 Nearly half of all patients with heart failure have a preserved ejection fraction HFpEF1-3. This group is increasing in prevalence, has similar morbidity and mortality to systolic HF, and, despite increasing awareness of the healthcare burden, is without proven treatments1. This is related largely to a limited understanding of the basic mechanisms causing the disease3. Recent studies have added to contemporary understanding, but the pathophysiology remains controversial and incompletely understood4-8. A limitation of most prior studies is that the noninvasive measurements employed are merely surrogates for gold standard, invasive hemodynamic assessment9. There is general consensus that HFpEF patients have increased left ventricular filling pressures LVDP and relatively normal systolic function at rest5,8,10, but two critical questions remain: what causes the increase in LVDP, and, are there important deficits in the cardiovascular response to exercise stress in HFpEF patients3,4? The current study will resolve these questions by performing comprehensive hemodynamic analysis in HFpEF patients referred to the cardiac cath lab, compared to age and gender matched controls without HF. LV systolic, diastolic, and vascular function will be examined at rest and during graded supine exercise at fixed and varied preload to definitively characterize both baseline differences and discrepancies in cardiovascular reserve function that only become apparent during stress, when HFpEF patients typically become symptomatic11. This study will yield valuable information describing the roles for systolic, diastolic and pericardial abnormalities in the pathogenesis of HFpEF, providing critical preliminary data upon which better targeted therapeutic trials of this common disorder can be based. 