Horses show promise as a model for studying sudden cardiac death in athletes

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Horses could prove to be a useful model for studying sudden cardiac death in human athletes, according to researchers.

Researcher Kamalan Jeevaratnam and his University of Cambridge colleagues said horses had advantages over laboratory rats and mice.

“Firstly, from an athletic life course perspective, horses, but not mice or rats, undergo a similar sequence to humans involving training, peak performances, and retirement from active participation.

“This allows investigation into underlying electrophysiological changes in the veteran athlete.”

Secondly, resting heart rates in rates and mice ranged from 500 to 700 beats per minute (bpm), 10-fold higher than resting human heart rates.

During exercise, the heart rate of rats and mice can increase 10% to 50%, whereas human heart rates can increase by up to 300%.

“Equine heart rates follow a more similar pattern to humans, ranging from as low as 20 bpm to around 229 bpm during maximum exertion.”

Horses, they said, were also more amenable to physiological analysis of exercise-induced heart rate changes.

Thirdly, there were greater similarities in electrical repolarization between the human and horse heart than between the human and rodent heart.

“Finally, amongst veterinary species, horse hearts have a unique ability of displaying spontaneous atrial fibrillation in the absence of gross structural abnormalities, as occurs in humans.”

Several studies, they said, had investigated the prevalence of such arrhythmias in horses.

An international study found that identifiable cardiac lesions accounted for only 54% of sudden cardiac death cases in Thoroughbreds, leaving a possibility of fatal cardiac arrhythmia in the remaining cases.

“Our limited understanding of equine cardiac electrophysiology compromises development of reliable diagnostic, therapeutic or prognostic strategies,” they said.

“This lack of understanding also prevents the equine athlete from being explored as a model for understanding physiological and pathological cardiac electrophysiology in the human athlete.”

The researchers, writing in the open-access journal PLOS ONE, said that to validate the equine athlete as an electrophysiological model for human athletes, it was important to determine whether the equine system is amenable to standard electrophysiological analysis techniques at incremental heart rates.

The study involved examination of 34 electrocardiograms (ECGs) previously recorded from healthy Thoroughbreds in race training presented for work-ups at Rossdales Equine Hospital and Diagnostic Centre in Suffolk.

All ECGs been recorded while horses were exercised as part of their established performance assessment programs. All were of racing age (aged 2–8) at the time of testing.

For each horse, the recordings were acquired before and during a period of acceleration from walk to canter. The protocol yielded a systematic series of recordings at a range of relatively steady incremental heart rates.

All ECGs were screened retrospectively for possible further study.

Data from seven horses spanning between 10–18 minutes of ECG recording fulfilled the necessary criteria for full analysis in the study.

The study team traversed their analysis work.

“We explored the electrophysiological capacity for the heart in the intact horse to function through its normal range of low, baseline through high, exercising heart rates.”

Their work identified a physiological basis for the large range possible with the equine heart, explaining why horses are capable of showing high heart rates without compromising their electrophysiological stability.

“This ability of a horse’s heart to increase heart rate allows the heart to meet the metabolic demands during exercise and racing.”

They concluded: “This study suggests a strong potential of the equine athlete being used to model the cardiac electrophysiology of the human athlete and future studies should aim to validate this further.”

The study team comprised Mengye Li, Karan Chadda, Gareth Matthews, Christopher Huang and Kamalan Jeevaratnam, all affiliated with the University of Cambridge; and Celia Marr, with Rossdales Equine Hospital and Diagnostic Centre in Suffolk.

Li M, Chadda KR, Matthews GDK, Marr CM, Huang CL-H, Jeevaratnam K (2018) Cardiac electrophysiological adaptations in the equine athlete—Restitution analysis of electrocardiographic features. PLoS ONE 13(3): e0194008. https://doi.org/10.1371/journal.pone.0194008

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

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