Atrial fibrillation explored by researchers in a group of Thoroughbreds

Thoroughbreds in Hong Kong that showed episodes of atrial fibrillation continued to win races, with no effect on the length of their careers or performance, other than compulsory retirement after three bouts.
File image. Photo by Chris Kendall

Thoroughbreds in Hong Kong that showed episodes of atrial fibrillation continued to win races, with no effect on the length of their careers or performance, other than compulsory retirement after three bouts.

Atrial fibrillation is characterized by an irregular, often rapid heart rate that can cause poor blood flow. It occurs when the heart’s upper chambers, the atria, beat out of coordination with the lower ventricles.

Laura Nath and her colleagues, writing in the Journal of Veterinary Internal Medicine, noted that atrial fibrillation is the most commonly recognized arrhythmia in poorly performing horses, with an estimated prevalence among racehorses of 0.11% to 0.29%.

Because of concerns for the health and welfare of horses and jockeys, and the impact of erratic performance on betting markets, many racing jurisdictions impose sanctions on horses after episodes of atrial fibrillation.

The Hong Kong Jockey Club provides a unique opportunity to investigate the heart condition because it manages a large population of horses with detailed veterinary records.

About 22 percent of horses are typically examined after a race by a veterinarian, which is a substantial group for the evaluation of post-race atrial fibrillation.

In their study, the researchers set out to describe the incidence, recurrence, and outcome of post-race atrial fibrillation, and describe the ECG characteristics of the condition in the post-race period.

They hypothesized that horses diagnosed with post-race atrial fibrillation were at increased risk for future episodes compared to horses without a history of arrhythmia.

The study team examined the medical records of Thoroughbreds racing in Hong Kong from July 31, 2007, until July 31, 2017.

The study population comprised 4684 horses, covering 96,135 race starts.

The Hong Kong Jockey Club clinical records were searched for all starts in which an episode of atrial fibrillation was diagnosed during a post-race veterinary inspection.

Atrial fibrillation was identified in 4.9% of horses, with an overall incidence of 2.7 episodes per 1000 starts.

The incidence of atrial fibrillation in horses after any previous episode was higher (12.8 per 1000 starts) than for horses with no previous episode (2.4 per 1000 starts).

Recurrence was seen in 64% of horses previously treated for persistent atrial fibrillation, which was higher than recurrence in horses with sudden bouts of arrhythmia.

The median duration between episodes was 343 days, ranging from 34 to 1065 days.

The authors concluded that Thoroughbreds were at increased risk of recurrent atrial fibrillation after sudden bouts or persistent episodes, but the time between episodes varied widely.

Of the 230 horses identified as having atrial fibrillation, 46 were retired immediately. Of the 184 that continued to race, 138 retired without a second episode and one had not yet retired. Thirty-six had a second episode during the study period and six had a second episode outside the study period. Three had a second episode post-race outside the study period.

Of the 45 horses with at least two episodes of cardiac arrhythmia, 30 retired immediately after the second episode.

Of the 15 that continued racing, nine retired without a third episode and one had not yet retired. One had a third episode post-race during the study and four had a third episode outside racing.

The authors noted that, after two episodes of post-race atrial fibrillation, 15 horses were embargoed and not allowed to start in any race for six months. Five horses were compulsorily retired after three episodes.

They noted that career duration was not significantly different between groups when measured by either days or the number of starts after the first episode.

The number of recurrent episodes did not impact career longevity or overall career performance, other than the handful of horses who had three episodes and were compulsorily retired.

One horse died suddenly 735 days after a sudden bout of atrial fibrillation, giving an incidence of sudden death in horses with a previous episode of the condition of 0.35 per 1000 starts.

Of the 182 horses that presented for a race start after an episode of post-race atrial fibrillation, 117 (64%) won at least one race. The median number of races won was two.

The authors said they had identified a high rate of recurrence in Thoroughbred racehorses after both sudden and persistent episodes of atrial fibrillation.

“This substantial atrial fibrillation burden could arise from underlying microstructural myocardial lesions and electrical remodeling.

“Although horses can have long and successful careers after atrial fibrillation, the arrhythmia should not be considered benign, and the suitability of horses to continue their racing careers should be assessed on an individual basis.”

The study team comprised Nath, Adrian Elliott and Samantha Franklin, with the University of Adelaide; Joe Weir and Peter Curl, with the Hong Kong Jockey Club; and Sarah Rosanowski, with EVC Global Ltd in Hong Kong.

The study received backing from Racing Victoria and Agrifutures Australia.

Nath LC, Elliott AD, Weir J, Curl P, Rosanowski SM, Franklin S. Incidence, recurrence, and outcome of postrace atrial fibrillation in Thoroughbred horses. J Vet Intern Med. 2021;1–10. NATH ET AL.

The study, published under a Creative Commons License,  can be read here.

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