Efficacy of CPAP Therapy on Pulmonary Function Test in Patients With COPD-OSA Overlap Syndrome

Inclusion Criteria: * Diagnosis with COPD by GOLD guideline criteria (Post bronchodilator FEV1/FVC \< 0.7 or \< LLN) with post bronchodilator FEV1 = 30-80% * Diagnosis with obstructive sleep apnea with AHI \> 15 event/hr. (Moderate OSA or more) * Age between 40-80 years * Stable disease prior to inclusion. Exclusion Criteria: * Diagnosis with \> 50% of central sleep apnea event * History of COPD exacerbation during the past 8 week prior to inclusion. * Subjects who cannot perform spirometry test or walking test * Subjects with recording history of chronic heart failure, significant arrhythmias, acute myocardial infarction, moderate to severe mitral or aortic valve disease and pulmonary hypertension. * Subjects with interstitial lung disease. * Subjects with neuromuscular disease. * Subjects with morbid obesity (BMI \>/=35) * Subjects with chronic respiratory infection. * Subjects with chronic CO2 retension (PaCO2 \>/= 45 from arterial blood gas analysis) or Transcutaneous CO2 during sleep test \>/= 45 or Serum bicarbonate level \> 27 * Subjects with acute or active respiratory infecton during 2 week prior to inclusion.