Blood samples (about 2 tablespoons) will also be collected after exercise, during your surgery, 24 hours after your surgery, and at 5 days after surgery or the time you discharge from the hospital (which ever is shorter). These blood samples will also be collected at your routinely schedule post operative clinic visits at 1 month, 6 months, and 1 year after surgery. The samples will be used to determine the amount of endothelial cells in the blood stream.