The goal of this observational study is to assess the clinical impact following Fontan stent placement for extracardiac Fontan conduit (EFC) stenosis in patients with single-ventricle Fontan-palliated patients ages 12-21 years old.

The main question it aims to:

-Assess changes in exercise capacity by cardiopulmonary exercise testing and liver stiffness assessed by shear wave elastography before and after EFC stenting.

Participants will undergo two cardiopulmonary exercise tests and three shear wave elastography scans.

Inclusion Criteria:

• Functionally single-ventricle Fontan-palliated patients undergoing clinically indicated cardiac catheterization
• Anticipated Fontan stent placement for extracardiac Fontan conduit stenosis at the discretion of the primary operator
• Weight ≥ 50 kg
• 12-21 years of age

Exclusion Criteria:

• Interrupted IVC
• Greater than mild atrioventricular valve regurgitation on most recent echocardiogram or MRI (RF > 20%).
• Greater than mild systemic ventricular dysfunction on most recent echo or MRI (EF < 40%).
• Non-sinus rhythm
• Pacemaker
• Inability to perform exercise test due to developmental or physical disability

Patients 12-21 years old with functioning single-ventricle Fontan-palliated who are scheduled to undergo clinically indicated fontan conduit stenting.