Cardiac computed tomography analysis of 975 patients revealed that the left atrium's booster pump function follows an inverted U-curve, reaching peak performance at 107mL preload volume before declining. Beyond this threshold, patients developed "atrial failure" characterized by reduced pumping despite increased chamber filling. Those with this failure pattern faced a 43% rate of heart failure, stroke, or cardiovascular death. The atrium compensates for chronic pressure elevation by enlarging to maintain its booster function, but this mechanism eventually fails when preload exceeds the 107mL threshold. This finding reframes our understanding of atrial dysfunction from simple enlargement to a specific failure of the Frank-Starling mechanism in cardiac chambers. The research provides clinicians with a quantitative marker for identifying high-risk patients who might benefit from earlier intervention. However, this preprint study awaits peer review, and its retrospective design limits causal conclusions. The work represents a significant advance in cardiovascular risk stratification, offering a novel biomarker that could transform how we monitor and treat atrial dysfunction before it progresses to clinical heart failure.