David Epstein of Sports Illustrated (SI.com) writes a very good article explaining how an enlarged heart leads to cardiac arrest in young athletes. Excerpts:
that the finding of an "enlarged heart" is quite normal for athletes, whose hearts, like any muscle, grow from exercise. There is a difference in a heart enlarged by exercise and one enlarged by disease, and the two can sometimes be difficult to tell apart.
Hypertrophic cardiomyopathy (HCM), a genetic disease and the most common cause of sudden death in athletes. (HCM is suspected in the deaths of Adams and Lewis, but an official cause of death is still unknown). At least one athlete in America, from high school to pros, will die from HCM about every other week while playing or practicing.
One treatment is to use an ICD (implantable cardioverter-defibrillator). The ICD, implanted in the chest, would shock the heart back to its normal rhythm after an arrest. Most athletes engaged in very strenuous activity do not get ICD's (Dr.'s do not normally recommend them for athletes because of the stresses they put on their heart). Anthony Van Loo, a professional Belgium soccer player, has an ICD. He collapsed during a soccer game. (See video here.) After being on the ground for about 5 seconds not moving, you see his body jolt and jump off the grass from the ICD shocking him (@27 seconds on the video). He quickly regained consciousness. The ICD probably saved his life.