Inclusion Criteria:  Individuals with Fontan physiology being seen in the Boston Children's Hospital cardiology clinic for outpatient care.  Exclusion Criteria:  Current pregnancy Decompensated heart failure/pulmonary edema Severe aortic insufficiency Active tachyarrhythmias Frequent atrial or ventricular ectopy Symptomatic peripheral vascular disease Thrombophlebitis or history of deep vein thrombosis or stasis ulcer Aortic aneurysm Uncontrolled hypertension (systolic blood pressure >180 mmHg or diastolic blood pressure>110 mmHg) Systolic blood pressure <90 mmHg Active femoral site bleeding or hematoma