The Fontan Dapagliflozin Pilot Study

The goal of this clinical trial is to study if an investigational study drug called Dapagliflozin could prevent heart failure from getting worse in adults with Fontan circulation. The main questions it aims to answer are: 1. Does Dapagliflozin decrease Fontan pressure? 2. Does Dapagliflozin improve exercise capacity and heart failure symptoms? Participants will have 4 study visits and 2 follow-up phone calls. The total duration of participation in the study will be up to 5 weeks from the time of screening to the completion of the final safety evaluation. Study procedures include the collection of study-related health information and blood samples, physical examination, exercise testing, total body water assessment, blood laboratory testing, health status survey, safety evaluation phone calls, and home blood pressure monitoring.