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 SpO₂, 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 SpO₂ 90), 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 TST ₉₀-PaO₂ feed-back structural equations modeling showed that daytime PaO₂ was inversely related (P2 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.