 Chronic obstructive pulmonary disease COPD and obstructive sleep apnea OSA are very common disorders associated with considerable morbidity, mortality, and healthcare costs. The prevalence of both co-existing conditions is estimated to be 4 of the general population. This COPD-OSA overlap syndrome causes more severe hypoxemia than either COPD or OSA alone and has important clinical consequences, including death. COPD is usually excluded in OSA research and OSA is typically excluded or not assessed in studies of COPD; thus, available information about the overlap syndrome is limited. Therefore, it is important to identify patients with both COPD and OSA and determine the mechanisms of poor outcomes for these patients in order to optimize therapy. The pathophysiology of the COPD-OSA syndrome is not well understood. The investigators propose to investigate upper airway UA anatomic characteristics and collapsibility as potential underlying mechanisms that may help to explain the negative additive effect of having both conditions. The objectives are to study CT measures of airway anatomy and the critical closing pressure of the upper airway Pcrit, a gold standard measure of upper airway collapsibility, in patients with COPD-OSA compared with COPD only and normal controls. CT scan of upper airway and chest will allow precise measures of upper airway characteristics and COPD associated alveolar and lower airway ch. angesMeasures of upper airway collapsibility will provide us information about the mechanical nature of the airway and if the patients are more likely to have OSA. Subjects with COPD-OSA may exhibit more upper airway inflammation possibly due to their pre-existing COPD disease and the reoccurring opening and closing of the upper airway due to the OSA. Therefore the investigators would like to assess the degree of inflammation in these patients compared to normal controls. 