Part I

Text: Dr Christina Eberhardt

Illustrations: Merlynn Trichardt

“The horse – an extraordinary athlete with an efficient heart.”

Horses are amazing athletes performing at a level that surpasses that of most other animals of similar body size. Thoroughbreds race at speeds of 64 km/h in flat races; Quarter horses reach speeds as high as 72 km/h over 400 m sprints in barrel racing; Arabians and other endurance breeds finish a 160 km race in less than 8 hours with an average speed of 22 km/h; draft horses pull weights of 1000 kg or more; and Warmbloods jump heights of 1.6 m and spreads of up to 2.0 m with a high jump record of 2.47 m. But what is the basis for the equine athlete’s extraordinary performance in this variety of disciplines?

In part, because of the remarkable equine heart, says Dr Christina Eberhardt, Dr.med.vet., Dipl. ACEIM, Dipl. ECEIM, who specializes in equine cardiology and internal medicine at Onderstepoort Veterinary Academic Hospital of the University of Pretoria and who is the founder of EQUICARDIO, an equine cardiology consulting service. The equine heart beats 28-44 times per minute at rest depending on the size and fitness of the horse and pumps about 40 litres of blood to the body every minute, which is called the cardiac output. During exercise the heart rate increases up to an exceptional 220-240 beats per minute resulting in a cardiac output of 8-12 times over resting value, an astonishing 300-400 litres per minute. The blood volume of a normal size horse is 45-50 L (10% of bodyweight), which means the horse can circulate its entire blood volume 6-8 times within one minute during peak exercise. The horse’s heart is exceptionally well developed and weighs 4-5 kg on average (about 1% of body weight). Dr Eberhardt continues, “It is a robust pump that continuously works to circulate blood throughout the body to ensure delivery of sufficient blood, hence oxygen, to the body where it is used to generate energy.”

About the equine heart

The heart is comprised of four chambers – the right and left ventricles and the right and left atria. Each chamber has its unique function. The left atrium receives oxygen-rich blood from the lungs and pumps it into the left ventricle. The left ventricle is the largest and most powerful chamber and pumps the oxygen-rich blood into the aorta, the main artery, and the body’s circulatory system. Oxygen is extracted from blood when it circulates throughout the body eventually reaching the right side of the heart. The right atrium receives the oxygen-poor blood from the body and pumps it to the right ventricle. The right ventricle pumps the oxygen-poor blood through the pulmonary artery to the lungs for oxygenation which completes the cardiac cycle.

The right heart chambers are separated from the left heart chambers with a muscular wall – the septum. In the healthy heart, blood flow is smooth and one-directional and is facilitated by heart valves. Blood flow from the left atrium to the left ventricle and from the right atrium to right ventricle is controlled by the mitral and tricuspid valves, respectively. The aortic and pulmonary valves control blood flow from the ventricles into their respective arteries, the aorta and pulmonary artery.

In the healthy heart, the movement of the heart and heart valves throughout the cardiac cycle results in up to four sounds (S1-S4), with short quiet phases in between, which veterinarians can hear with a stethoscope, says Dr. Eberhardt. A natural pacemaker generates electrical impulses in the heart muscle resulting in a regular heartbeat. She continues, “When listening to the heart, you can typically expect when the next beat will come.”

When things go wrong

Heart diseases in horses often do not cause any obvious clinical signs unless they are or have become relatively severe. Therefore, heart diseases tend to be overlooked or historically were considered unimportant, says Dr. Eberhardt. However, heart diseases may cause poor performance in equine athletes and, although rare, horses with certain heart diseases may experience events of collapse and even sudden cardiac death often during or just after intense exercise. Some heart diseases progress and eventually lead to heart failure. Today, with researchers and specialists leading the way, there is more recognition about the importance of cardiac diseases as well as concerns from veterinarians and the public about rider safety and horse welfare.

How do I know if my horse has a heart disease?

Heart diseases usually result in an additional sound in the usually quiet phases of the cardiac cycle, called a heart murmur or an irregular heart rhythm, called arrhythmia. Often times an arrhythmia or heart murmur is detected incidentally during a routine health check or a pre-purchase examination. You might recognise that your horse does not perform to your expectations (anymore), like uncharacteristically slowing down mid-race or repeatedly knocking down jumps. However, besides heart diseases, poor performance may also be caused by diseases of the musculoskeletal or respiratory system. Rarely, heart diseases progress to heart failure leading to typical signs such as lethargy, exercise intolerance, edema, rapid breathing, cough, and pulsating or congested veins.

No one likes to hear their horse has a heart disease and it can raise many questions: Is my horse safe to ride? What is the likelihood of the disease impairing performance? Is the horse I am buying suitable for the intended use? Is there any influence on the quality of life for my horse? What is my horse’s life expectancy? Is there anything I can do? To answer those questions and determine the type and significance of a heart disease referral to a specialist for a comprehensive examination is usually required.

Referral – When and what to expect

Some heart murmurs and arrhythmias are physiologic or benign and some are significant and associated with a heart disease. It can be very difficult to differentiate between physiological and pathological abnormalities and it is impossible to evaluate the severity of a heart disease without further examination, says Dr. Eberhardt. Therefore, referral to a specialist is indicated to evaluate the presence and severity of a heart disease. A comprehensive examination of the cardiovascular system should include an ultrasound examination of the heart (echocardiography) and a resting ECG recording. Depending on the suspected type of cardiac disease veterinarians may also suggest a 24-hour ECG recording, an exercise test and ECG or specific blood tests. The specific abnormalities, cardiac size and function, the severity of the disease and the questions relating to rider safety, impact on athletic performance, quality of life as well as treatment or management options should be addressed during the examination.

Take home message

The horse is an extraordinary athlete in part because of its remarkable and highly efficient heart. Heart diseases in horses often do not cause any obvious clinical signs but some horse may show poor performance or even experience episodes of collapse and sudden cardiac death. If a heart disease is suspected, referral to a specialist is indicated for a in depth evaluation of the heart and address the questions relating to the horse’s safety for riding, impact on performance as well as life expectancy and treatment options.

In the next two issues of HQ Magazine, Dr Eberhardt will cover the most important cardiac diseases in horses and how to manage horse with heart problems.

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For more health-related content, check out our article on anaemia in equines here.