Advanced cardiac imaging analysis of 225 patients reveals that heart failure with preserved ejection fraction (HFpEF) and reduced ejection fraction (HFrEF) exhibit fundamentally opposite mechanisms of diastolic dysfunction. Using parameterized diastolic filling analysis, researchers found that while both conditions showed similar left ventricular stiffness compared to healthy controls, damping patterns diverged dramatically—increased in HFpEF but reduced in HFrEF patients. This finding challenges the prevailing assumption that myocardial stiffness drives diastolic dysfunction across all heart failure types. The discovery has profound implications for cardiovascular health and longevity, as it suggests these conditions require entirely different therapeutic approaches. HFpEF affects millions globally and has historically been difficult to treat, partly because treatments developed for HFrEF often fail in HFpEF patients. Understanding that damping—the heart's ability to dissipate energy during filling—differs between phenotypes could guide development of targeted therapies. However, this preprint awaits peer review, and the retrospective design limits causal inferences. The findings represent a potentially paradigm-shifting insight that could reshape heart failure treatment strategies and improve outcomes for aging populations at risk for cardiovascular decline.