Technically, it's not a heart problem; he has an aortic aneurysm. My dad had the same condition. The abdominal aorta is the main artery from the heart to the lower portion of the body. An aneurysm results when a wall of the aorta thins out and allows a ballooning of the vessel. If it gets too large and bursts, you bleed to death. Green's fortunate that they found it so that it can be repaired. It's a tough surgery, but it sounds like they expect he'll be able to return to playing next season.