They did some crazy stuff to her heart. Lucy's biggest problem with her heart was her pulmonary stenosis (that is the narrowing of her pulmonary artery). Hers was so narrowed that a lot of doctors just refer to it as pulmonary atresia(that's where the artery is completely closed off or missing. They were planning on removing the narrowed section of her pulmonary artery and replace it with a human donors. When they got in there they were able to save her artery and place the human donors next to it. The grandpas have been referring to it as a double barreled shotgun. The idea is that hopefully her real artery will keep growing so that she doesn't need replacements her whole life.
Wow, if you made it through all of that you're a trooper.