It is common for a vet to hear a heart murmur on routine examination of a healthy dog. This can come as an alarming surprise. But, don’t panic. A heart murmur can be present without serious implications and your dog may never develop full-blown heart disease. This article will review what a murmur is, what investigations are helpful and how heart disease can be effectively treated.

What is a heart murmur?

A heart murmur is caused by turbulent blood flow in the heart. Normal heart sounds are heard as a regular lub-dub which coincide with valves closing. If the flow of blood in the heart is disrupted, turbulent flow makes an additional pshhh or whoosh sound.  Turbulence can be caused by disease of the heart valves, heart muscle or a structural heart defect.

Valvular disease

The heart is a four-chambered pump. The chambers pump blood around a one-way system with valves controlling the flow. The valves shut behind the blood as it is pumped forward. This ensures that enough blood flows to the lungs to receive oxygen and then around the body to deliver that oxygen. 

Most canine heart murmurs are caused by valvular disease. 10% of dogs that visit the vet have heart disease and 75% of those have valve disease (1). It is more common in small breeds (under 20 kg). Male dogs have a higher risk and up to 70% of cases occur on the left side of the heart, affecting the mitral valve. 

Valve disease occurs when the valves become thickened and stiff. The valves no longer close properly and leak, reducing the output of the heart.

Most toy and small breeds develop valve disease as middle-aged or older dogs. Cavalier King Charles Spaniels can develop it when they are young. The disease is slowly but unpredictably progressive. Most dogs develop a murmur many years before they show signs of heart failure. 

Cardiomyopathy 

Cardiomyopathy means disease of the heart muscle, this can cause structural change in the heart and turbulence. Dilated cardiomyopathy causes the heart muscle to stretch preventing the valves from closing. This disease is common in Doberman Pinschers. 

Heart defects

Puppies can be born with structural heart defects, these are called congenital defects. The defects may involve a hole between two chambers of the heart or an abnormally shaped blood vessel. The murmurs are usually heard at vaccination. Some congenital heart defects can be successfully treated by surgery preventing future disease. Therefore early diagnosis is essential. 

Puppies can also have ‘innocent’ murmurs. These murmurs are usually soft murmurs (quieter than the heart sounds) and disappear by 5 months of age. They are not caused by underlying heart disease. 

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Anaemia can also cause a heart murmur. Anaemic dogs have pale gums, a simple blood test can be used to rule this out.

A heart murmur can be quiet, and difficult to hear in a panting, unsettled dog. A vet may need to auscultate the chest for a long time in a cool, peaceful room. They may check certain parameters of the murmur to determine the likely cause. These parameters include; volume, loudest point, any vibration or thrill, timing in the heart cycle and the dynamic of the sound. 

An increase in resting heart rate can be an indicator of heart disease.  

Further investigation is necessary to determine the significance of the murmur. There is strong evidence that if the heart shows early signs of remodelling, intervention will extend your dog’s lifespan considerably (1). Treating early changes will prolong the period before heart disease causes symptoms (2). Pimobendan is commonly used in this situation. Studies have shown that pimobendan can reduce heart size and heart rate in remodelled hearts (2,3). It has also been shown to prolong the symptom free period in Doberman Pinschers with dilated cardiomyopathy (4).

Cardiac ultrasound/echocardiogram

This is the best way to investigate a murmur. When heart disease affects heart function the chambers become dilated. The chambers can be measured on an echocardiogram to determine if the chamber is stretched. The speed of blood flow across the valves can also be measured and the valves examined.  If abnormalities are found, treatment is indicated. If the heart is normal, then the scan can be repeated in 6-12 months. This means effective treatment is prescribed when it is needed. Some heart conditions require specialist scanning by a cardiologist. 

X-ray

Heart enlargement will also show up on x-ray. X-ray allows examination of the lungs, this can be helpful as some dogs with heart disease also have or go on to develop respiratory disease later in life.

Blood pressure measurement

High blood pressure can be present with heart disease. 

Blood tests

Blood tests can be used to check markers which indicate heart muscle stretch. Nutritional deficiencies can cause heart disease particularly in large dogs, so blood tests can be used to check nutrient levels. 

What happens in heart failure? 

If the heart cannot pump blood effectively around the body, symptoms of heart failure are seen. Initially, loss of appetite and lethargy are common. As the heart rate increases and fluid accumulates in the lungs, breathing becomes rapid, laboured and noisy. The abdomen may swell with fluid collecting behind the heart.

When heart function becomes poor, the gums appear grey or blue. The dog may be restless and anxious, unable to lie down. As their blood pressure drops, they may become weak, faint or collapse. 

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This usually progresses slowly. Rarely, the cords attached to the heart valves rupture and heart failure occurs suddenly.  

Treatment for heart failure in dogs

If your dog develops heart failure, oxygen therapy and diuretics will be used to stabilise them for investigation. There are effective medication protocols to manage heart disease and dogs can often have a good quality of life for some time.

In dogs with no symptoms, investigation and pre-emptive treatment can mean that they never develop full-blown heart disease despite their murmur. 

The diagnosis of a heart murmur signals a possible problem. This allows your vet to thoroughly investigate and treat any heart disease or monitor your dog to keep them well. As early intervention is so important, diagnosing a heart murmur can be positive.    

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References

  1. Keene BW, Bonagura JD, Fox PR, Haagstrom J, Luis Fuentes V, Oyama MA, Rush JE, Steipen R and Uechi M. ACVIM guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs, (2019) JVIM 33(3):1127-1140
  2. Boswood A, Häggström J, Gordon SG, et al. Effect of pimobendan in dogs with preclinical myxomatous mitral valve disease and cardiomegaly: the EPIC Study—a randomized clinical trial. J Vet Intern Med. 2016;30:1765–1779
  3. Boswood A, Gordon SG, Häggström J, et al. Longitudinal analysis of quality of life, clinical, radiographic, echocardiographic, and laboratory variables in dogs with preclinical myxomatous mitral valve disease receiving pimobendan or placebo: the EPIC Study. J Vet Intern Med. 2018;32(1):72–85.
  4. Summerfield NJ, Boswood A, O’Grady SG, Gordon SG, Dukes-McEwan J, Oyama MA, Smith S, Patterson M, French AT, Culshaw GJ, Braz-Ruivo L, Estrada A, O’Sullivan ML, Loureiro J, Willis R and Watson P. Efficacy of Pimobendan in the prevention of congestive heart failure or sudden death in Doberman Pinschers with preclinical dilated cardiomyopathy. (The PROTECT Study). JVIM 2012;26(6):1337-49