This study is being done to try to learn more about how a child's body breaks down bupivacaine, a local anesthetic medication that is being used for pain control. This study will provide information about how much medication gets into the bloodstream and how long it remains in the blood. Newborns and young infants often process drugs in their body in a different way than older children. This study will help the investigators determine how bupivacaine is broken down in the body of infants.

A transverse abdominal plane (TAP_ block will be performed by one of the investigators who is adept in using ultrasound guidance after induction of general anesthesia. The dose of bupivacaine administered will be based on the weight of the patient. This dose is consistent with routine levels of local anesthetic used for peripheral nerve blocks and is lower than toxic levels so there will be no additional change in renal or liver function. Whole blood samples will be collected on a piece of filter paper from either an arterial, central, peripheral line or heel stick. The extracts from dried blood samples (DBS; calculated volume 20 µL) collected on filter paper will be analyzed using an LC-MS/MS system in combination with online extraction (LC/LC-MS/MS). Blood samples will be obtained at 0, 5, 15, 30, 60, 120 minutes, 4, 24, 48 hours. If obtained from a heel stick, these samples will be obtained at time of glucose sampling or a clinical indication. Subjects will be considered evaluable if they have 5 samples. All sampling for the study will be based on concurrent sampling so there will be no additional blood draws.

Pain will be assessed by the bedside nurse or study research assistant using the Neonatal Infant Pain Scale (NIPS). As part of the standard of care in this institution, nurses will have the opportunity to administer additional pain medications including but not limited to intravenous fentanyl or morphine. This will be done as per standing orders if the neonate meets criteria for additional analgesia, in other words, if there is an increase in pain as indicated by the NIPS pain scores. NIPS score will be recorded prior to each blood draw and every 8 hours while the subject is enrolled in the study. The NIPS will be the primary clinical outcome measure for this study. Blood pressure and heart rate will also be recorded as a physiologic correlate for drug level at these times.

Inclusion Criteria:

• Neonates under 28 days of age undergoing any abdominal surgery including laparotomy, colostomy placement. In addition neonates who have procedures performed using laparoscopic as well as open techniques will be included
• Written informed consent from parent or guardian

Exclusion Criteria:

• Neonates with significant cardiovascular disease (any child with any congenital heart disease will be excluded from the study)
• Neonates with significant liver disease (any neonate with ALT/AST elevated 30% above normal values as determined by the lab at Lurie Children's at the time of testing)
• Neonates under 1.65 kg

Neonates younger than 28 days undergoing any abdominal surgery.