Gluck Equine Research Center, 2006
"Heart Attack and Heart Disease in Horses"
Discussion of sudden death in horses, including "Swale Syndrome" (yes, it's a named "disease process" recognized in veterinary medicine) and how there are often detailed pathology found that makes such deaths more specifically determinant of a cause. In other words, what can be wrong when a horse just "drops dead" of non-traumatic causes, and no other organ system dysfunction is apparent.
http://www.ca.uky.edu/gluck/q_oct10.asp