The purpose of this study is to determine the factors that are associated with improved cardiovascular function with the use of CPAP therapy on subjects diagnosed with moderate to severe obstructive sleep apnea.

We will monitor non-invasively changes in arterial stiffness, cardiac mass, pulmonary pressures, endothelial function, etc. We will also assess the reversibility of these changes after only 5-7 days of not using the CPAP mask. In a subgroup of subjects we will explore changes in coronary flow response after 12 weeks of CPAP use compared with baseline.

Inclusion Criteria:

• subjects naïve of CPAP therapy just recently diagnosed with moderate to severe obstructive sleep apnea (OSA). The degree of OSA is defined by results of a comprehensive polysomnogram requiring an apnea hypopnea index (AHI) >15 events/hour and Epworth score >10, or an AHI >20 events/hour
• either sex
• any race
• between 21-50 years old.

Exclusion Criteria:

• Systolic blood pressure >160 mmHg, or diastolic blood pressure >100 mmHg
• Changes in antihypertensive medications in the last 6 weeks
• Restless leg syndrome
• Average overnight oxygen saturation below 80%
• Current use of beta-blockers
• History of coronary artery disease
• History of Stroke
• Atrial fibrillation
• Peripheral vascular disease
• Suspected cardiac valve abnormality
• Ejection fraction <50%
• Type I and type II Diabetes Mellitus (DM)
• Asthma or confirmed Chronic Obstructive Pulmonary Disease
• Cigarette smoking in the last 6 months
• Raynaud's disease
• Pregnancy (the normal hormonal changes that occur in pregnancy affect greatly arterial stiffness parameters) If a subject becomes pregnant we will discontinue data collection.
• Physically incapable of resting on left lateral decubitus for 40 minutes.
• Mastectomy with lymph node removal that might preclude us to monitor blood pressures on both arms