The investigators will investigate the safety and preliminary efficacy of ISDN therapy to
reduce venous congestion and improve exercise tolerance in children and adults after the
Fontan operation. This will be accomplished by recruiting 15 Fontan physiology patients from
the Cincinnati Children's Fontan clinic and University of Kentucky Pediatric Cardiology
clinic for the investigation. The investigators will non-invasively measure both central
venous pressures at rest and during graded cardiopulmonary exercise testing. In addition the
study team will obtain a measurement of liver stiffness before and after a 4-week regimen of
ISDN therapy. Patients will be seen twice in clinic, once before and after ISDN therapy, and
phone calls will be made to ensure safety, compliance, and make appropriate alterations to
medications throughout the study period. Pre- and post-intervention central venous pressure
and exercise tolerance will be compared.