Submitted on November 23, 2004
Accepted on May 24, 2005
Left ventricular size and systolic function in thoroughbred racehorses and their relationships to race performance
L. E. Young1*, K. Rogers2, and J. L.N. Wood3
1 Centre for Equine Studies, Animal Health Trust, Newmarket, Suffolk, United Kingdom
2 Department of Epidemiology, Animal Health Trust, Newmarket, Suffolk, United Kingdom
3 Department of Veterinary Medicine, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom
* To whom correspondence should be addressed. E-mail: lesley.young{at}aht.org.uk.
Cardiac morphology in human athletes is known to differ depending upon the sports-specific endurance component of their primary event, whilst anecdotes abound about superlative athletes with large hearts. As the heart is a key determinant of stroke volume and VO2max in mammals, we undertook a study to test the hypothesis that the morphology of the equine heart would also differ between trained equine athletes, depending upon their race type, and that left ventricular size would be greatest in horses racing successfully.
Echocardiography was performed in 482 conditioned and race fit Thoroughbred racehorses engaged in either flat (race distance 1000-2500m) or jump racing (3200-6400m). Associations between different indices of left ventricular size and function and objective measures of race performance were determined using a standard regression approach.
Body weight and gender adjusted measures of left ventricular size were largest in horses engaged in jump racing over fixed steeplechase fences compared to horses running shorter distances on the flat (range 8 - 16%). The observed differences in cardiac morphologies between horses suggest that subtle differences in training and competition also result in differing cardiac adaptations that are appropriate to the endurance component of the event.
Derived left ventricular mass was strongly associated with published rating (quality) in horses racing over longer distances in jump races (p
0.001), but less so for horses in flat races. Rather, left ventricular ejection fraction and left ventricular mass combined were positively associated with race rating in older flat race horses running over sprint (<1408m) and longer distances (>1408 m), explaining 25-35% of overall variation in performance, as well as being closely associated with performance in longer races over jumps (23%). Predicted differences between otherwise equivalent horses with small and large hearts could thus explain a significant proportion of the difference between elite and non-elite racehorse performance, providing the first direct evidence that cardiac size influences athletic performance in a group of mammalian running athletes.
