Interesting. And I'm glad to hear that research is being done into exercise related arrhythmias in racehorses.
But racehorses to showhorses is not exactly an apples to apples comparision. Racehorses are in an aerobic state for most of a race, and their peak workout is both longer and more intense than show jumpers. Also, the strain on a racehorse is primarily cardiovascular and skeletal; show jumpers use more slow twitch muscle to perform their jobs. A fit jumper going around a standard indoor stadium course (Usually less than a minute in duration; longer in at outdoor stadium or derby cours) may not show *any* shortness of breath or sweating, and his heart rate may only rise slightly, but he will have expended incredible muscle energy.
If I were to make a guess, even a wildass guess, I would say that what happened to Hickstead was a massive heart attack, but that it was related to some sort of underlying defect and was not a exercise induced arrhythmia, because I doubt he raised his heart rate to the point that it would induce arhyhtmia.. Just my guess, but at this point, before the results of the necropsy are released, my guess is as good as anyone else's.