Daytime Hypercapnia in Obstructive Sleep Apnea Syndrome

BackgroundThe pathogenesis of daytime hypercapnia (Paco2 ≥ 45 mm Hg) may be directly linked to the existence of obstructive sleep apnea syndrome (OSAS) per se, although only some patients with OSAS exhibit daytime hypercapnia.ObjectiveTo investigate the prevalence of daytime hypercapnia in patients with OSAS; the association of daytime hypercapnia and obesity, obstructive airflow limitation, restrictive lung impairment, and severity of sleep apnea; and the response to continuous positive airway pressure (CPAP) therapy in a subset of subjects.MethodsThe study involved 1,227 patients with OSAS who visited a sleep clinic and were examined using polysomnography. As for the response to CPAP therapy, the patients were considered good responders if their daytime Paco2 decreased ≥ 5 mm Hg and poor responders if it decreased < 5 mm Hg.ResultsFourteen percent (168 of 1,227 patients) exhibited daytime hypercapnia. These patients had significantly higher body mass index (BMI) and apnea-hypopnea index (AHI) values compared with normocapnic patients, while percentage of predicted vital capacity (%VC) and FEV1/FVC ratio did not differ between the two groups. Logistic regression analysis showed that only AHI was a predictor of daytime hypercapnia (p < 0.0001), while BMI (p = 0.051) and %VC (p = 0.062) were borderline predictors of daytime hypercapnia. Daytime hypercapnia was corrected in some patients (51%, 19 of 37 patients) with severe OSAS after 3 months of CPAP therapy.ConclusionThe pathogenesis of daytime hypercapnia may be directly linked to sleep apnea in a subgroup of patients with OSAS.