The purpose of this study is to see if the study drug called spironolactone reduces fibrous (stiffening) in heart muscle tissue and improves heart function. Subjects from the study titled "Heart Failure in Congenital Heart Disease: the role of myocardial fibrosis" who have evidence of heart dysfunction and/or evidence of fibrosis (stiffening) in the heart muscle will be asked to take part in this study.

This study will include randomizing the subject to receive the study drug or placebo. Randomization will occur during visit 1.

Visit 1 will include the following:

• Subjects will answer questions about how well they can breathe.
• An MRI. Dye called gadolinium will be injected into the subject's vein.
• They will go to the Oregon Clinical and Translational Research Institute where 2 tablespoons of blood will be drawn from an intravenous (IV) catheter (tube).
• They will do a 6 minute walk test..
• They will also have an echocardiogram, which is a test that looks at the movement of the subject's heart. A technician will place a cool jelly on their chest and use a small wand to take pictures through the skin.

Subjects' will also have visits 3-6 weeks, 3, 6, 9 months, and 12 months after randomization. Visits 2-5 will include the following:

• They will go to the Oregon Clinical and Translational Research Institute where 1/2 tablespoon of blood will be drawn.
• They will do a 6 minute walk test. During this test, they will walk back and forth in a hallway. The goal is to walk as far as possible for 6 minutes. Subjects will probably get out of breath or become exhausted. If they do, they can slow down, stop, or rest as they need to. Blood pressure will be taken before the walk.
• They will have a health review at 6 months.

Visit 6 will be identical to visit 1 and include the following:

• Subjects will answer questions about how well they can breathe.
• An MRI. Dye called gadolinium will be injected into the subject's vein.
• They will go to the Oregon Clinical and Translational Research Institute where 2 tablespoons of blood will be drawn from an intravenous (IV) catheter (tube).
• They will do a 6 minute walk test..
• They will also have an echocardiogram, which is a test that looks at the movement of the subject's heart. A technician will place a cool jelly on their chest and use a small wand to take pictures through the skin.

The investigators will compare the study drug, called spironolactone, to placebo with regard to any changes in heart stiffening and function of the heart.

Inclusion Criteria:

1. Fibrosis index ≥29%, or

2. Evidence of cardiovascular dysfunction including any of the following:

   • Systemic ejection fraction <55%,
   • NYHA 2-3
   • 6-minute walk distance <500 m.

3. Completion of Visit 1 of the study Heart Failure in Congenital Heart Disease: the role of myocardial fibrosis" (eIRB # 3665) including meeting all inclusion for that study (Aged 18-80, Known congenital heart disease).

4. Tetralogy of Fallot, cyanotic congenital heart disease, or a systemic right ventricle.

Exclusion Criteria:

1. Patient currently taking spironolactone or previously taking spironolactone within the last 6 months.
2. Serum potassium ≥5.0 mmol/L at the initial visit, if not taking potassium supplements. Patients will be eligible if a repeat potassium is <5.0 mmol/L after potassium supplements have been discontinued.
3. Moderate/severe systemic atrioventricular valve regurgitation,
4. Likely to undergo cardiac surgery, pacemaker implantation, or possible transplantation within one year (all self-reported),
5. Unwilling to commit to return visits including mandatory blood draws for potassium,
6. Renal insufficiency (estimated creatinine clearance < 30 ml/min/1.73m2),
7. Positive urine pregnancy test.
8. Any contraindication to MRI.