THE SINGLE-BREATH DIFFUSING-CAPACITY FOR CARBON-MONOXIDE IN OBSTRUCTIVE SLEEP-APNEA AND OBESITY

Background: We have observed high values of single-breath diffusing ca pacity for carbon monoxide (Dco) in patients with obstructive sleep ap nea (OSA). Study objectives: To confirm this observation and to determ ine the factors influencing diffusion indices in patients with OSA, As weight excess was found to be the major determinant of increased diff usion indices, the adequacy of various prediction equations was evalua ted in obese subjects. Design: Retrospective data collection. Setting: University hospital, Methods: Dco was measured in patients with OSA a nd in nonapneic subjects who served as control subjects, The results w ere expressed with respect to our own reference values and also to oth er prediction equations, Results: In moderate/severe OSA (apnea/hypopn ea index [AHI]greater than or equal to 30), both Dco and transfer coef ficient (KCO) were increased and KCO was positively correlated with AH I and body mass index (BMI). Diffusion indices were not different betw een patients with moderate/severe OSA and nonapneic control subjects m atched for age and BMI. In the latter group, KCO was also correlated w ith BMI. In our obese patients (BMI, 36.7+/-5.6 kg/m(2)), Dco was abou t 10% and KCO was 20 to 25% greater than predicted by standard referen ce equations, Conclusions: Dco and KCO are increased in a typical popu lation of patients with moderate/severe OSA. This appears to be due to weight excess and not to OSA itself, In obese subjects, whether apnei c or not, BMI is a determinant of diffusion indices that are higher th an predicted by equations relying on age and stature alone.