Daytime PaO2 in OSAS, COPD and the combination of the two (overlap syndrome)

OSAS and COPD are often associated with day-time hypoxemia. Overlap Syndrome (OS), the association between both diseases, increases the risk of day-time hypoxemia. The aim of this study was to investigate the mechanisms which could justify the low oxygen level and the effect of CPAP.

We performed a retrospective analysis in all patients referred to our institutes for suspected OSAS and who also underwent spirometry and blood gas analysis during our evaluation. Thus, 720 patients were selected. According to pulmonary function test parameters they were divided into 3 groups: OSAS (N = 466,65%); OS (N = 168,23%) and COPD (N = 86,12%). In order to evaluate the differences between the three groups, ANOVA analyses were carried out, whereas a multivariate analysis was performed in order to evaluate which factors determine the diurnal PaO(2). In 90 patients we also have the data on blood gas analysis after one year of CPAP treatment, so we evaluate the PaO(2) improvement in accordance with compliance to treatment in these patient subgroups.

The OS group showed a lower level of daytime PaO(2) compared with OSAS patients and T90 was higher in OS compared with OSAS. A multivariate analysis showed that in the OS diurnal PaO(2) correlated with age (β = -0.20) and moreover with FEV(1) (β = 0.31) and T90 (β = 0.37), while in the OSAS a correlation was found with FEV(1) (β = -0.11) and mostly with BMI (β = 0.25), age and T90. In all patients with good compliance to CPAP day-time PaO(2) improved.

Our data suggest that day-time hypoxemia in OSA patients is largely determined by the increase of body weight and severity of nocturnal hypoxia. However, CPAP therapy has been shown to improve daytime PaO(2) values both in OSAS and in OS.