The single-breath diffusing capacity for carbon monoxide in obstructive sleep apnea and obesity

Background: We have observed high values of single-breath diffusing capacity for carbon monoxide (Dco) in patients with obstructive sleep apnea (OSA). Study objectives: To confirm this observation and to determine the factors influencing diffusion indices in patients with OSA, As weight excess was found to be the major determinant of increased diffusion indices, the adequacy of various prediction equations was evaluated in obese subjects. Design: Retrospective data collection. Setting: University hospital, Methods: Dco was measured in patients with OSA and in nonapneic subjects who served as control subjects, The results were expressed with respect to our own reference values and also to other prediction equations, Results: In moderate/severe OSA (apnea/hypopnea index [AHI]greater than or equal to 30), both Dco and transfer coefficient (KCO) were increased and KCO was positively correlated with AHI and body mass index (BMI). Diffusion indices were not different between patients with moderate/severe OSA and nonapneic control subjects matched for age and BMI. In the latter group, KCO was also correlated with BMI. In our obese patients (BMI, 36.7+/-5.6 kg/m(2)), Dco was about 10% and KCO was 20 to 25% greater than predicted by standard reference equations, Conclusions: Dco and KCO are increased in a typical population of patients with moderate/severe OSA. This appears to be due to weight excess and not to OSA itself, In obese subjects, whether apneic or not, BMI is a determinant of diffusion indices that are higher than predicted by equations relying on age and stature alone.