleep on CPAP. (2) Brief dial-downs of CPAP (dial-downs) both during air breathing and when ventilation was increased to different levels. MEASUREMENTS AND RESULTS: T(A): the increase in ventilation associated with a 50% probability of arousal (T(A)50). Dynamic Response: the increase in ventilation on the 5th breath following breathing 3% CO2 in 11% to 15% O2. T(ER): the increase in ventilation prior to dial-downs that was associated with an arousal-free airway opening during dial-down. CPAP therapy (10.5 +/- 4.3 months) resulted in marked reduction in Dynamic Response (131% +/- 95% to 52% +/- 34% baseline ventilation, P < 0.005), a decrease in T(A)50 (134% +/- 78% to 86% +/- 47% baseline ventilation, P < 0.05), and no change in T(ER). CONCLUSIONS: T(ER) may be an inherent characteristic. Untreated OSA results in an increase in dynamic response to asphyxia and an increase in arousal threshold.