Showjumping horses are more likely to develop ventricular arrhythmias compared to horses in other disciplines.
University of Copenhagen researcher Rikke Buhl, reporting on exercised-induced cardiac changes in equine athletes to delegates at the recent International Equitation Science conference in Denmark, suggested the higher rate could be explained by the substantial heart-rate fluctuations experienced by sport horses during jumping.
Professor Buhl, an equine cardiologist at the Danish university, told delegates that well-trained horses would, over time, develop a larger heart, mild low-grade cardiac murmurs and most likely some cardiac arrhythmias during and after exercise – all characteristic of the phenomenon known as Athlete’s Heart.
“Whether these changes could lead to potential fatal arrhythmias and sudden death in equine athletes remains speculation,” she said.
Buhl said prolonged exercise-induced dramatic changes in the body.
With increased oxygen demands during exercise the demand for blood flow is met by increased stroke volume and increased heart rate, resulting in increased cardiac output.
“For the short-term, the heart can easily cope with the greater volume and/or pressure loads,” she explained.
“However, when the overload is repeated over time, other mechanisms becomes activated, leading to an increase in cardiac muscle mass.
“This physiological phenomenon is described as the ‘Athlete’s heart,” she said.
Buhl said there had been considerable scientific debate over the years as to whether large hearts in horses were associated with racing success.
“The rationale behind this hypothesis is that stroke volume is increased in athletes with a larger heart and maximum oxygen uptake will increase, which has been documented in thoroughbreds.”
These results, she said, supported findings in studies of racehorses showing a positive relationship between heart sizes and racing performance.
In addition to cardiac hypertrophy (enlargement), equine athletes developed valvular regurgitations – leaking cardiac valves which are heard as cardiac murmurs.
“The physiological explanation for the development of valvular regurgitations is unclear,” she said, “but in general these regurgitations are small and do not increase in severity over time.
Moreover, it has been shown that the majority tended to disappear after exercise.
“Exercise-induced cardiac changes also result in cardiac arrhythmias,” she told delegates.
Studies of arrhythmias in normal performing racehorses and riding horses have been published recently and quite high amounts of cardiac arrhythmias were observed, she said.
Buhl said high-performance horses were at risk of dying from cardiac failure while performing at maximum capacity; and horses have collapsed and died during or after exercise.
The pathophysiological changes underlying collapse or sudden cardiac death in horses were unknown, she said.
“Sudden death of horses has a big impact on animal welfare, rider safety, finances and public relations.
“Most horses that suddenly die do not show sufficient lesions to account for death on necropsy.
“As spontaneous arrhythmias are common in the exercising horse it can be speculated that cardiac repolarization disorders may be responsible for sudden death as described in humans, but this area need to be elucidated further.”