The Equine Heart

TEXT: DR. CHRISTINA EBERHARDT, DR.MED.VET., DIPL. ACEIM & ECEIM

PART 2 THE EQUINE HEART

EQUINE HEART DISEASES - HEART MURMURS

Dr Christina Eberhardt

In the HQ August edition, we learned about the remarkable heart of the horse and how it contributes to the equine athlete’s extraordinary performance. But what happens when things go wrong? Horses generally do not experience the same heart diseases that are associated with cardiovascular health in people. Horses do not, for example, build up cholesterol plaques in their coronary arteries, and therefore do not experience heart attacks like people. Horses also rarely develop enlarged hearts as dogs and cats can. Nevertheless, heart diseases are relatively common in horses and are often detected incidentally during a routine physical examination.

Fortunately, many heart diseases in horses are mild, well-tolerated and have little or no impact on the horse’s performance. However, some heart diseases cause poor performance, result in a reduced lifespan, or generate safety concerns for both the rider and the horse. Two common findings on a physical examination of the horse will raise the veterinarian’s suspicion of a potential heart disease – a heart murmur or an arrhythmia.

In this month’s edition of HQ Magazine, Dr Christina Eberhardt, Dr.med.vet., Dipl. ACEIM & ECEIM highlights heart murmurs in horses. Dr Eberhardt specialises in equine cardiology and internal medicine and is based at the Onderstepoort Veterinary Academic Hospital of the University of Pretoria. She is also the founder of EQUICARDIO, an equine cardiology consulting service.

WHAT IS A HEART MURMUR?

The healthy heart of the horse beats 28 – 44 times per minute, and each heartbeat makes up to four separate sounds, referred to as S1, S2, S3 and S4. Two of them, S1 and S2, can easily be heard when listening to the heart with a stethoscope. The heart goes “lub-dub, lub-dub, lubdub”. Click on the audio file to listen to the heartbeat of a healthy horse.

The “lub” is associated with the simultaneous closing of the mitral and tricuspid valves, which ends blood flow from the left atrium to the left ventricle and from the right atrium to the right ventricle. Then you hear the “dub”, which is associated with the simultaneous closing of the aortic and pulmonary valves and ends the blood flow from the left ventricle into the aorta and from the right ventricle into the pulmonary artery. The phases between the “lub” and “dub” are usually quiet. A heart murmur describes an extra atypical sound, such as whooshing or swishing, heard in addition to the "lub-dub."

DETERMINING THE TYPE OF HEART MURMUR

In up to 30% of horses, a heart murmur is detected during a physical examination or during a pre-purchase examination. Heart murmurs can simply be the sound of changes in blood flow through a normal healthy heart that may occur due to colic, fever, exercise, excitement, pain or

anaemia. Such heart murmurs are called “functional”and they disappear when the blood flow returns to normal, for example, after exercise or when the colic is resolved.

If a heart murmur is caused by abnormal turbulent blood flow because of heart disease, then it is called a “pathological” heart murmur. A grading system helps the veterinarian to characterise and better understand the heart murmur, says Dr Eberhardt. Heart murmurs are graded on a scale of 1 to 6 based on their loudness and are further characterised based on their timing and their location. “A grade 1 is a very soft murmur that requires experience and a quiet environment to be heard. A grade 2 is soft but readily heard. A grade 3 is a readily audible murmur that is moderately loud. A grade 4 is louder than the “lub-dub” and radiates widely. Grades 5 and 6 are even louder, and you’re going to actually feel a thrill on the chest.”

The timing refers to whether the murmur is heard between the “lub” (S1) and the “dub” (S2), in which case it is referred to as a systolic murmur or between the “dub” (S2) and the “lub” (S1), in which case it is called a diastolic murmur. It is also possible for a murmur to be heard in both quiet phases, in which case is it called a continuous murmur.

Murmurs are further classified by their location - the left or the right side of the heart or the lower or upper half of the heart.

Specific forms of heart disease are associated with characteristic murmurs. Therefore, your veterinarian may suspect a specific disease after carefully listening to the heart with a stethoscope.

WHAT ARE THE CAUSES OF PATHOLOGICAL HEART MURMURS?

