Heart Murmurs in Dogs and Cats

By: Dr. Caroline Kiss

What is a heart murmur?

heart murmurs in dogs and catsA heart murmur is an abnormal sound heard on auscultation (listening) of the heart with a stethoscope.   The normal heart makes two distinct sounds often described as "lub" and "dub", which are the sounds of the heart valves closing.  A murmur is often described as a "shooshing" or "whooshing" sound that occurs in between the normal lub-dub-lub-dub sounds of the heart.  If severe, some murmurs can drown out the normal sounds of the heart and can even cause a vibration that can be felt when placing your hand on the chest wall.

Blood normally flows through the heart in a very smooth manner much like a calm, smoothly flowing river with no rapids.   Murmurs are caused by an increase in turbulence or a change in the way the blood flows through the heart, similar to when a river narrows or hits rocks.   

Once a heart murmur is heard, it is characterized by its loudness, the location on the chest at which it is the loudest (point of maximal intensity), and when it can be heard during the normal heart cycle.  It is important to remember that while loudness can be an indicator of the severity of a problem this is not always the case. 

What does a heart murmur mean?

Murmurs are sounds and not diagnoses.  There are multiple causes of murmurs, some of which are more concerning than others.  Murmurs can be caused by structural changes or defects within the heart, anemia, fever, infection, hyperthyroidism, heartworm disease and hypertension.  When we treat a patient with a heart murmur, we are not treating the murmur itself.  We are treating the underlying cause or the effects of the changes in blood flow through the heart and body.

heart murmurs in dogs and catsAnatomy of the heart

In order to understand heart murmurs, you need to know the basic anatomy of the heart. There is a right and a left side of the heart, each made up of one atrium and one ventricle. The right and left sides of the heart are divided by muscle while the atria and ventricles are divided by valves that open and close with the beating of the heart.  The right atrium and ventricle is separated by a valve called the tricuspid valve and the left side of the heart is separated by a valve called the mitral valve.   Blood from our veins that needs oxygen enters into right side of the heart in order to be transported to the lungs to be oxygenated.   The oxygenated blood then returns to the left side of the heart to be pumped out to the rest of the body.  Two major vessels that can be associated with some murmurs are the pulmonary artery and the aorta. Blood flows through the pulmonary artery as it exits the right side of the heart and blood flows through the aorta as it exits the left side of the heart.  

Classification and types of murmurs

heart murmurs in dogs and catsMurmurs are classified as congenital, acquired and innocent or benign.

Congenital murmurs are present at birth and do not resolve or get worse as the pet grows up to 6 months of age.  These are most commonly caused by defects or disease within the heart that the puppy or kitten was born with.  Some of these defects are more serious than others and may include leaky mitral or tricuspid valves, a narrowed pulmonary vein or aorta, a hole in the heart, or a condition called hypertrophic cardiomyopathy in cats.   Some pets with congenital defects will remain without any signs of problems for years or all their lives while others will develop serious problems within several months.

Young puppies and kittens under the age of 6 months can also have soft murmurs that are considered "innocent" murmurs.  These are murmurs that occur because the heart and major vessels of the heart are still undergoing development.  These murmurs can be difficult to distinguish from congenital murmurs but are most commonly more quiet than congenital murmurs and resolve by the age of 4 - 6 months.  If the murmur is still present at the age of 6 months, it is likely due to a congenital defect of the heart.  

An innocent murmur is a type of benign murmur that is not caused by any structural abnormality of the heart.  Other benign murmurs exist that are not caused by structural abnormalities of the heart (see an example below under common causes of murmurs in cats).

Acquired murmurs are those that dogs and cats are not born with but that develop and are heard later in their lives. These are the most common type of murmurs that are heard.   

Diagnostics used to determine the cause of a heart murmur

Depending upon the age, breed, and size of your pet, the character of the heart murmur, and physical examination findings, your veterinarian here at Animal Hospital of North Asheville may recommend a number of tests to help determine the cause of a murmur.  In some cases your veterinarian may recommend monitoring and reevaluating the murmur within a specified time frame before doing diagnostic tests.  

