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The horse’s heart is one of the most efficient pumps found in nature and is a major reason why the horse is such an excellent athlete. If we consider the equine ‘power train’ (lungs, heart and muscle), the lungs absorb fuel (oxygen) and transfer it to the blood for the heart to pump it to the engine (muscle) where it is burnt to create the power to drive the skeleton.
Few animals can match the horse for the efficiency of this power train and even the most elite human athletes have only half the power train efficiency of the average horse.
The equine heart is a remarkably hard-working pump and has a wider range of rates than humans, from a very slow 30 beats per minute at rest to an enormously impressive 240 bpm when galloping. The rapid speed at which it returns to resting rate after exertion is also impressive and is an indicator of fitness. The heart of a thoroughbred at rest pumps 35 litres of blood per minute compared to five by a human heart. The ability to maintain high exercising heart rates combined with the large size of the equine heart allows it pump huge volumes of blood for extended periods during strenuous exercise and so deliver more oxygen to muscles.
Horses with larger hearts generally have greater athletic ability with some of the world’s most famous racehorses including Eclipse, Phar Lap and Secretariat having much larger hearts than average. A large heart is not the only factor creating a successful racehorse and elite horses have not just an efficient power train but also excellent skeletal function and a desire to win.
Common cardiac problems that hinder equine performance, particularly in athlete horses, include murmurs (abnormal blood flow) and arrhythmias (abnormal cardiac rhythms). Unlike in other species (for example dogs) where the assessment of cardiac disease mainly relates to whether they are likely to be life threatening, in horses the assessment of cardiac disease is more challenging because it must take into account two key factors: the likelihood of the abnormality impairing athletic performance and the risk to rider safety.
Murmurs are the result of abnormal blood flow as a result of leakage through the valves of the heart or a turbulent blood flow in major blood vessels. They are common in fit horses and can be perfectly normal. For example, between 30 and 40% of fit thoroughbreds may have heart murmurs. The reason for this is that the relatively small volume of blood flowing through major vessels at rest can result in turbulence and hence generate a murmur. A further reason is that training increases heart size and power, generating pressures that exceed valve tolerances, which then leak and generate murmurs.
However, as in other species, cardiac murmurs may occur because of cardiac pathology and it is essential that a veterinary opinion is sought to identify those horses with murmurs because of pathology and to separate them from horses with ‘normal’ (physiological) murmurs. Pathological murmurs can occur because of heart valve disease and also because of congenital (developmental) defects in the heart and the major vessels around it. Many pathological murmurs are progressive and so get worse over time, which is another reason why veterinary assessment and monitoring of murmurs is important.
As with murmurs, changes in rhythm (arrhythmias) frequently occur in horses and many are not significant. Most fit horses have a rhythm with occasional pauses which is a cardiac adaptation allowing the heart to maintain a slow rate at rest.
Atrial fibrillation is the most common and important pathological arrhythmia in horses. While in humans and dogs atrial fibrillation reduces cardiac output and causes symptoms, the large capacity and slow rate of the equine heart means that horses that are not doing high intensity exercise might show no obvious clinical signs. This is the case in many leisure horses. It is much more likely to have a noticeable impact on athlete horses, for which it is a quite common cause of loss of performance. Atrial fibrillation can occur in all types of horse but is most common in large horses and in fit horses.
Assessing the equine heart
Veterinary assessment of a suspected cardiac problem includes taking a complete history of the horse, conducting a physical examination, carrying out a stethoscope exam of the heart and assessing the impact of exercise in standardised exercise tests.
Other diagnostic techniques include:
- Ultrasound – imaging of the heart in real time to measure its size and visualise murmurs
- Resting and exercising electrocardiography (ECG) – to characterise for arrhythmias
- Laboratory tests, including enzymes, electrolytes and heart-specific tests and those focused on other organs
- The equine heart has evolved to allow the horse high levels of athleticism and to be relatively free of debilitating cardiac diseases
- Although many horses have murmurs and arrhythmias, many of these are not clinically significant and the important challenge is to identify those murmurs and arrhythmias that are significant
- Murmurs are caused by turbulent flow of blood, either because of blood leakage through the valves of the heart or turbulent blood flow within major vessels
- Arrhythmia is a variation in the heart’s rhythm can be a cause of poor performance. Atrial fibrillation is the most common arrhythmia in horses, especially in athletic horses for which it can be a significant performance limiting condition.