Heart murmurs in horses are distinct sounds between the normal beats of the heart that can be heard with a stethoscope. They arise from the abnormal flow of blood through the heart, often due to conditions affecting the heart’s structure or function.

Not all heart murmurs are problematic. They can range from benign with no impact on the horse’s health or performance, to severe indicating a serious heart defect.

Murmurs are often diagnosed during routine veterinary exams. They may also be detected in a horse presenting with other symptoms, including poor performance or weakness, which can be signs of an underlying heart condition.

Upon detecting a heart murmur, your veterinarian may recommend an echocardiogram to further assess the condition. This examination helps to determine the impact of the murmur on your horse’s health or performance, and whether treatment is necessary.

Heart Murmurs in Horses

Heart murmurs are unusual sounds during the heartbeat cycle that can vary in intensity, sound, and timing.

Most murmurs that develop during the horse’s lifetime are due to valvular insufficiency, a condition that involves weakness of the heart valves. [1] Around one-third of all horses develop valvular weakness during their lifetime. [2]

Weak heart valves fail to close completely, allowing blood to leak through them during the heart’s contraction phases. [1] This valvular regurgitation produces the murmur sound that veterinarians identify with a stethoscope. [1]

Valvular Regurgitation

There are three main types of valvular disorders that can affect horses, distinguished based on the specific heart valve that is malfunctioning: [1]

  • Aortic regurgitation
  • Mitral regurgitation
  • Tricuspid regurgitation

Not all valvular regurgitation is problematic for the horse. Some evidence suggest that this phenomenon may be an adaptive response to high intensity work, such as steeplechasing. [1]

Physiologic vs. Pathologic Murmurs

Heart murmurs in horses are classified as either physiologic, often arising from intense training, or pathologic, associated with underlying heart diseases.

Physiologic heart murmurs show no symptoms, other than an audible heart murmur when listening to the heart with a stethoscope. [1] In contrast, pathologic murmurs are associated with symptoms indicating a heart condition.

The role of the veterinarian is to evaluate the horse’s heart’s function and the presence of any symptoms to diagnose whether a heart murmur is physiologic or pathologic. [1]

Symptoms

Clinical signs of pathologic murmurs vary depending on the extent and severity of the underlying heart disease. Common symptoms include: [3]

  • Poor performance
  • Exercise intolerance
  • Poor recovery times after exercising
  • Weakness
  • Collapse
  • Swelling of the lower abdomen
  • Rapid heart rate
  • Pulsation within the jugular vein that extends more than halfway up the neck

The most common symptom is poor performance, which may be subtle. [1] Regular examination by your veterinarian can help identify heart murmurs early in their development.

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Diagnosis

Diagnosing heart murmurs in horses requires a combination of clinical examination and advanced imaging techniques. The veterinarian will listen to the horse’s heart to determine the murmur’s location, timing, and intensity.

The primary goal is to assess whether the heart murmur is physiologic or pathologic, and which valve is affected. [1] Additional diagnostic tests may be necessary to make these distinctions.

Auscultation

Auscultation is the process of using a stethoscope to listen to the sound of the heartbeat. On auscultation, the veterinarian can identify which heart valve is affected and assess various characteristics of the murmur, providing insights into its severity.

Features of a heart murmur evaluated by the veterinarian include: [3]

  • Timing: When during the heart beat the murmur occurs
  • Grade: The severity of the murmur
  • Point of maximum intensity: Where the murmur is the loudest
  • Radiation: Whether the sound of the murmur “spreads” to other locations of the heart
  • Character: The sound the murmur makes, such as squeaking or honking

The veterinarian can also monitor the horse’s heart rate (pulse) to gather more information about the impact of the murmur on the horse’s performance. Evaluating the heart rate before exercise, immediately post-exercise, and during the recovery period can offer valuable insights. [1]

However, cardiac auscultation alone is not sufficient for predicting the prognosis of a heart murmur. [4] Additional diagnostic testing is required to ensure an appropriate diagnosis.