Common diagnostics used to help determine the cause of a heart murmur include:

  • a complete blood count (CBC) to check for anemia or infection
  • a heartworm test to check for heartworm disease
  • blood pressure to check for hypertension
  • a thyroid test (in cats) to check for hyperthyroidism
  • chest radiographs (x-rays) to evaluate the heart and vessel size and shape and to evaluate lungs
  • an ultrasound of the heart called an echocardiogram that allows visualization of the internal structures of the heart
  • a blood test called proBNP which can help distinguish a benign versus pathologic murmur in cats and can help monitor heart disease in a dog
  • an ECG to help determine if there is an irregular rate or rhythm occurring

Common causes of heart murmurs in dogs

In small to medium sized dogs, the most common cause of an acquired murmur is a leaky mitral valve.  This occurs due to a degenerative process of the valve that causes the valve to become thickened and irregular.  Once this occurs, the valve no longer closes completely and will allow leaking of blood through the valve when the valve is supposed to be closed.   If the leaking is mild, your dog will remain asymptomatic and demonstrate no signs of illness.  If the degenerative process continues and the leaking becomes greater, your pet's heart may no longer be able to compensate for this defect and  congestive heart failure can develop.  However, if your pet is diagnosed  with mitral valve disease do not panic.  A majority of dogs with mitral valve disease will never develop clinical signs of a problem or congestive heart failure.  30-40% of dogs with mitral valve disease will develop congestive heart failure but it can take years for this to occur. If your pet does develop congestive heart failure, most can be medically managed and have an excellent quality of life.

If your pet's age, breed, and size and type of murmur are consistent with mitral valve disease, extensive diagnostics are generally not needed to determine the cause of the murmur.    However, chest radiographs are recommended on a routine basis to monitor heart size and shape.  In some circumstances, an echocardiogram, EKG, and blood pressure may be recommended.

A leaky tricuspid valve can also be a cause of an acquired heart murmur.  This occurs due to a similar degenerative process that occurs with the mitral valve.  A leaky tricuspid valve can occur alone or concurrent with mitral valve disease.  Unless it is very severe, tricuspid valve disease rarely leads to further problems or congestive heart failure.

Acquired heart murmurs in larger breed dogs can also be due to a leaky mitral or tricuspid valve, but this is a less common cause of murmurs in larger breeds than it is in small to medium breeds.  The more common cause of murmurs in larger breeds is dilated cardiomyopathy.  This disease is characterized by thinning of the heart muscle walls, which eventually lose their ability to contract.  This disease has a poorer prognosis than mitral or tricuspid valve disease and requires early intervention for the best outcome.  Therefore, if a murmur is present in certain breeds of dogs (Boxers, Dobermans, Labrador Retrievers, Golden Retrievers) or larger dogs, an echocardiogram should be performed to determine its cause.  

Common causes of heart murmurs in cats

heart murmurs in dogs and catsAs in dogs, heart murmurs in cats can be caused by leaky valves, but this is not common.  More commonly, an acquired heart murmur in a cat is due to hypertension, hyperthyroidism, or several forms of cardiomyopathy.   The most common form of cardiomyopathy is hypertrophic cardiomyopathy.   This is a disease in which the walls of the heart become thickened, leading to restriction of blood flow through the ventricles. When the disease is severe, it can lead to congestive heart failure and thromboembolisms (clots).  However, the course of this disease can be quite variable.  Some cats with milder forms of hypertrophic cardiomyopathy live normal and long lives with no signs of problems while others with more severe disease will develop congestive heart failure, throw blood clots, or even die suddenly at a young age.  

Unique to cats, heart murmurs can also occur in nervous cats with increased heart rates.  The increased heart rate of a nervous cat increases the amount and speed of blood that flows through the heart and this can result in a murmur.  This type of murmur is a benign murmur and does not result in heart disease or problems later in life.  This is why your veterinarian may hear a murmur in your cat during one visit and not another.

Unfortunately, auscultation alone cannot distinguish benign causes of heart murmurs in cats from more severe causes.  The only way to distinguish these heart murmurs is through further testing.  The two most beneficial tests in helping us distinguish benign versus pathologic heart murmurs in cats are the proBNP blood test and an echocardiogram.  One or both may be recommended to you by your veterinarian.

This article was not meant to be an all-inclusive article on the causes of heart murmurs in dogs and cats but was meant to give you a better understanding of terminology and common causes of heart murmurs in your pets. If we hear a heart murmur in your pet, we at Animal Hospital of North Asheville will discuss the potential causes and best way to diagnose its cause in much more detail.  While a heart murmur is a concern not to be ignored, it is also not a reason to panic.  Many pets with heart murmurs can lead long lives, especially if the cause of the murmur is known and the underlying cause managed.