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.

Inclusion Criteria: 1. Diagnosis of single ventricle congenital heart disease 2. Status post-surgical or hybrid stage 1 palliation but prior to surgical stage 2 palliation 3. Age ≤ 30 days of life at time of stage 1 palliation 4. Age \< 6 months at time of enrollment 5. Require treatment with enteral digoxin per their treating medical provider 6. Informed consent obtained from parent(s) or legal guardian(s) Exclusion Criteria: 1. Gestational age at birth \ 2 mg/dL at enrollment 3. Diagnosis of second degree or higher atrioventricular conduction block at enrollment 4. Diagnosis of clinically significant sinus bradycardia requiring intervention at enrollment 5. Known hypersensitivity to digoxin or other forms of digitalis 6. Extracorporeal life support (i.e., ECMO, dialysis, ventricular assist device) at enrollment 7. Received digoxin prior to enrollment 8. Any condition that would make the participant, in the opinion of the investigator, unsuitable for the study