Inclusion Criteria:  Males and females with Fontan physiology who are 13 to <19 years of age at enrollment Own a mobile device capable of installing the University of Michigan Patient Portal application Fluent in English Participant consent or parental/guardian consent and participant assent  Exclusion Criteria:  Height < 130 centimeters Current intravenous inotropic drugs Severe ventricular dysfunction assessed qualitatively by clinical echocardiography within six months prior to enrollment. Severe valvar regurgitation, ventricular outflow obstruction, or aortic arch obstruction assessed by clinical echocardiography within six months prior to enrollment. History of arrhythmia with exercise (excluding isolated supraventricular or ventricular ectopy without symptoms). Inability to complete exercise testing at baseline screening. Noncardiac medical, developmental, and/or social disorder that would prevent successful completion of planned study testing or would invalidate its results. Suicidality or homicidality in the past 6 months.