Equine Athletes, The Equine Athlete's Heart and Racing Success
Dr Lesley E. Young,
Dr Lesley E. Young,
BVSc, DVA, DVC.PhD, DipECVA, MRCVS,
Animal Health Trust,
Newmarket,UK
Although the horse is often considered to be the premier athlete amongst mammals, VO2 relative to body mass of elite racehorses (~200mls/kg/min) pales into insignificance when compared to that of the diminutive Etruscan shrew (400mls/kg/min), or the Pronghorn antelope (300mls/kg/min). Nevertheless the cardiovascular system of the Thoroughbred racehorse has evolved to allow it consume more oxygen per kilogram than most other large mammals. The superiority of the Thoroughbred cardiovascular system rests in a proportionately larger heart and spleen per unit body mass than other large mammals. The equine cardiovascular system is hugely compliant with a heart rate range from 20 – 240 beats per minute and a splenic red cell reserve able to double packed cell volume and oxygen delivery during maximal exercise. Whilst heart rate is important in determining maximal cardiac output, stroke volume will be determined principally by heart size. In the Thoroughbred industry, it has long been believed that large hearts were associated with racing success. This is a topical issue and has lead to increasing use of echocardiographic and electrocardiographic methods to assess heart size at prestigious Thoroughbred yearling sales across the world. Anecdotes and the historical post-mortem records of elite racehorses encouraged the practice. Eclipse was unbeaten in 26 races and like Pharlap, the winner of 57 races, his heart after death weighed over 6 kg. This is 20% larger than that of an average racehorse. Secretariat, a record-breaking American racehorse was alleged to have had a heart that weighed over 10kg, and based on this heart size, it was suggested that his maximal cardiac output would have exceeded 500 L/min and his VO2max 240 ml/kg/min!
Our recent data have demonstrated a significant linear relationship between British Horseracing Board Official rating or Timeformârating and heart size measured by echocardiography in 400 horses engaged in National Hunt racing. Interestingly no such relationships are found when horses employed in flat racing are examined, suggesting that, as might be expected, VO2max and heart size are more important predictors of performance for equine athletes running longer distances. These data are corroborated by the discovery of a much stronger relationship between left ventricular mass and other measurements of cardiac size and VO2max in 18 Thoroughbred racehorses exercising on a high speed treadmill.
Selective breeding of performance horses for superior athletic ability has resulted in a mammal with a huge heart and improved aerobic capacity. It seems increasingly likely that these features are also responsible for many of the problems of today’s performance horse. Exercise-induced pulmonary haemorrhage occurs in all racehorses and visible epistaxis occurs not uncommonly both during training and racing. High intraluminal pressures and very negative alveolar pressure generated during peak exercise combine to produce extreme transmural pressures and stress failure of the pulmonary capillaries. The prevalence of audible murmurs of atrioventricular valve regurgitation varies between 54 (tricuspid valve) and 21% (mitral valve) in mature National Hunt steeplechasers, and atrial fibrillation is the commonest cardiovascular cause of poor performance in the Thoroughbred. The sustained form the arrhythmia is present in 1% of National Hunt horses in training, while paroxysmal fibrillation during exercise occurs in racehorses of all types. The prevalence of paroxysmal atrial fibrillation during racing, training and competition is difficult to assess, since riders are often unaware of the condition, unless the horse’s performance is affected. In the majority of horses affected with sustained and paroxysmal atrial fibrillation, there is no evidence of significant underlying cardiac disease. It seems likely that large atrial mass and changing autonomic influence during exercise provide the anatomical and physiological substrates for the re-entrant rhythm to develop and be sustained.
Acknowledgements
The Animal Health Trust and a Research grant of the Horserace betting Levy Board supported our epidemiological studies.
Animal Health Trust,
Newmarket,UK
Although the horse is often considered to be the premier athlete amongst mammals, VO2 relative to body mass of elite racehorses (~200mls/kg/min) pales into insignificance when compared to that of the diminutive Etruscan shrew (400mls/kg/min), or the Pronghorn antelope (300mls/kg/min). Nevertheless the cardiovascular system of the Thoroughbred racehorse has evolved to allow it consume more oxygen per kilogram than most other large mammals. The superiority of the Thoroughbred cardiovascular system rests in a proportionately larger heart and spleen per unit body mass than other large mammals. The equine cardiovascular system is hugely compliant with a heart rate range from 20 – 240 beats per minute and a splenic red cell reserve able to double packed cell volume and oxygen delivery during maximal exercise. Whilst heart rate is important in determining maximal cardiac output, stroke volume will be determined principally by heart size. In the Thoroughbred industry, it has long been believed that large hearts were associated with racing success. This is a topical issue and has lead to increasing use of echocardiographic and electrocardiographic methods to assess heart size at prestigious Thoroughbred yearling sales across the world. Anecdotes and the historical post-mortem records of elite racehorses encouraged the practice. Eclipse was unbeaten in 26 races and like Pharlap, the winner of 57 races, his heart after death weighed over 6 kg. This is 20% larger than that of an average racehorse. Secretariat, a record-breaking American racehorse was alleged to have had a heart that weighed over 10kg, and based on this heart size, it was suggested that his maximal cardiac output would have exceeded 500 L/min and his VO2max 240 ml/kg/min!
Our recent data have demonstrated a significant linear relationship between British Horseracing Board Official rating or Timeformârating and heart size measured by echocardiography in 400 horses engaged in National Hunt racing. Interestingly no such relationships are found when horses employed in flat racing are examined, suggesting that, as might be expected, VO2max and heart size are more important predictors of performance for equine athletes running longer distances. These data are corroborated by the discovery of a much stronger relationship between left ventricular mass and other measurements of cardiac size and VO2max in 18 Thoroughbred racehorses exercising on a high speed treadmill.
Selective breeding of performance horses for superior athletic ability has resulted in a mammal with a huge heart and improved aerobic capacity. It seems increasingly likely that these features are also responsible for many of the problems of today’s performance horse. Exercise-induced pulmonary haemorrhage occurs in all racehorses and visible epistaxis occurs not uncommonly both during training and racing. High intraluminal pressures and very negative alveolar pressure generated during peak exercise combine to produce extreme transmural pressures and stress failure of the pulmonary capillaries. The prevalence of audible murmurs of atrioventricular valve regurgitation varies between 54 (tricuspid valve) and 21% (mitral valve) in mature National Hunt steeplechasers, and atrial fibrillation is the commonest cardiovascular cause of poor performance in the Thoroughbred. The sustained form the arrhythmia is present in 1% of National Hunt horses in training, while paroxysmal fibrillation during exercise occurs in racehorses of all types. The prevalence of paroxysmal atrial fibrillation during racing, training and competition is difficult to assess, since riders are often unaware of the condition, unless the horse’s performance is affected. In the majority of horses affected with sustained and paroxysmal atrial fibrillation, there is no evidence of significant underlying cardiac disease. It seems likely that large atrial mass and changing autonomic influence during exercise provide the anatomical and physiological substrates for the re-entrant rhythm to develop and be sustained.
Acknowledgements
The Animal Health Trust and a Research grant of the Horserace betting Levy Board supported our epidemiological studies.
posted by Luis Felipe Triana at 11:54 AM
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