One in a hundred children is born with a heart defect. Some children require heart surgery within the first few days of life, while others can wait until they are older. A complication of open-heart surgery is low blood flow due to the heart-lung machine that can cause sudden loss of kidney function known as acute kidney injury (AKI). AKI causes complications that can increase hospital length of stay and increase risk of death. Current ways to identify AKI are not able to it until 2 or 3 days after it has occurred. Because of this, there is not a specific treatment for AKI. If the investigators diagnose AKI early, they might be able to treat it and improve outcomes in children. NIRS is a skin monitor that can detect low blood flow to the kidney and might help diagnose AKI when it occurs in the operating room. The use of NIRS to diagnose AKI early is the focus of this study.