The association of obstructive sleep apnea (OSA) and cardiac diastolic dysfunction have been reported, and improvement of diastolic function after continuous positive airway pressure (CPAP) in OSA patients was observed. However, more detailed analysis of diastolic function by supine bicycle exercise echocardiography is lacking. The investigators hypothesized that 3 months of CPAP therapy in OSA patients will significantly improve diastolic functional parameters measured by exercise stress echocardiography. Patients with severe OSA (Apnea-hypopnea index > 30) will be included in this study, and randomized to CPAP versus sham-CPAP group by 1:1 ratio. Supine bicycle exercise echocardiography, pulse wave velocity, 24 hours ambulatory blood pressure (BP), central BP will be checked before and after CPAP therapy and parameters will be compared.

Inclusion Criteria:

• Patietns≥20 and ≤75 years-old
• Patients who diagnosed as obstructive sleep apnea and AHI>30 on polysomnography

Exclusion Criteria:

• Age < 20 years-old
• Left ventricular dysfunction (LVEF<50%) or structural heart disease on transthoracic echocardiography
• Diabetes mellitus
• Significant cardiac valvular disease (≥moderate)
• Severe hypertension (>180/110mmHg)
• Moderate or severe kidney disease (eGFR < 60 mL/min )
• Clinically significant liver disease
• History of coronary artery disease
• malignancy or autoimmune disease
• Acute/chronic infection status
• Pregnant status
• Patients taking anxiolytics or sedatives