The relationship of daytime hypoxemia and nocturnal hypoxia in obstructive sleep apnea syndrome

Question of the study: Prevalence and determinants of daytime hypoxemia in patients with obstructive sleep apnea (OSA) syndrome are not well established. The aims of this study, conducted in a large series of OSA patients, were to estimate the prevalence of daytime hypoxemia, to assess the reciprocal effects between daytime PaO2 and nocturnal SpO(2), and to investigate the direct and indirect role of sleep apnea severity in determining feedback gas exchange abnormalities. Materials and methods: In 456 patients a daytime hypoxemia-nocturnal hypoxia feedback structural equations model was designed. PaO2 adjusted for age (% of predicted), percent sleep time spent with SpO(2) <90% (TST90), oxygen desaturation index and the apnea-hypopnea index, were determined as the measures of daytime hypoxemia, nocturnal hypoxia, and sleep apnea severity, respectively, after adjusting for the severity of obesity and lung volumes. Results: The TST90-PaO2 feed-back structural equations modeling showed that daytime PaO2 was inversely related (P<0.001) to nocturnal hypoxia (-4.0% of PaO2 per 1 SD of TST90). The severity of OSA (-1.0%) was an indirect determinant of daytime PaO2 via the TST90 pathway. In contrast, daytime PaO2 did not influence (P>0.05) the extent of nocturnal hypoxia. Conclusions: In OSA patients, the extent of nocturnal hypoxia seems to be both a direct determinant and a mediator of the indirect effect of sleep apnea on the development of daytime hypoxemia.