Echocardiogram

An echocardiogram is an ultrasound of the heart, which allows the veterinarian to examine the heart’s structure and function. Features of the heart visible on ultrasound include: [1][5]

  • Velocity, direction, and amount of blood flow
  • Presence of turbulent blood flow that can indicate regurgitation
  • Integrity of the heart valves
  • Thickness of the heart walls
  • Volume of blood within the heart chambers
  • Size of the large vessels supplying the heart
  • Estimation of cardiac output (amount of blood pumped by the heart)

Electrocardiogram

Electrocardiograms (ECG) measure electrical currents within the heart to identify cardiac arrhythmias, which are abnormalities in the heart’s rhythm.

Many heart murmurs are linked to cardiac arrhythmias, leading to a poor prognosis for athletic performance. For this reason, ECG evaluation is recommended for any horse with evidence of a heart murmur. [1][5]

Pulmonary Artery Pressures

Horses showing signs of severe cardiac disease may have their pulmonary artery pressure measured as part of their diagnostic work-up. [5]

The pulmonary artery is the major vessel carrying oxygenated blood from the lungs back to the heart. In horses with heart disease, blood pressure in the pulmonary artery can increase, resulting in a poor prognosis. [1]

Cardiac Enzymes

Blood testing may be performed to measure cardiac enzyme levels. These enzymes are biomarkers that are released into the blood when the heart muscle is damaged.

Valvular disease can result from damage to the heart muscle, so testing cardiac enzyme levels may help provide clues as to the underlying cause. [6][7]

Aortic Regurgitation

Aortic regurgitation (AR) is a type of valvular disorder that can cause heart murmurs in horses. This condition affects the aortic valve, which controls blood flow from the left side of the heart to the body.

Diagnostic Findings

Murmurs caused by aortic regurgitation typically occur during the diastolic phase of the heartbeat, which is when the heart chambers fill with blood. [1] These murmurs are loudest on the left side of the chest, right above the aortic valve, and often radiate downward towards the base of the heart. [1]

On echocardiogram, common findings include nodules or thickened areas on the aortic valve leaflets, and prolapse(failure of the valve leaflets to close completely). [1] A turbulent jet of blood flow may be present, helping to confirm the diagnosis. [1]

In severe cases, the left ventricle may enlarge due to the increased blood volume passing through the compromised valve. [1]

Many horses with aortic regurgitation have cardiac arrhythmias affecting the ventricles, which may result in sudden death. [1] The cause of arrhythmias linked to aortic regurgitation is unknown. [1]

Causes

Aortic regurgitation usually results from degeneration of the valve due to old age. [1][8] Usually, the degeneration causes nodules or scar tissue to form on the edge of the valve, preventing complete closure. [8]

Other potential causes of aortic regurgitation include: [1]

  • Infective endocarditis
  • Malformation of the aortic valve at birth

Prevalence

The main risk factors for developing aortic regurgitation are: [1][2]

The prevalence of aortic regurgitation in horses is estimated between 2 – 8%, with a median age of 14 years. [1] Aortic regurgitation is uncommon in racing horses, unlike mitral and tricuspid regurgitation. [9]

Prognosis

The majority of aortic regurgitation (AR) cases in horses are mild, and horses have a normal life expectancy and performance capacity. [7][10] Horses diagnosed with AR should undergo annual examinations to follow the progression of heart disease. [7][10]

Factors associated with a poorer prognosis include: [10]

  • Diagnosis at a young age (under 10 years old)
  • Bounding arterial pulses
  • Elevated blood pressure
  • Severe damage to the aortic valve
  • Mitral regurgitation present at the same time
  • Presence of cardiac arrhythmias

Horses with severe AR should not be used for high performance sports, or be ridden by children, as there is a risk of sudden death. [10]

Mitral Regurgitation

Mitral regurgitation (MR) is another form of valvular disorder that can result in heart murmurs. MR affects the mitral valve, which regulates the flow of blood from the left atrium — the chamber that receives oxygen-rich blood from the lungs — into the left ventricle, the chamber responsible for pumping blood out to the body’s tissues.

