Venous Compression in Fontan

The aim of this study is to investigate whether venous compression garments increase exercise stroke volume in patients with Fontan circulation. To address this aim, we will test the following hypotheses: 1. Acute and chronic external venous compression will increase exercise stroke volume in patients with Fontan circulation. 2. Patients with significant venous varicosities will have a greater response to venous compression. Participants will: * Undergo submaximal exercise testing in MRI to measure venous return and exercise stroke volume with and without the wearing of compression garments * Undergo submaximal exercise testing on a seated upright exercise ergometer with concurrent measurement of stroke volume with and without the wearing of compression garments * A subset of participants will repeat both testing visits after wearing compression garments for 2-weeks during waking hours. Participants will: * Undergo submaximal exercise testing in MRI to measure venous return and exercise stroke volume with and without the wearing of compression garments * Undergo submaximal exercise testing on a seated upright exercise ergometer with concurrent measurement of stroke volume with and without the wearing of compression garments * A subset of participants will repeat both testing visits after wearing compression garments for 2-weeks during waking hours.

Study Day 1 - Consent, Screening and Garment Measurements: Following assessment of participant eligibility, all participants will undergo leg venous ultrasound assessment and abdominal-foot measurements for the custom making of compression garments. Study Day 2 - MRI: Study participants will undergo comprehensive cardiovascular MRI using a 3T MRI scanner with and without compression garments and exercise. Study Day 3 - Cardiovascular \& Exercise Testing Day: All participants will undergo comprehensive cardiovascular structure and function assessments at rest and during exercise with and without the wearing of compression garments. Baseline testing will include dual x-ray absorptiometry (DEXA), pulmonary function testing, comprensive vascular assessments and echocardiography. During exercise at two submaximal workloads, the study team will measure blood pressure, oxygen uptake and cardiac output.

Patients with a Fontan circulation - Inclusion criteria: • Patients at UTSW aged 18-45 years old with a Fontan circulation. Exclusion Criteria: * NYHA Class 4 * Severe AV valve regurgitation * Severe ventricular dysfunction * Large ascites * Significant varicosities * Atrio-pulmonary (AP) Fontan * Known Fontan thrombus * Poorly controlled arrythmia * Unplanned cardiac admission in the last 6 months * Cyanosis (resting O2 saturation \<85%) * Pregnant * Common femoral vein obstruction \>50% to be performed following completion of informed consent. Healthy Controls: Healthy control participants will be recruited on the basis of having no known chronic diseases and not currently taking any cardiovascular medications.