Lactate Acidosis in Postoperative Hearts

Investigators will characterize the etiologies of hyperlactatemia predominantly present at serial postoperative timepoints in children recovering from cardiac surgery by following lactate levels and the lactate-to-pyruvate ratio over the first 48 post-operative hours and correlating them with cardiac output as determined by conventional markers and physical exam. Investigators expect to validate the assumption that in children recovering from heart surgery whose cardiac output is apparently normal, hyperlactatemia (if seen) is predominantly due to impaired lactate clearance or metabolism, while in those whose cardiac output is low, it is predominantly due to inadequate oxygen delivery. Blood samples will be obtained up to 12 timepoints (arrival at the intensive care unit postoperatively, hour 1, hour 2, hour 3, hour 4, hour 6, hour 8, hour 10, hour 12, hour 24, hour 36 and hour 48 postoperative) or only at up to 6 time points (approximately Hour 0, 2, 4, 6, 8, and 12) for infants under 10 kg as described below. . Samples will be analyzed for concentration of pyruvate. Results will be compared to standard of care laboratory results for lactate and other direct and indirect measures of cardiac output.