Effect of Emphysema Severity on the Apnea-Hypopnea Index in Smokers with Obstructive Sleep Apnea.

RATIONALE The presence of obstructive sleep apnea (OSA) in patients with chronic obstructive pulmonary disease (COPD) is referred to as the OSA-COPD overlap syndrome. While lung inflation has been shown to be an important factor in determining upper airway stability, its role in determining OSA severity in smokers, including those with emphysema, has not been evaluated. OBJECTIVES To evaluate the importance of lung inflation on OSA severity (apnea-hypopnea index [AHI]) in smokers with suspected OSA. METHODS Fifty-one smokers (18 males; mean [±SD] age, 59 ± 9 yr; body mass index [BMI], 32 ± 9 kg/m(2)) who were part of the Genetic Epidemiology of COPD (COPDGene) project were studied. Patients underwent a full-night polysomnography for suspected OSA. Other testing included spirometry and volumetric chest computed tomography (CT) for quantitative measurement of CT-derived percent emphysema and CT-derived percent gas trapping. MEASUREMENTS AND MAIN RESULTS For the group overall, there was evidence of obstructive airway disease by spirometry (FEV1, 1.4 ± 0.5 L, 58 ± 14% predicted) and emphysema by quantitative CT (CT-derived percent emphysema, 11 ± 13%; CT-derived percent gas trapping, 31.6 ± 24.1%). Twenty-nine (57%) of the patients had OSA (AHI, 18 ± 12 events/h). Patients with OSA had a higher BMI but were younger than those without OSA (BMI, 35 ± 9 kg/m(2) vs. 29 ± 7 kg/m(2), respectively [P = 0.007]; age, 56 ± 8 yr vs. 62 ± 9 yr, respectively [P = 0.01]). There was an inverse correlation between the AHI and the CT-derived percent emphysema and CT-derived percent gas trapping, both for the entire group (r = -0.41 [P < 0.01] and r = -0.44 [P < 0.01], respectively) and when just those patients with OSA were evaluated (r = -0.43 [P = 0.04] and r = -0.49 [P = 0.03], respectively). Multiple linear regression revealed that, in addition to CT-derived percent emphysema and CT-derived percent gas trapping, sex and BMI were important in determining the AHI in these patients. CONCLUSIONS In smokers with OSA, increased gas trapping and emphysema as assessed by CT are associated with a decreased AHI. Along with sex and BMI, these measurements may be important in determining the severity of OSA in patients with COPD and may offer a protective mechanism in patients with more advanced disease.