Fontan-palliated patients comprise an interesting group of single-ventricle patients who are dependent upon passive return of blood from the body to the lungs. Systemic venous pressures are therefore quite elevated, and one study demonstrated that venous capacitance was diminished. In addition, these patients have markedly diminished aerobic capacity (Max VO2). Although one recent study demonstrated that Fontan patients can increase their Max VO2 through diligent cardiac rehabilitation, their ultimate results were still well below average.

It has previously been described that normal, healthy subjects have decreased exercise performance in a supine vs upright position. However, due to high venous pressures and the need for systemic venous return to overcome gravity in order to perfuse the lungs, one might hypothesize that Fontan patients would have increased exercise performance in a supine vs upright position. The investigators are interested in studying this hypothesis.

Inclusion Criteria:

• S/p Fontan palliation > 2 years prior
• Age 8 to 30 years

Exclusion Criteria:

• Pacemaker dependence
• Severe hypoxemia (O2 saturation <80%)
• Recent atrial flutter or other arrhythmias
• Protein losing enteropathy (PLE)
• Severe atrioventricular valve regurgitation
• History of cardiac arrest
• Significant comorbidities (renal failure, asthma, etc).

Fontan-palliated patients aged 8-30 will be recruited for this study. Eligible patients will be identified from the cardiology and surgery database here. Patients will be contacted by phone and/or formal letter informing them of the study. If their primary cardiologist is not in our group, a courtesy call will be made or letter sent. Age and gender matched healthy controls will be sought as well through local recruitment (schools, university campus, local community postings).