Diagnostic Findings

Mitral regurgitation produces to a systolic murmur, which is audible during the heart’s contraction phase. [1] The murmur is loudest on the left side of the chest, directly above the mitral valve. [1]

An echocardiogram for horses with MR typically reveals nodules or scar tissue formation on the mitral valve. [1] Additionally, valve prolapse is often observed, where the valve leaflets fail to close completely. [1]

Some severely affected horses show an enlarged left atrium or left ventricle, a result of these chambers being overloaded with blood due to the compromised valve. [1] These horse may also exhibit a widened pulmonary artery, indicating increased blood pressure within the vessel. [1]

Horses with significant left atrium enlargement frequently show signs of atrial fibrillation in electrocardiogram tests. [1]

Causes

Several factors can lead to mitral regurgitation in horses, including: [1][8]

  • Intensive training routines typical in high-performance equine athletes
  • Age-related degeneration of the valve
  • Infective endocarditis, an infection of the heart’s inner lining
  • Congenital malformations of the valve present from birth
  • Rupture of the chordae tendinae, the fibrous cords connecting the valve to heart muscle

Prevalence

Mitral regurgitation is seen in approximately 3% of the general horse population. However, its incidence is significantly higher in performance horses, such as Thoroughbreds involved in racing and steeplechasing, reaching up to 23%. [1]

Prognosis

Many horses with mild cases of MR maintain normal cardiac function and have a typical lifespan. MR can be a normal physiologic development in exercising horses. [1][10] These horses have no performance limitations so long as they do not develop symptoms of cardiac disease.

Studies in racehorses show that the presence and severity of MR does not impact racing performance compared to horses without MR. [9]

However, horses displaying signs such as poor performance or exercise intolerance may have pathological MR and need thorough veterinary assessment. [10] Diagnostic findings indicating severe MR include: [10]

  • Weak pulses
  • Severe damage to the mitral valve
  • Changes to the heart shape
  • Increased pulmonary artery pressure
  • Presence of cardiac arrhythmias
  • Increased heart rate

Affected horses should have an annual cardiac examination to monitor the progression of disease. [10] Regular evaluation of the horse’s heart rate and rhythm are also recommended, as affected horses are prone to developing atrial fibrillation. [10]

Horses with MR are susceptible to developing congestive heart failure due to their valvular disease. [1] The increased blood pressure within the lungs also puts them at risk of blood vessel rupture, leading to sudden death. [1][6]

Tricuspid Regurgitation

Tricuspid regurgitation (TR) is another valvular disorder in horses that result in heart murmurs. TR affects the tricuspid valve, situated between the right atrium — the chamber that gathers deoxygenated blood returning from the body — and the right ventricle, responsible for pumping blood to the lungs.

Diagnostic Findings

Tricuspid regurgitation is usually loudest on the right side of the chest, above the tricuspid valve. [1] Most of the murmurs associated with TR occur during the systolic phase of the heartbeat. [1]

Echocardiographic examination often reveals prolapse of the tricuspid valve, in which the valve leaflets do not close completely to form a seal. [1] Some horses may also have an enlarged right ventricle due to overflow of blood through the leaking tricuspid valve. [1]

Many horses with TR also have atrial fibrillation on electrocardiogram. However, the atrial fibrillation is believed to be related to cardiac damage rather than the TR itself. [1]

Causes

TR is commonly found in athletic horses, but the exact cause of the murmur in these cases is unclear. [1] Other rare causes of TR include: [1][8]

  • Degeneration of the valve due to aging
  • Infective endocarditis
  • Increased pressure in the pulmonary artery
  • Rupture of chordae tendinae
  • Malformation of the valve from birth

