The proposed study will be conducted at Children's National Health System (CNHS). Pediatric subjects undergoing cardiac surgery requiring cardiopulmonary bypass will be recruited. Eligible and consenting participants will be assigned a unique identification number and will be randomly assigned to receive either methadone or fentanyl using a computer-generated randomization scheme. A total of 52 participants will be enrolled to each arm of the study, providing 80% power to detect a 30% difference between groups.