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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.
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]
- Male horses
- Thoroughbred or Arabian lineage
- Advanced age of the horse
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 resul