Inclusion Criteria:  Experimental group: (15 patients)  18-75-year-old, AND With mild to severe OSA (mild: Apnea Hypopnea Index: AHI between 5-15, Moderate AHI= 15-30, severe AHI>30), diagnosed by polysomnography. Patient with a creatinine measurement within the last 6 months. Control group (5 patients): patients with no lung or heart disease, matched by age and BMI.  Exclusion Criteria:  History of chronic heart failure (Left Ventricle Ejection fraction (LVEF) < 50%), AF, myocardial infarction, valvular disease. Prior cardiac or chest surgery. Other advanced pulmonary disease (severe Chronic Obstructive Pulmonary Disease (COPD) or asthma, pulmonary hypertension) or central sleep apnea. Contraindications to DE-MRI (e.g. allergy to gadolinium, pacemakers, defibrillators (ICD's), other devices/implants contraindicated for MRI, glomerular filtration rate <30 ml/min, etc.). BMI > 35 kg/m2 (affects DE-MRI quality). Pregnancy. Contraindications to WATCHPAT device (peripheral neuropathy). Use of one of the following medications: alpha blockers, short acting nitrates (less than 3 hours before the use of WATCHPAT). Inability to give informed consent. No access to proper smartphone technology and/or internet. Inability to return to follow up visit.