Dexmedetomidine is increasingly used for sedation in children placed on cardiopulmonary bypass for heart surgery. It is also often used after surgery and may be particularly helpful for children with heart disease. In order for this medication to be helpful and to minimize risks associated with taking the medication, it is important to provide correct dosing for this medication. Based on evidence from studies of other medications used during heart surgery, it is likely that dosing of dexmedetomidine while on cardiopulmonary bypass is different from dosing in other settings. The purpose of this study is to evaluate how the heart-lung bypass machine affects dexmedetomidine levels in the body.

Inclusion Criteria:

• < 2 years of age
• Sufficient venous access to permit administration of study medication.
• Supported with cardiopulmonary bypass (CPB)
• Receiving dexmedetomidine per standard of care.
• Availability and willingness of the parent/legal guardian to provide written informed consent.

Exclusion Criteria:

• <38 weeks post menstrual age
• Any concomitant condition, which in the opinion of the investigator would preclude a subject's participation in the study.
• Previous participation in this study
• CPB circuit primed with clear fluid

The study population will include children less than or equal to 2 years of age who undergo cardiopulmonary bypass and receive dexmedetomidine per standard of care during the bypass session.