Racehorses and other performance horses occasionally succumb to cardiac death during or after exertion, and in most cases equine researchers are at a loss as to why.

"It's an infrequent phenomenon, but it has an impact way out of proportion to the frequency with which it occurs," said Peter W. Physick-Sheard, BVSc, MSc, FRCVS, of the University of Guelph, referring to the welfare of the horse, cost to the owner, and public perception of these incidents.

To study how exercise affects a horse's heart, researchers recently hooked up portable electrocardiogram (ECG) devices, called Holter monitors, to continuously monitor Standardbreds at an Ontario racetrack. The monitors enabled them to check the horses' heart rates and rhythm variations from rest, through warm-up, and to the end of actual races. The special electrode harness did not interfere with normal equipment or the race in any way, he said.