Inclusion Criteria:  Age ≥ 18 years Diagnosis of HFpEF by catheterization with resting PCWP>15 mmHg or exercise PCWP>25 mmHg Ambulatory (not wheelchair/scooter dependent) If no known diagnosis of OSA, must not be pacemaker dependent with either atrial pacing or VVI without sinus rhythm If no known diagnosis of OSA, must be able to temporarily hold nitrates or alpha channel blockers for home WatchPAT testing (3 hour washout period)  Exclusion Criteria:  Ejection fraction <40% Obstructive hypertrophic cardiomyopathy Constrictive pericarditis or tamponade Active myocarditis Complex congenital heart disease Other valve disease requiring surgical intervention Terminal illness (other than HF) with expected survival of less than 1 year Inability to comply with planned study procedures Pregnancy or breastfeeding mothers