The most common heart disease causing a heart murmur in horses is valve regurgitation – the heart valves do not close tightly and leak. This often affects only one valve in the heart, is usually acquired and can occur at any stage of a horse’s life. In the healthy heart, blood flow through the heart valves is one-directional. A “leaky heart valve” allows backwards blood flow. Depending on how much the valve leaks (the severity of the regurgitation), the heart operates less efficiently and can affect the horse's performance.

A less common cause of a heart murmur is a shunt – an abnormal blood flow pattern in the heart. The most common shunt in a horse is a hole in the muscular wall between the right and the left ventricle, a so-called ventricular septal defect. This is a congenital heart defect, which means the horse is born with it. If the hole is small, it may not affect the horse, but if it is large, the horse will perform poorly.

A very rare cause of a heart murmur in horses is valve stenosis – the heart valves do not open completely and hinder blood flow through the heart. Click on the audio file to listen to a heart murmur.

Top image: An echocardiogram that shows the heart chambers – the left and right ventricle and the left and right atrium - and the mitral and tricuspid valves. Lower image: This is an echocardiogram showing aortic regurgitation. It shows a moderately “leaking” aortic valve (aortic regurgitation). The green colour shows blood flowing in an abnormal direction through the aortic valve.

Aortic regurgitation allowing blood to flow back into the heart i.e. the blood is travelling in the wrong direction.

DETERMINING THE SIGNIFICANCE OF A HEART MURMUR

Mild heart murmurs are commonly associated with insignificant heart disease, are well tolerated, and have little or no effect on the horse’s performance or life expectancy. Although progression tends to be slow, a mild insignificant heart murmur in a young horse might become more significant in older horses. This factor is especially important when a murmur is detected during a pre-purchase examination. Louder murmurs might be more significant, associated with more severe disease and may be performance limiting. In such cases, a comprehensive cardiac examination is important to accurately determine the significance of a heart murmur and assess the severity of the heart disease. For example, if your horse has aortic regurgitation, a “leaky aortic valve”, you want to know how much it is leaking and if the heart size and function is still normal, says Dr Eberhardt.

There is no cure or treatment for a heart murmur. Further diagnostics are directed towards the evaluation of the horse’s future performance, the suitability for intended use, the assessment of horse and rider safety, the risk for progression to heart failure and the influence on the quality of life.

Echocardiography – an ultrasound examination of the heart - is the primary diagnostic tool veterinarians use to assess heart murmurs. It uses high-frequency sound waves to produce two-dimensional images of the heart’s chambers, valves, walls and blood vessels. With echocardiography the heart size, structure and function as well as normal and abnormal blood flow through the heart and the valves can be assessed. This allows for an accurate diagnosis of a horse’s heart condition and advice on appropriate management of the horse’s heart health.

MANAGEMENT OF HORSES WITH HEART DISEASES

If your horse was diagnosed with a mild heart disease, it is likely that you can continue to ride your horse safely and that the disease has no impact on the horse‘s performance. In such cases, Dr. Eberhardt recommends yearly cardiac examinations, where a veterinarian listens to your horse’s heart on both sides of the body to readily detect any changes of the murmur that indicates progression of the disease. If your horse was diagnosed with moderate to severe heart disease, the heart disease should be monitored closely for progression by yearly ultrasound examinations. Further examinations such as a 24-hour resting ECG or an ECG recording during exercise are often necessary to evaluate your horse’s safety for riding. Depending on the severity, veterinarians might recommend that the horse is only ridden on a low-intensity work level, is not ridden at all or is specifically not ridden by children.

TAKE HOME MESSAGE

Heart murmurs are common in horses. Many murmurs do not significantly affect a horse’s performance and do not generate a safety concern for horse and rider. However, some do. The best way to determine whether your horse has significant heart disease, and to answer the questions relating to rider and horse safety, impact on athletic performance, quality of life as well as appropriate management, is by having an echocardiogram performed.

DID YOU KNOW?

HORSES CAN’T VOMIT! UNLIKE CATTLE AND OTHER RUMINANTS WHO REGURGITATE FOOD TO RE-CHEW IT, HORSES CAN ONLY ALLOW THE FOOD TO PASS IN ONE DIRECTION THROUGH THE DIGESTIVE SYSTEM. THIS ONEDIRECTIONAL SYSTEM CAN CAUSE PROBLEMS THAT RESULT IN COLIC.