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Heart Murmurs in Dogs: What It Is, Symptoms & More

If you’re reading this article, you may be wondering, “my dog was just diagnosed with a heart murmur-now what?!” You may be feeling scared, worried, and wondering what exactly a heart murmur means for your pet’s health, now and in the future. At AskVet, we’re here to help you understand what the diagnosis of a heart murmur means-and what it doesn’t mean-as well as certain symptoms of heart disease in dogs to discuss with your family veterinarian.

Normal Heart Function

Thinking back to high school biology, you probably remember that the heart pumps blood, which goes around the body in tiny tubes called blood vessels. The blood absorbs oxygen when it passes through the blood vessels in the lungs, and the oxygen-rich blood is then pumped by the heart to the rest of the body. Your body—and that of your dog—needs oxygen in order to maintain the health of every organ and tissue. 

What happens when blood flows into the heart itself? It helps to think of your dog’s heart as a house, with both plumbing and electricity. The main blood vessels that lead into the heart and away from it are just like pipes or plumbing leading into your home!  The heart has four rooms (called “chambers”), which are each separated by walls (or “septae”). Each septum/wall has a set of  swinging doors in the middle that only swing open and closed in one direction. These doors are called “valves.” 

Once blood vessels, or veins, deliver blood back to the heart, the blood is moved through each heart chamber in a specific order by contractions of the heart muscle. While the heart’s electrical system controls the timing of each chamber’s muscular “squeeze” movement, the valves (doors) open in one direction in a carefully-choreographed rhythm to create the proper blood flow through the heart. The bottom line: a high level of coordination allows blood to flow through the heart in only one-forward-direction.

The heart muscle is so powerful that, as the different chambers squeeze in sequence to move blood through the heart, a noise can be heard using a stethoscope. When the heartbeat is normal, the noise produced by those valves closing in unison is described as a “lub-dub” sound. When you think about the level of coordination required to make such a simple sound, it’s really quite incredible! 

So, What IS a Heart Murmur? 

Now that you understand how a normal heartbeat is created, you can understand what a heart murmur represents. To put it simply, a murmur simply refers to a sound heard with a stethoscope that indicates turbulent blood flow. 

Imagine going for a hike and finding a quiet, still creek with a surface like glass—that’s like the flow within a normal heart. The fluid flows in one direction very quietly and without any interruptions. A heart murmur, on the other hand, sounds like a “whoosh” sound—as though someone put a big rock in the middle of that creek! 

Turbulent blood flow within the heart can be caused by many things, as you can probably imagine! In a nutshell, sometimes the valves are leaky or do not close fully, allowing blood  to flow backwards. In other pets, the vessels leading out of the heart are too narrow in spots and we can hear an extra sound where there is usually silence. There can even be holes in the heart walls, creating bloodflow where there shouldn’t be any! 

As you can probably tell, this means that not all heart murmurs are created equal. Some heart murmurs indicate a type of problem that does not affect a dog’s lifespan at all- and unfortunately, some murmurs indicate a life-threatening heart problem. These types of heart disease require close monitoring and medical intervention. How worried we should be about your pup’s heart murmur depends on what is creating the extra noise. If there is an underlying cause of the murmur, it may impact how hard the heart needs to work, and affect normal blood flow throughout the body.

The bottom line: sometimes, the extra noise of a heart murmur is the first clue that your pup has a heart condition requiring medical attention–and sometimes, it truly is nothing to worry about. Further testing is required to find out the cause of your dog’s heart murmur and your veterinarian can discuss these with you. 

Next, we’ll talk about the most common causes of heart murmurs in dogs, as well as what your veterinarian may recommend. 

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Congenital Defects

When your pup was growing inside of the womb, many changes were happening to help him develop into his full potential! Along the way, the structures that eventually became your pooch’s circulatory system were growing, dividing, and changing. Sometimes a pup is born with part of this complicated process incomplete, and they may have an extra blood vessel in a place where it isn’t needed, or a hole in a part of the heart that should have been closed. Some of these defects may have familiar names like “Patent Ductus Arteriosus,” “Aortic Stenosis,” and “Septal Defects.”

Some puppies are born with defects that are inconsequential (leading to “innocent murmurs” that actually go away as your little one grows up), but some of these can be life-threatening without prompt intervention. If your family veterinarian suspects a more serious congenital heart defect, they may recommend x-rays (radiographs) of your pet’s heart and lungs, and may refer your puppy to a veterinary cardiologist for an ultrasound of his heart. Some of the more serious congenital heart defects can be fixed with appropriate surgery or a non-invasive procedure, helping your pup to live a normal life.

Degenerative Valve Disease

In a normal heart, the valves are like tightly-fitting swinging doors. Their edges are smooth and straight, and there is no blood allowed to flow the wrong direction! (Pretty amazing stuff, when you think about it!) 

However, in some dogs, the smooth edges of these valves become gnarled over time, leading to irregular edges. As you can imagine, this means that the edges of the valves fail to create a tight seal when they are closed—allowing blood to flow backwards as well as forwards. This is called regurgitation, and it is this regurgitation that causes the “whoosh” of a heart murmur heard by your veterinarian. Over time, blood continuing to regurgitate in the wrong direction causes the heart to work harder just to get enough blood moving forward in the right direction. Just like any muscle that works harder, the heart grows in size. Sometimes, the noise generated by a small leak may be very loud, as the extra blood rushes through a tiny space and creates lots of turbulence. Conversely, larger leaks can also be more quiet, as the blood is whooshing through a larger opening. For this reason, it’s impossible to judge the severity of your dog’s heart disease based on how loud or severe her  heart murmur sounds–the only way to know is to have an ultrasound (echocardiogram) performed to look at the “rooms” and “doors” (chambers and valves) of your dog’s heart. . 

