Context: Daytime hypercapnia in patients with obstructive sleep apnea syndrome (OSAS) has a highly variable prevalence in the published studies, and is usually thought to be the consequence of an associated disease, COPD, or severe obesity.
Study objectives: To assess the prevalence of daytime hypercapnia in a very large population of adult patients with OSAS, free of associated COPD, and with a wide range of body mass index (BMI), and to evaluate the relationship between daytime hypercapnia and the severity of obesity and obesity-related impairment in lung function.
Design: Retrospective analysis of prospectively collected data.
Methods: The database of the observatory of a national nonprofit network for home treatment of patients with chronic respiratory insufficiency (Association Nationale pour le Traitement à Domicile de l’Insuffisance Respiratoire Chronique) was used. Collected data at treatment initiation were age, apnea-hypopnea index, BMI, FEV1, vital capacity (VC), and arterial blood gases. The study included 1,141 adult patients with OSAS treated in France with nocturnal nasal continuous positive airway pressure (CPAP), FEV1 ≥ 80% predicted, FEV1/VC ≥ 70%, and absence of restrictive respiratory disease other than related to obesity.
Results: The prevalence of daytime hypercapnia (Paco2 ≥ 45 mm Hg) before initiating CPAP therapy was 11% in the whole study population. The prevalence of daytime hypercapnia was 7.2% (27 of 377 patients) with BMI < 30, 9.8% (58 of 590 patients) with BMI from 30 to 40, and 23.6% (41 of 174 patients) with BMI > 40. Patients with daytime hypercapnia had significantly higher BMI values and significantly lower VC, FEV1, and Pao2 values than the normocapnic patients. Stepwise multiple regression showed that Pao2, BMI, and either VC or FEV1 were the best predictors of hypercapnia, but these variables explained only 9% of the variance in Paco2 levels.
Conclusion: Daytime hypercapnia was observed in > 1 of 10 patients with OSAS needing CPAP therapy and free of COPD, and was related to the severity of obesity and obesity-related impairment in lung function. However, other mechanisms than obesity are probably involved in the pathogenesis of daytime hypercapnia in OSAS.