Evening and morning blood gases in patients with obstructive sleep apnea

The purpose of this study was to see if blood oxygen levels deteriorate overnight during obstructive sleep apnea (OSA). Before and after sleep, arterial blood gases (ABGs) in OSA subjects and controls were drawn during a diagnostic night, as well as during a continuous positive airway pressure (CPAP) night for the OSA subjects.

Subjects, both male and female, were referred to our sleep laboratory for symptoms of daytime somnolence. Subjects consisted of a control group (N=13) with a mean apnea hypopnea index (AHI) of 3.3 events/h and a study group (N=22) with a mean baseline AHI of 57 events/h.

With the subject supine, resting room air ABGs were drawn at 'lights out' on the evening before (PM) nocturnal polysomnography and in the morning (AM) at discontinuation ('lights on') of the sleep study. In controls, PM PaO(2) (79.4+/-9.7 mmHg) was not significantly different from AM PaO(2) (80.2+/-8.9 mmHg, P=0.5). In apneic subjects, the PM PaO(2) was 78.7+/-7.2 mmHg compared to an AM PaO(2) of 72.6+/-8.3 mmHg (P<0.05). The AM PaO(2) after a night of CPAP treatment in the OSA subjects was 77.5+/-10.2 mmHg compared to the PM PaO(2) of 76.0+/-6.0 mmHg (NS). The PM and AM PaCO(2)s were not different in controls or in study subjects under baseline conditions. However, during titration with nasal CPAP, the PaCO(2) was significantly higher in the morning after CPAP treatment [43.1+/-4.8 vs. 46.1+/-4.8 mmHg, respectively (P<0.05)].

OSA subjects showed a fall in overnight resting oxygenation. This could be accounted for by overnight deterioration of gas exchange and is ameliorated by CPAP.