Regenerative therapies could provide new ways of treating heart failure. Unlike many organs in the human body, such as the skin and the GI tract, the ability to regenerate heart muscle decreases after birth, but the precise timing of this decrease and how this decrease is altered in heart disease are uncertain. The investigators will use an innovative approach to quantify cellular heart regeneration in pediatric patients, an appropriate population for determining this decline as well as the potential for reactivating heart muscle regeneration. The study has now been registered on ClinicalTrials.gov, despite its initiation on July 23, 2015, as registration was not mandated at the original study site, the University of Pittsburgh Medical Center. However, following the transfer of the study to Weill Cornell Medicine, adherence to institutional requirements necessitated its registration on ClinicalTrials.gov.