Beta-blocker Administration for Cardiomyocyte Division

15N-thymidine will be administered to patients in order to quantify cardiomyocyte proliferation. 15N-thymidine will be administered as 5 separate syringes. Each syringe will contain the recommended daily dose based on participants weight. Subjects will be given the labeled thymidine by mouth once a day for 5 consecutive days. The dosing will begin one week after starting the study drug.The caregiver (parent or nurse) will administer the labeled thymidine by mouth via a syringe once a day for 5 consecutive days. They will be provided with a diary to keep track of study drug dosing.

Parents will be asked to keep a diary of the child's feeding and diaper changes during the administration of thymidine. To monitor how the labeled thymidine is processed in the body, investigators will analyze urine from the child's diapers. Each time a diaper is changed, a cotton ball will be placed in the diaper. At the next diaper change, this cotton ball will be removed from the diaper, placed in a Ziploc bag, and stored in the freezer. They will be collected and stored cotton balls starting with the first administration of thymidine until 24 hours after the last administration. The frozen cotton balls may be collected from the home by a nurse or brought by parent to the next clinic visit.

On Study Day 1 to obtain baseline assessment of cardiac size. A second research dedicated echocardiogram will be done after being anesthetized for surgery. Echocardiography is routinely performed to determine right ventricle (RV) structure of the heart and function in Tetrology of Fallot/ Pulmonary Stenosis (ToF/PS) and is non-invasive.

A limited study protocol will be performed using the feed-and-bundle technique at the time of study enrollment, The feed and bundle technique is utilized so that the infant will be as comfortable as possible and asleep during the procedure. This is done to prevent the risk of sedation and allow for a more natural sleep. The baby is undressed, prepared with ECG-leads and an oxygen saturation probe, and fed. With this "feed, swaddle, and sleep" technique, scanning times of 30-60 minutes can be achieved, which provides the stillness needed for accurate visualization of the heart. A second CMR, with the same limited study protocol, will be performed under general anesthesia prior to surgical repair. It does not pose any risk of radiation exposure and will not require IV placement or contrast administration for a limited assessement of the right ventricle.

For patients who are randomized to receive propranolol, The target dose for this study will be 4 mg/kg/day divided in 4 doses. The first dose will be given in the Pediatric Clinical and Translational Research Center (PCTRC) at 1mg/kg. Parents will administer propranolol (1 mg/kg po x 4 doses per day). Parents will be instructed to administer propranolol with feeding or soon after feeding, but not before feeding. families will also be given instructions for regarding 15N-thymidine dosing frequency and interval and provided with a log sheet to record administered and missed doses of propranolol and 15N-thymidine.

For patients who are randomized to receive placebo, the first dose will be given in the Pediatric Translational Research Center (PCTRC) and will be based on the patients weight. Parents will administer placebo 4 doses per day. Treatment with placebo will begin at 1 month of age and continue until 4 months of age or 24hrs prior to surgical repair, whichever comes first.

On Study Day 1; monthly until surgery in conjunction with patient's clinical cardiology appointment; and at the time of surgery. Will include vital signs.

On the morning of surgery, after IV access is obtained for anesthesia 2 ml of blood will be collected for analysis. A small piece of heart tissue and fat that has already been removed during surgery will be collected for analysis.