The central hypothesis is that plasma biomarkers of intestinal EBF can be used to initiate safe delivery of early EN for infants after surgical repair of CHD. Investigators will identify clinical variables which act as effect modifiers of intestinal EBF and create research tools to assess feeding intolerance in post-operative infants with left-sided CHD. Based on our pilot data, we suspect that pressor dose, mean airway pressure, hyperglycemia and time to initiation of EN are important variables which may alter intestinal barrier profile. Given the emerging data for the importance of the intestinal microbiome on intestinal barrier function, we will evaluate change in the intestinal microbiome from pre-op and over the first 5 post-operative days to determine how microbiome changes correlate with intestinal EBF profile. This proposal will generate pilot and feasibility data for a future comparative effectiveness trial to compare the effect of a biomarker based decision rule to a clinical decision rule for EN initiation on daily percent of goal nutrition delivered, feeding intolerance, and incidence of gastrointestinal complications after surgical repair of left-sided congenital cardiac defects. To determine the feasibility of plasma intestinal EBF profile to assess readiness for early post-operative EN in infants with left sided congenital cardiac defects, investigators will complete the following 2 specific aims.