Nitrogen Balance in Infants After Post Cardiothoracic Surgery

Inclusion Criteria: * Clinical decision to initiate parenteral nutrition based on determination by medical team * Gestational age ≥ 35 weeks * Birth weight ≥ 2000 grams * Indwelling urinary catheters for urine collection * Central venous access for parenteral nutrition Exclusion Criteria: * Chromosomal abnormalities known to effect protein metabolism * Hepatic failure defined as ALT and AST \> 500 UL with an INR \> 2.5, not accounted for by therapeutic anticoagulation * Renal failure defined as creatinine 2x the upper limit of normal for age. * Sepsis or suspected sepsis, defined as clinical suspicion or confirmation of a systemic infection treated with antibiotics * Excessive blood loss from chest tubes (5 ml/kg/hr) that has not resolved within six hours following admission to the CCCU; as indicated by the need for frequent blood transfusion these subjects will be withdrawn from the study * Requiring Extra Corporeal Membrane Oxygenation (ECMO) support