Adaptation to Hypoxia in Sleep Apnea Patients: Multifaceted Mechanisms

Obstructive sleep apnea syndrome (OSA) is a form of sleep disordered breathing. The key phenomena are multiple repetitive pauses or restriction of airflow in the airway, defined as apnea and hypopnea. This study was based on a retrospective analysis of the results of polysomnographic (PSG) recordings of 230 adult patients (62 women and 168 men), being evaluated for OSA. The mean age, body mass index (BMI), apnea-hypopnea index (AHI), and pulse oximetry (SO₂ nadir) of all patients were 50.4 ± 12.7 years, 30.4 ± 5.4 kg/m², 33.6 ± 26.6 1/h, and 80.7 ± 12.1%, respectively. Statistical analysis was performed taking into account the moment of obstructive apnea occurrence and duration, sleep stage, and the SaO₂ nadir associated with these events. We found a lengthening of apneic episodes with progressing of sleep-time, which depended on the sleep-time. There was rather a fast increase in apnea duration in the first quartile of sleep-time. In the later sleep phases, the dynamics of the increase were four-fold weaker. The lengthening of apnea duration was dependent on the severity of AHI, but was hardly related to the sleep stage or arterial desaturation. In conclusion, the results revealed two time scales of a lengthening of apneic episodes as a function of sleep-time in OSA patients. Sympathetic activation and spleen reflex may be involved in the observed phenomenon. Although the exact mechanisms of increasing duration of apneic episodes with the passage of sleep remain elusive, we believe these mechanisms are liable to be multifaceted.