Degenerative valve disease is thought to be genetic, and is most common in smaller dog breeds, like Cavalier King Charles Spaniels, Chihuahuas, toy Poodles, Cocker Spaniels and Shih-tzus. However, any dog breed can be affected. In dogs with valve disease, we most often see problems with the mitral valve and the tricuspid valve. 

Degenerative valve disease can eventually lead to congestive heart failure as the blood starts to severely back up and the heart struggles more and more to keep blood flowing in the right direction. However, these changes to the heart muscle can be slowed down–and the risk of heart failure can be decreased–by starting heart medications at the appropriate time. Follow your veterinarian’s recommendations for chest x-rays and/or referral to a cardiologist to maximize the length of time your dog lives with valve disease!

Dilated Cardiomyopathy

Sometimes, the walls of the heart muscle become thinner over time, due to a degeneration or weakening of the muscle tissue itself. Since the volume of blood the heart is squeezing through the chambers does not change, the pressure of the blood inside the walls causes them to stretch and thin out even more. Unfortunately, the valves remain the same size—so the effect is like widening a doorway in your home while keeping the same size door in the frame! Blood can now regurgitate back and forth through this gap, creating the “whoosh” sound your veterinarian hears as a heart murmur. This type of heart disease is called dilated cardiomyopathy, or DCM.

Dilated cardiomyopathy is thought to be largely genetic, and tends to affect medium- and large-breed dogs such as Doberman Pinschers, Golden Retrievers, Boxers, and Labradors. 

However, there is an important environmental cause of dilated cardiomyopathy that can affect ANY dog—and this type of dilated cardiomyopathy is caused by feeding certain foods. Specifically, grain-free diets (especially those containing legumes) have been linked to this type of heart disease. The specific food ingredients that cause DCM are being studied, but for now, veterinarians recommend avoiding ANY grain-free or “boutique” dog food, especially one that has been linked to cases of DCM. (Link to FDA article regarding a possible diet-related link to canine dilated cardiomyopathy)

Dilated cardiomyopathy is a serious disease and sadly shortens a dog’s lifespan. The electrical system of the heart is eventually affected, and most dogs with this disease will develop heart failure. However, because medications can delay the onset of heart failure and/or a fatal heart electrical storm (arrythmia), it is worth pursuing a diagnosis to optimize treatment recommendations. This will help to maximize your dog’s quality of life for as long as possible. 

Symptoms of Heart Failure

If your pet has been diagnosed with a heart murmur, it’s important to remember that your pet’s heart murmur is a SYMPTOM—and that if your pet has a murmur, this does NOT mean that your pet is in heart failure! They may actually be doing just fine! (Thank goodness!)

Fortunately, the vast majority of pets with heart murmurs will never go on to develop heart failure. For those that do, heart failure is a life-threatening and very dangerous illness—but can often be controlled IF it is detected early enough for medical intervention. 

One of the most important ways to monitor your pet for early signs of heart failure is to watch your dog’s chest closely and count how many breaths he takes per minute while he is sleeping. This is called your pet’s Sleeping Respiratory Rate, and anything over 30 breaths per minute is a reason to contact a veterinarian as soon as possible. (See Veterinary Partner for how to take the sleeping respiratory rate). You can reach out to your AskVet veterinarians for advice on whether seeking emergency care is needed for your pet, or contact your family veterinarian if they are open! Read our dog physical exam checklist at home guide for more information.

More obvious symptoms of heart failure include coughing and difficulty breathing (often, we’ll see panting/gasping for air with a tongue that is blue-ish in color instead of bubblegum-pink). Also, dogs in heart failure start getting tired and having to rest after short periods of activity, such as walking across the room. If your pet is exhibiting these signs, it is best to take him to the nearest emergency veterinary hospital immediately.

Read our guide on ‘Why is my dog breathing heavy’ for more information if you think your dog is having abnormal breathing.

The Bottom Line

If your pet is diagnosed with a heart murmur, it does NOT mean that your pet is seriously ill, or that she is in heart failure. Instead, it means that your astute veterinarian has found a symptom that is worth investigating further to determine what your dog’s heart murmur means for his health. Often, some yearly monitoring is all that will be needed-but if your pup needs more intensive testing and medication, rest assured that you are intervening as early as possible to ensure she lives a long, healthy, and happy life.

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Written by:

Allison Ward, DVM
Dr. Allison Ward grew up in the suburbs of Washington, D.C. and started working in veterinary hospitals when she was 14 years old. After graduating from veterinary school in 2011, she completed a small animal rotating internship in New Jersey, followed by a neurology/neurosurgery internship in Miami. After completing this advanced training, Dr. Ward then moved on to general small animal practice. Dr. Ward’s professional interests include feline medicine, neurology, and pain management. Her passion for educating pet owners carries over into her work with AskVet, and she loves being able to help pets and their parents at all times of the day (and night!). She currently resides in sunny south Florida with her two cats, Larry and George.


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