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Daytime Hypercapnia in Adult Patients With Obstructive Sleep Apnea Syndrome in France, Before Initiating Nocturnal Nasal Continuous Positive Airway Pressure Therapy^*

^* From the Association Nationale pour le Traitement à Domicile de l’Insuffisance Respiratoire Chronique, Paris, France.

Correspondence to: Jean-Pierre Laaban, MD, Department of Pneumology, Hôtel-Dieu, 1 place du Parvis Notre-Dame, 75004 Paris, France; e-mail: j-pierre.laaban{at}htd.ap-hop-paris.fr

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.

Key Words: alveolar hypoventilation • carbon dioxide • hypercapnia • obesity • obesity-hypoventilation syndrome • Pickwickian syndrome • respiratory insufficiency • sleep apnea syndrome

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