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Heart Murmur in Dogs

 

Cute Pug getting a check up at the vet

Pug getting checked for a heart murmur


What is a Heart Murmur?

 

Most people would be familiar with the normal “lub dub” sounds of a heartbeat. A heart murmur is an abnormal sound caused by turbulent blood flow through the heart. Most heart murmurs in dogs are heard between the “lub” and “dub” and they make a noise like a swish or a whoosh.

Heart murmurs are quite a common problem in dogs and can be congenital or acquired. Congenital murmurs are not common and are caused by birth defects – there may be a hole between the chambers of the heart, such as an atrial septal defect, or a narrowing of an artery carrying blood out of the heart, for example pulmonic stenosis.  A retrospective study of several thousand dogs published in 2015 found that just 0.13 % of dogs evaluated had a congenital heart defect.

Acquired murmurs, as the name suggests, develop during a dog’s life. These are the more common murmurs that dog owners and veterinarians encounter.

There are many causes of a heart murmur in dogs, both congenital and acquired and further testing is needed to reach a diagnosis and develop a treatment plan to manage it.

 

What Causes a Heart Murmur?

 

Cavalier King Charles Spaniel Dog

Cavalier King Charles Spaniels are susceptible

 

The normal “lub dub” heart sounds are caused by valves in the heart closing to make sure blood flows along a one way route through the heart and around the body. The blood usually moves smoothly through the blood vessels but if the flow is turbulent, it is noisy and the resulting murmur can be heard with a stethoscope.

 

Heart Murmur in Puppies

Puppies may have what is known as an innocent murmur which is completely harmless and usually resolves by four to five months of age. A veterinarian may notice this when a pup has their first or second vaccination. Dogs with anaemia (fewer than normal red blood cells) may also have a heart murmur. However the most common cause of heart murmurs in dogs is a leaky valve between the left atrium and the left ventricle.  This valve, known as the mitral valve, prevents blood flowing from the ventricle back into to the atrium when the ventricle contracts. When the valve degenerates and thickens with age, it doesn’t close properly so blood goes the wrong way with every contraction of the heart, resulting in a murmur.

 

Mirtral Murmurs in Dogs

These mitral murmurs are most common in middle aged toy or small dog breeds. There appears to be a genetic component in Cavalier King Charles Spaniels but how it is inherited isn’t yet understood. While mild valve disease doesn’t usually cause any problems, the condition is progressive and the leak will worsen over time, ultimately causing congestive heart failure in dogs.

 

Dilated Cardiomyopathy

Another cause of dog heart murmurs and valve failure is dilated cardiomyopathy. This is a disease of the heart muscle rather than the heart valves and often affects larger dogs, including Boxers and Doberman Pinschers. It is a hereditary condition in some breeds and the genetic mutation that causes it has been identified. In dilated cardiomyopathy, the heart muscle becomes weak and thin and loses its ability to contract effectively. These changes in the muscle can distort the mitral valve so it doesn’t close properly. The result is a heart murmur.

When the heart muscle can’t pump blood effectively and the mitral valve isn’t working, the result is ultimately congestive heart failure in dogs.

To understand the effects of valvular disease and its associated heart murmur, it’s essential to have a general awareness of how blood flows through the heart and body. Oxygenated blood from the lungs is pumped through the left side of the heart and around the body where the oxygen is delivered to the cells and carbon dioxide is collected. This blood returns through the circulation and into the right side of the heart where it is pumped back to the lungs to offload its carbon dioxide and pick up another load of oxygen. This the process then repeats itself.

 

Congestive Heart Failure

Congestive heart failure in dogs occurs when the distorted heart valve slows blood flow in a forward direction.  This allows fluid to leak from the small blood vessels “upstream” from the heart. When the left side of the heart fails, fluid accumulates in the lungs causing coughing, breathlessness and lethargy. When the right side of the heart fails, fluid accumulates in the abdomen, known as ascites, and a dog will develop a swollen tummy.


Diagnosis and Testing


Dog getting checked for heart murmur

Dog undergoing standard veterinary exam


Often the first indication that your pup has a dog heart murmur is when a veterinarian examines them during a routine check-up, perhaps when they’re presented for a vaccination. Dog owners can be surprised by the news that their pet has a murmur because they may have no symptoms at all of heart disease at this stage.

