Clues About Cardiac Arrest in Racehorses

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.

Holter Monitor worn

Researchers at the University of Guelph used Holter Monitors, shown above and below, to track heart rates and rhythm variations in Standardbred racehorses.

Holter Monitor device

They found that in 16% of horses, heart rate slowing after the race was associated with complex and potentially serious ventricular rhythm disturbances. In one-third of horses with these rhythm disturbances, heart rate slowing after the race was initially gradual, then rate slowed suddenly, then sped up, and then slowed down gradually again. Arrhythmias occurred during these rapid slowing events in these horses. "If you plotted individual heartbeats on a graph it would look like the heart rate was coming down in steps, with the surface of the steps being tilted upward instead of being flat," he said. In these horses the rhythm disturbances were closely associated with the sudden slowing.

"We don't know at this point whether these rhythm disturbances are associated with sudden death; we just know they could be," he said. "They occur at a stage of competition known to be a risk period for sudden death."

Physick-Sheard said experts always assumed horses that died suddenly during a competition had experienced serious abnormal heart rhythms, but they didn't know for sure.

"Our data support this interpretation, but don't prove it," he said. "Our findings suggest that complex ventricular rhythm disturbances may be quite common immediately post-race and indicate the vast majority of horses spontaneously recover."

Because the work is preliminary, Physick-Sheard said he could not suggest any other tactics for after-race care than what riders and trainers are already doing: "cooling a horse out gradually after work, particularly after intense work (and on hot/humid days), and paying close attention to hydration, electrolyte, and mental status and avoiding letting a horse get exhausted."

The study, "Ventricular arrhythmias during race recovery in Standardbred racehorses and associations with autonomic activity," was published in June online ahead of print in The Journal of Veterinary Internal Medicine. The abstract is available on PubMed.

About the Author

Marie Rosenthal, MS

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