Heart murmurs in dogs

Content by Dr Siân Burwood MA VetMB MRCVS

A heart murmur is a symptom of heart disease, and is something your vet hears as a ‘whooshing’ noise with a stethoscope. They are the sound of abnormal blood flow in the heart, and are usually caused by problems in the valves between chambers – although the heart muscle itself can also be a cause.

Heart murmurs are either graded from 1 to 6, or as quiet or loud. The problems that cause murmurs can stop blood and oxygen being efficiently pumped around the body, and could lead to congestive heart failure.

• Heart murmurs are a common symptom of heart disease in dogs (although not all heart diseases cause murmurs)
• Dogs can be born with the heart problems that cause murmurs, or they can develop over time
• Your vet hears a murmur as a ‘whooshing noise’ when listening to the heart, and will then grade the murmur as quiet or loud, or from 1 to 6
• When diagnosed early, many causes of murmurs and heart disease can be managed well
• Left untreated, murmurs lead to congestive heart failure, where blood and oxygen are not pumped around the body efficiently and fluid builds up in the lungs or belly

A murmur is the sound created by blood flowing the wrong way through the heart, either through a deformed heart valve or through a hole in the muscle.

• Valve dysplasia is where the tissue that makes up a valve becomes scarred over time, and so does not seal itself properly when shut. This is the most common cause of mitral valve disease (MVD), often seen in small breed dogs as they get older. The more blood that leaks, the louder the murmur is.
• Valve stenosis is where a valve is narrowed and stiff, and so cannot open and shut properly. This most commonly affects the aortic and pulmonic valves, and is something that affected dogs are usually born with. 
• Septal defects are holes in the walls between chambers of the heart, and are something that dogs are born with. In this type, smaller holes cause louder murmurs, but larger ones cause quieter murmurs.
• Physiological murmurs are quiet murmurs heard in young puppies as the structures they had in the uterus (when they get oxygen from the placenta rather than their lungs) have not fully closed off yet. These murmurs often disappear by the time of the second vaccination.
• Other birth defects can occur where structures needed in the uterus do not close off at all when the puppy is born. Patent Ductus Arteriosus (PDA) is the most well known, causes very loud murmurs, but can be surgically corrected if caught in time.

Most heart diseases that cause murmurs are hereditary and are commonly seen in certain breeds:

• Cavalier King Charles Spaniels are the most commonly affected breed with mitral valve disease, and are often affected from a very young age.
• Boxers, Golden Retrievers and Rottweilers are at increased risk of aortic stenosis.
• Bulldogs of all types and Terriers are at increased risk of pulmonic stenosis.
• Bulldogs of all types, Terriers, English Springer Spaniels and Basset Hounds are at increased risk of ventricular septal defects.
• Small toy breeds are at increased risk of patent ductus arteriosus.

Any breed can be born with or develop any cause of a murmur. As such, breeders should be screening parents to ensure they don’t breed from affected dogs, or should stop breeding from affected lines.

Quiet murmurs (graded 1-3) are classified as when the ‘whooshing’ noise is quieter than the heart sounds themselves, and sometimes can only be heard in a quiet room with a good stethoscope. They’re often the only symptom, and are one of the main reasons that regular check-ups with your vet are important.

Diagnosis

Listening with a stethoscope diagnoses the murmur, but the cause of the murmur can only be diagnosed with an echocardiogram. This is an ultrasound scan of the heart that looks at the shape and function of the heart valves, as well as how well blood is flowing through the chambers.

Most GP vets can do a standard level heart scan, but they may recommend you see a vet (either in their practice or at a referral centre) who has further training and expertise to do a more detailed scan.

Most quiet heart murmurs don’t need treatment if there are no signs of progression to congestive heart failure. Regular monitoring by your vet may be the only treatment recommended, but in some cases your vet will advise a scan first. This is to accurately diagnose the problem and provide baseline measurements, against which future values can be compared.

Prognosis

• Physiological ‘puppy’ murmurs are often heard at the first vaccination, but disappear by the second one. In this case, the prognosis is excellent.
• Murmurs that remain stable over time with no signs of congestive heart failure carry a good prognosis.
• The quicker a murmur gets louder, the worse the prognosis is and the more urgent it is to get treatment.

Loud heart murmurs (grade 4-6) are when the ‘whooshing noise’ is louder than the heart sounds. They can sometimes be felt on the side of the chest as a ‘buzzing’ sensation, or even heard without a stethoscope.

These may be accompanied by signs of congestive heart failure, when the blood is not effectively pumped around the body and fluid starts to build up where it should not. The symptoms range from:

• A throat clearing cough, especially after rest or at exercise
• Reduced ability to exercise
• Lethargy
• Pale gums and tongue
• A swollen belly
• Difficulty breathing
• In puppies with developmental conditions, they may fail to grow properly and be small and weak

Diagnosis

History and clinical examination are very important for diagnosis, as is an echocardiogram and X-rays of the chest. These will help find the source of the murmur, but also assess for the degree of fluid build up in the lungs and other parts of the body.

When a loud heart murmur is diagnosed but the dog has no other symptoms, a medication called ‘pimobendan’ will be started. This is a safe, lifelong medication and helps each beat of the heart to be more efficient.

If milder symptoms are present that indicate fluid is starting to build up (such as a cough or exercise intolerance), pimobendan will be started – along with 2 or 3 other drugs that help the body get rid of the extra fluid and slow changes in the heart muscle.

If the dog is already in serious congestive heart failure when they’re first seen by the vet (with breathing difficulty and pale or purple gums), then they’ll need emergency treatment to get rid of the excess fluid building up. They’ll often be kept in hospital to be given oxygen.

Prognosis

Whilst loud heart murmurs are almost always progressive and cannot be reversed, with early diagnosis and treatment, many dogs go on to have a good quality of life while on medication. Regular vet checks are essential to monitor for any change in condition.

By the time a dog is presented in congestive heart failure, the prognosis is more guarded as the heart has already stopped coping. This is why regular vet checks are vital to pick up any causes of murmurs early.

Some developmental causes of loud puppy murmurs (like PDA) can be surgically reversed if caught early enough that can lead to normal lives for these dogs. Others (like stenosis) can have surgical treatments that can really improve expected lifespan. In both cases, these are always specialist surgeries.

“It’s important to understand that a canine heart murmur is a symptom of heart disease, rather than the diagnosis itself. As such, working with your vet is really important to find the cause. 

“Over the last 10 years, treatment has been revolutionised by starting to treat murmurs without symptoms in small-breed dogs. By not waiting for symptoms to develop, these dogs live on average 18 months longer!

“My own dog benefited from this treatment. And while it always seems scary to put a seemingly healthy dog on lifelong medication, I always encourage my owners to do it, as I know first-hand what a difference this can make”

Dr Siân Burwood, MA VetMB MRCVS

With a Petgevity Lifetime Plus policy, we’ll consider all pre-existing conditions when offering cover for your dog*. 

When you apply for dog insurance, you need to let us know: 

• Which medical conditions they’ve had in the last two years – including those which need ongoing treatment, monitoring or check-ups
• Their health status now

Find out more about our dog insurance for pre-existing medical conditions.