Heart surgery in young children involves some risks. This study looks at a new type of monitor that may reduce these risks. Near Infrared Reflectance Spectroscopy (NIRS) aims to measure if enough blood reaches vital organs in the body. The investigators goal is to see if this shows problems developing sooner than usual. The investigators do not know at this time if they can improve the blood supply to the vital organs. The investigators main aim is to see which responses work the best. The NIRS monitor is approved by the U.S. Food and Drug Administration (FDA).

The investigators will also follow the progress of recovery after surgery in the Cardiac Intensive Care Unit (CICU). This will find out if using the NIRS monitor improves the outcome for children.

The NIRS monitor will record information from the patient until he/she has been in the CICU for about 24 hours. The monitoring consists of small sticky pads placed on the child's forehead and trunk. These work like the pulse oximeter (oxygen monitor) that has been used for for the child already, but have a more sensitive detector.

NIRS data will be recorded in the operating room and in the CICU for all babies in the study. The NIRS values can be seen by the surgical and anesthesia teams in the operating room for all babies in the study. Once the patient is in the CICU, the NIRS values will be visible to the CICU team only if he/she is assigned to the intervention group. If he/she is assigned to the control group the NIRS values will not be visible to the CICU team. Currently, use of the NIRS monitor is not standard care in the CICU.

For half of the children in the study (intervention group) the team will follow a carefully designed plan on how to react to any changes seen. For the other half of the children (control group) the team will respond to any changes seen according to their standard practice. For the control group NIRS monitoring will occur in the operating room but not in the CICU. By using two groups, the investigators can compare the NIRS recordings. In this way the investigators will find out if blood supply to the tissues is altered.

No standard monitoring or treatments will be withheld from the patient.

Some extra urine tests will be taken in the 24 hours following surgery. They will be taken from the catheter inserted into the patient's bladder, as is standard for all heart surgery. No genetic tests will be done.

The patient will have usual medical care following the 24 hours of the study. The investigators will use the hospital's electronic notes to collect further information. This will include medications, routine labs and vital signs. The investigators may also call the family 60 days after surgery with a few simple questions about the patient's current state of health.

Inclusion Criteria:

• undergoing elective or emergency heart surgery
• under 6 months of age
• operation requiring cardiopulmonary bypass
• complex operation, defined as RACH-1 score 3 to 6 inclusive

Exclusion Criteria:

• weight less than 2 kg
• prematurity defined as less than or equal to 36 weeks post-concepional age at tme of operation
• preoperative renal failure defined as serum creatinine greater than 132 mmol/l
• major non-cardiac congenital anomalies or preoperative non-cardiac disease
• operation or any of its components not classified in the RACHS-1 system
• receiving preoperative mechanical circulatory support
• presence of known intracranial hematomas or cerebral arteriovenous malformations
• reoperation if previously enrolled