Prevalence

Tricuspid regurgitation is a common condition in horses, especially those used for performance disciplines. [1] An estimated 5 – 9% of the general horse population is affected by this condition, with a significantly higher prevalence of 40 – 50% in Thoroughbreds involved in steeplechasing. [1]

Horses participating in exercise-intensive disciplines typically have a higher rate of TR. [9]

Prognosis

Tricuspid regurgitation does not usually affect athletic performance. [1][10] Studies in racehorses showed no consistent association between racing performance and severity or presence of TR. [9]

Horses with tricuspid valve damage or exhibiting signs of heart failure should undergo annual cardiac examinations to monitor their condition. [7][10] Additionally, some horses develop TR as a consequence of mitral regurgitation, which carries a poor prognosis. [10]

Treatment of Heart Murmus

The primary approach for managing horses with valvular heart disease involves regular monitoring to track changes in the severity of the condition. [10]

Unlike other species, such as humans and dogs, most horses with valvular insufficiency are not treated, unless they are actively in heart failure. [2] This is mainly because the medications used for valvular disease in other species do not appear to affect disease progression or life expectancy in horses. [2][7]

Once heart failure develops, treatment options for horses may include: [1]

  • Vasodilating medications, such as ACE inhibitors or acepromazine, to reduce the heart’s workload
  • Diuretics, such as furosemide, to reduce blood pressure
  • Medications, such as digoxin, to improve heart contraction strength

However, many horses are euthanized due to a poor prognosis. [1]

Summary

  • Valvular weakness is very common in horses, with up to one-third of all horses developing a heart murmur during their lifetime
  • Not all heart murmurs in horses are problematic, as some occur due to intensive training
  • Many problematic heart murmurs occur due to age-related degenerative of the valve structure
  • There is no specific treatment for heart murmurs other than ongoing monitoring and lifestyle changes

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References

  1. Marr. C. M. and Bowen. I. M., Eds., Cardiology of the horse, 2nd ed. Edinburgh?; New York: Saunders, 2010.
  2. Marr. C. M., Equine Acquired Valvular Disease. Veterinary Clinics of North America: Equine Practice. 2019. doi: 10.1016/j.cveq.2018.12.001.
  3. Jago. R. and Keen. J., Identification of Common Equine Cardiac Murmurs. In Practice. 2017. doi: 10.1136/inp.j1769.
  4. ter Woort. F. et al., Cardiac Pre-Purchase Examination in Horses – Evaluation, Outcome and Athletic Follow-Up. Equine Veterinary Education. 2022. doi: 10.1111/eve.13507.
  5. Durando. M. M., Clinical Techniques for Diagnosing Cardiovascular Abnormalities in Performance Horses. Clinical Techniques in Equine Practice. 2003. doi: 10.1053/S1534-7516(03)00069-6.
  6. Reef. V. B. et al., Severe Mitral Regurgitation in Horses: Clinical, Echocardiographic and Pathological Findings. Equine Veterinary Journal. 1998. doi: 10.1111/j.2042-3306.1998.tb04084.x.
  7. Chope. K. B., Cardiac/Cardiovascular Conditions Affecting Sport Horses. Veterinary Clinics of North America: Equine Practice. 2018. doi: 10.1016/j.cveq.2018.04.001.
  8. Reed. S. M. et al., Equine internal medicine, 3rd ed. St. Louis, Mo: Saunders Elsevier, 2010.
  9. Young. L. e. et al., Heart Murmurs and Valvular Regurgitation in Thoroughbred Racehorses: Epidemiology and Associations with Athletic Performance. Journal of Veterinary Internal Medicine. 2008. doi: 10.1111/j.1939-1676.2008.0053.x.
  10. Reef. V. b. et al., Recommendations for Management of Equine Athletes with Cardiovascular Abnormalities. Journal of Veterinary Internal Medicine. 2014. doi: 10.1111/jvim.12340.