ABSTRACT - In the post-ischemic heart, relatively little is known about the injured-but-not-infarcted myocardium, which we call the intermediate zone as it is neither normal nor infarcted. We recently identified compelling evidence that the intermediate zone is not merely a "lesser infarct", but has a set of unique pathological characteristics and contributes significantly to cardiac impairment. These novel discoveries were made possible by overcoming a technological challenge. We developed a high-sensitivity phosphatidylethanolamine (PE)- based imaging technique, enabling the mapping of the intermediate zone which is otherwise missed by conventional methods. Using imaging-guided pathological analyses, we discovered that, in contrast to the infarct zone where there is necrosis across all cell types, in the intermediate zone different cell types survive differently. This disparity between surviving cardiomyocytes (residual contractility) and loss of sympathetic neurons (dysinnervation) creates chaos in electrophysiology. Chronically, the intermediate zone exhibits functional deficiency with signaling activation associated with hypertrophy. The data strongly support that the intermediate zone has significant contractile dysfunction as well as being a substrate for arrhythmias. As such, there are significant prognostic values both for assessing the full scope of myocardial impairment and for predicting the risk for arrhythmias. Based on these findings, we propose a central hypothesis that the intermediate zone constitutes a distinct pathological entity which contributes to cardiac dysfunction in the post-ischemic heart. The hypothesis will be tested in three integrated and synergistic Specific Aims: 1) to refine the in vivo imaging methodology for mapping the intermediate zone, and characterize the pathology of this tissue in an imaging- guided approach; 2) to determine the signaling changes in the intermediate zone; and 3) to investigate the roles of the intermediate zone in arrhythmogenesis. Collectively, the ability to positively identify the intermediate zone in vivo provides a critical technological breakthrough. By understanding the signaling, pathological and functional changes in this tissue, our findings will ultimately have a transformative impact on enriching the knowledge base and shaping clinical practices in ACS.