PROJECT ABSTRACT Sudden cardiac arrest (SCA) due to malignant ventricular arrhythmias remains a major cause of mortality in the US. While the majority of SCA patients have significant coronary artery disease (CAD), nearly one in three patients present with lethal arrhythmias as their first and only manifestation of heart disease without prior cardiovascular symptoms. Moreover, these arrhythmic events exhibit a diurnal variation but the mechanisms responsible for this remain unclear. We hypothesize that chronic ischemia results in circadian misalignment between ischemic and non-ischemic (healthy) regions and is a mechanism of increased cardiac arrhythmias mediated by circadian phase shift in the intracellular calcium-calcineurin signaling. To test this hypothesis, the proposed studies will use a combination of donor human hearts and swine model of chronic LAD stenosis that develops ischemia with high frequency of SCA. We will employ optical mapping of ex vivo cardiac slices and in vivo electrical mapping of intact hearts to establish whether a diurnal window of arrhythmia vulnerability exists in swine with chronic LAD stenosis (Aim 1). Next, we will investigate the extent to which cardiac circadian misalignment exists in swine with chronic LAD stenosis and determine whether pharmacological circadian realignment can attenuate incidence of arrhythmias and SCA (Aim 2). Finally, we will use donor human hearts to validate the findings from swine studies (Aim 3). Successful completion of the proposed studies will provide unique insight regarding the impact of circadian misalignment on arrhythmia vulnerability and illuminate opportunities for pharmacological interventions to manage, prevent, or even restore the loss of cardiac circadian alignment and consequently attenuate time-of-day incidence of cardiac arrhythmias and SCA.