Heart murmurs in dogs are graded based on how loud they are, with a grade 1 murmur being soft and a grade 6 murmur being quite loud. Some veterinarians prefer to describe murmurs as soft, moderate or loud instead of using the numerical grading. Studies suggest that the louder the murmur, the more severe the heart disease. Some murmurs are so severe that they can be felt by placing the hand on the dog’s chest.

If a young pup has a mild heart murmur, it’s appropriate to monitor it as the pup grows to adulthood. It should disappear, but if it persists then further diagnostics are necessary. Those pups with a loud, grade 3 to 6 murmur need immediate evaluation by a veterinary cardiologist.

 

Identifying Heart Failure in Dogs

There are a number of procedures that will help veterinarians to diagnose the cause of a heart murmur. Some can be performed by a general practice veterinarian but others need the services of a specialist cardiologist or ultrasonographer.

1. Listening with a stethoscope. Veterinarians use this tool to identify the location and intensity of a heart murmur as well as identify any abnormal heart rhythms. They can listen while feeling a dog’s pulse so they can make sure each heart beat results in a pulse beat – this is another way to identify an arrhythmia. A stethoscope can also allow them to listen to lung sounds. Fluid accumulation in the lungs will sound like soft crackles. This sound indicates that congestive heart failure is occurring.

2. Chest and abdominal radiographs. This procedure allows veterinarians to visualise and measure the heart. Because dogs of different size and conformation can have different chest shapes, the vertebral heart scale was developed to allow objective evaluation of heart size across dogs of all shapes and sizes. This is used in conjunction with other tests and procedures to evaluate heart function. Chest radiographs will also show a characteristic pattern in the lungs if there is fluid accumulation. If there is heart failure with abdominal enlargement, an x-ray will show fluid in the abdomen and a swollen liver.

3. Blood tests and urine analysis. These tests aren’t specifically used to evaluate heart murmurs but they do give information on how other organs are coping with poor heart function and if a dog has any other medical conditions that their veterinarian needs to be aware of.  A procedure known as abdominocentesis may be performed if there is fluid in the abdomen. This involves carefully passing a small needle into the abdominal cavity and collecting a sample of the fluid. It can then be evaluated for such things as protein content and if there are any cells present.

4. Electrocardiogram. This procedure assesses the electrical activity of the heart muscle. Abnormal rhythms can be seen on the ECG trace. Stress on the muscular walls of the heart associated with dilated cardiomyopathy can cause arrhythmias as well as heart murmurs. There can be difficulties with interpreting the results of this procedure, for example muscle tremors in a nervous dog or changes in their breathing rhythm can affect the trace. However a skilled operator will be able to take these into consideration and get an accurate result.

5. Echocardiogram or ultrasound. This is one of the most useful procedures in evaluating heart murmurs. The heart can be measured accurately and each chamber and valve can be looked at individually. Blood flow can be seen on the screen so veterinarians can measure the degree of back flow through valves.

6. Blood test. A very new test measures the amount of a chemical known as NT-proBNP in a dog’s blood. This chemical is an indicator of heart muscle stress and can be used in conjunction with the other listed tests to obtain an accurate picture of the severity of their heart disease.

 

Treatment of Heart Murmurs

 

If a dog has a heart murmur and no symptoms of congestive heart failure, it has been common procedure to not commence any treatment at that point. However, a 2016 study found that if treatment is started when a dog is diagnosed with an enlarged heart and no obvious signs of heart disease, it can delay the onset of symptoms by 15 months. This suggests that there may benefits to taking routine radiographs of the chests of high risk dogs or dogs that are found to have a heart murmur when they’re at the vet for a routine check-up. 

If a puppy is diagnosed with a congenital heart defect, it may be possible for a cardiac surgeon to repair this defect. For example, narrowing of the artery to the lung as seen in pulmonic stenosis can now be repaired by stenting the artery and allowing increased blood flow.  This is extremely specialised surgery and is performed by veterinarians and assistants with advanced training and skills.

For those dogs with mitral valve disease or dilated cardiomyopathy, treatment is medical.  There are four main classes of drugs that are used to alleviate their symptoms and prolong their lifespan. These drugs do this by reducing the fluid accumulation in the body and helping the heart to pump blood more effectively.

Diuretics encourage the kidneys to excrete accumulated fluid as urine. They are a very useful part of therapy for congestive heart failure in dogs because they remove the fluid in the lungs and alleviate the cough and rapid breathing. Dogs on long term diuretic therapy will need regular blood tests to check their electrolyte levels.

