Project Summary Twenty-five percent of children with congenital heart disease (CHD) undergo intervention in the first year of life, often requiring surgery with cardiopulmonary bypass (CPB). CPB and related techniques including deep hypothermic circulatory arrest (DHCA) are necessary but contribute to poor postoperative physiology. Mortality for high risk surgeries remains >10%. Major complications occur in 30% of these complex surgeries and are key drivers of hospital length of stay (LOS) and costs. Novel diagnostic, mechanistic, and therapeutic approaches are critically needed to impact this burden on our infants, families, and healthcare system. Metabolites are the small-molecule end products of the genome that collectively determine minute-to-minute cellular physiology. Individual metabolites (e.g. lactate) are commonly used in postoperative management, but the interrelated metabolomic changes induced by infant cardiac surgery remain poorly understood. Recently, the metabolic profile of infants undergoing CPB was shown to shift markedly during the first 24hrs postoperatively and metabolites from two related pathways (kynurenine and nicotinamide metabolism) were associated with mortality and ICU LOS. Evolution of the postoperative metabolic profile beyond 24hrs and comprehensive changes in circulating/tissue kynurenine and nicotinamide metabolites are unknown. Overall Hypothesis: Infant cardiac surgery with CPB induces pathologic changes in the circulating metabolome across multiple key metabolic pathways. These changes directly impact postoperative outcomes and organ injury through a combination of beneficial metabolite depletion and pathologic metabolite production. Proposal: The study will use a combined clinical and translational approach. The clinical arm will consist of a prospective cohort study of infants undergoing CPB, with serial targeted metabolic profiling and pathway mapping through 72hrs postoperatively. The complementary translational arm will consist of a piglet model of CPB/DHCA to evaluate and modulate organ-specific flux through kynurenine and nicotinamide metabolism. Specific Aim 1: Validate the association of the 24hr postoperative metabolic profile with the combined outcome of death, cardiac arrest, or mechanical circulatory support and determine the evolution of this pathologic metabolic profile through 72hrs postoperatively. Specific Aim 2: Perform quantitative mapping of the kynurenine and nicotinamide metabolic pathways in order to a) quantify individual metabolite abnormalities, b) identify contributing changes in pathway enzymes, and c) determine the association of specific pathway abnormalities with postoperative outcomes. Specific Aim 3: In a piglet model of CPB with DHCA, quantify circulating and organ-specific kynurenine and nicotinamide pathway metabolites and determine the effects of pathway blockade on development of postoperative acute organ injury using systemic indoleamine 2,3-dioxygenase inhibition.