PK/PD of Digoxin in Infants With SVHD

The primary participant will be an infant with single ventricle heart disease. This is a research study to learn more about how the medication digoxin, which is routinely prescribed to infants and children with heart disease in pediatric cardiac intensive care units is processed by their bodies and how it may help their cardiac function. The investigators will collect blood or will collect blood samples when bloodwork is checked as part of regular care ("opportunistic"). The investigators will also collect information from medical records. Being part of this study will not change treatment plan or medications. The risks of this study include loss of confidentiality and risks associated with having blood drawn. The study team will make every effort to minimize these risks.