The study will last two years. Patients will be placed on one of two pacemaker programs for the first year and another the second year. At 3- and 6-month follow-up visits, a pacemaker check and echocardiogram will be performed. After 1 year, patients will be admitted to NIH for 2 to 3 days for exercise tests, echocardiogram, and cardiac catheterization. Also, the pacemaker will be changed to the second program. At 15- and 18-month follow-up visits, a pacemaker check and echocardiogram will be performed. After 2 years, patients will again be admitted for 2 to 3 days for exercise tests, echocardiogram, and cardiac catheterization. A pregnancy test will be given to females of child-bearing age before each cardiac catheterization and electrophysiology study.