Obstructive sleep apnea can be associated with daytime chronic hypercapnia in some patients, but the prevalence of the phenomenon is highly variable in the published literature. The most often it is found in patients with coexisting COPD. There is also an evidence of persisting hypercapnia in OSA patients without other respiratory disease. In previous studies lung function impairment, obesity, gender, severity of OSAS have been considered to contribute to daytime hypercapnia. Several studies demonstrated that the defect in control of breathing can play a role in the development of chronic hypercapnia in patients with OSAS. The aim of the study was to estimate respiratory responses to hypercapnic stimulation in patients with OSAS and chronic daytime hypercapnia. Material consisted of 38 patients with OSAS and chronic hypercapnia (COPD was present in 24-group B, "pure" OSA in 14-group A) and 40 normocapnic OSA patients (group C). Lung function testing, blood gases and chemical control of breathing tests were performed in all of them before initiating therapy with nCPAP. Diagnosis of OSAS was stated with standard polisomnography and AHI was similar in mentioned groups.
Results: Respiratory responses to hypercapnic stimulation were significantly lower in hypercapnic patients (A 10.6+/-4.6; B 9.5+/-5.6) in opposition to normocapnics (C 23.3+/-14.0 l/min/kPa). In all studied patients PaCO2 level significantly correlated with respiratory responses to hypercapnic stimulation (r=-.61), lung function indices (VC r=-.69 and FEV1 r=-.71), mean SaO2 during sleep (r=-.68), and BMI (r=.49), but not with the factors like age, AHI or minimal SaO2 during sleep. Analysis with multiple regression revealed that hypercapnic drive, mean SaO2 during sleep, FEV1 and BMI were the best predictors of hypercapnia in studied group, being responsible for 72% of the total variance in PaCO2 in our OSA patients (R2=0.72; p<0.0001).
Conclusion: predisposition to daytime hypercapnia in our OSA patients was related to dimished chemosensitivity to CO2, mean desaturation during sleep, the severity of obesity and impairment of lung function mainly due to coexisting COPD.