Vasodilators open up the body’s arteries and veins and allow more efficient blood flow. They effectively lower the blood pressure and ease the amount of work the heart has to do to pump blood forward.

Positive inotropes improve heart function by increasing the ability of the heart muscle to contract, so it pumps more blood with each beat.

Enzyme blockers are used to interfere with one of the body’s mechanisms to maintain good blood pressure.  The result is dilation of blood vessels and a drop in blood pressure, which then leads to improvement in blood flow around the body.

A very important part of managing dogs with heart murmurs is maintaining a healthy lifestyle. These dogs must not be allowed to become overweight because obesity can contribute to heart disease and worsen the symptoms. Moderate exercise is beneficial but these dogs should avoid excessive activity to reduce their heart’s workload. This can be challenging for those with active dogs but instead of physical exercise, they can enjoy mental exercise such as nosework and trick training.

The right diet can also help – a dog with congestive heart failure should be fed a diet that has reduced salt content. Salt encourages water retention and worsens fluid accumulation. This dietary modification may make life difficult for owners because salt makes food taste better - dogs may not be too enthusiastic about eating their new low salt menu. Good protein levels and omega fatty acids can support the body and minimise the weight loss that can occur with heart disease. The easiest way to meet a dog’s nutritional needs when it has heart failure is to use a prescription diet that’s recommended by a veterinarian.

It can be frightening for an owner to be told their dog has a heart murmur. However, many dogs with murmurs live normal lives with no adverse effects. If heart failure develops, then treatment is available to give them a good quality of life, often for many years.

 

Cost to Treat: $100 up to $20,000

 

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References

Cummings School of Veterinary Medicine at Tufts University. (2017). HeartSmart: Information on Pets with Heart Disease. [online] Available at: https://vet.tufts.edu/heartsmart/ [Accessed 17 Mar. 2017].

Understanding Congestive Heart Failure. (2017). 1st ed. [ebook] Philadelphia: PennVet Ryan Hospital, pp.1-3. Available at: https://www.vet.upenn.edu/docs/default-source/ryan/cardiology-brochures-(ryan)/understanding-heart-failure [Accessed 17 Mar. 2017].

Schrope, D. (2015). Prevalence of congenital heart disease in 76,301 mixed-breed dogs and 57,025 mixed-breed cats. Journal of Veterinary Cardiology, 17(3), pp.192-202.

Ljungvall, I., Rishniw, M., Porciello, F., Ferasin, L. and Ohad, D. (2014). Murmur intensity in small-breed dogs with myxomatous mitral valve disease reflects disease severity. Journal of Small Animal Practice, 55(11), pp.545-550.

Fox, P., Oyama, M., Hezzell, M., Rush, J., Nguyenba, T., DeFrancesco, T., Lehmkuhl, L., Kellihan, H., Bulmer, B., Gordon, S., Cunningham, S., MacGregor, J., Stepien, R., Lefbom, B., Adin, D. and Lamb, K. (2014). Relationship of Plasma N-terminal Pro-brain Natriuretic Peptide Concentrations to Heart Failure Classification and Cause of Respiratory Distress in Dogs Using a 2nd Generation ELISA Assay. Journal of Veterinary Internal Medicine, 29(1), pp.171-179.

Scansen, B., Kent, A., Cheatham, S. and Cheatham, J. (2014). Stenting of the right ventricular outflow tract in 2 dogs for palliation of dysplastic pulmonary valve stenosis and right-to-left intracardiac shunting defects. Journal of Veterinary Cardiology, 16(3), pp.205-214.

Fox, P., Oyama, M., Hezzell, M., Rush, J., Nguyenba, T., DeFrancesco, T., Lehmkuhl, L., Kellihan, H., Bulmer, B., Gordon, S., Cunningham, S., MacGregor, J., Stepien, R., Lefbom, B., Adin, D. and Lamb, K. (2014). Relationship of Plasma N-terminal Pro-brain Natriuretic Peptide Concentrations to Heart Failure Classification and Cause of Respiratory Distress in Dogs Using a 2nd Generation ELISA Assay. Journal of Veterinary Internal Medicine, 29(1), pp.171-179.

Thengchaisri, N., Theerapun, W., Kaewmokul, S. and Sastravaha, A. (2014). Abdominal obesity is associated with heart disease in dogs. BMC Veterinary Research, 10(1), p.131

 

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