What is a heart murmur and what does it mean for your dog?
You’re at the veterinarian for an annual checkup with your Pomeranian. Your vet has just finished listening to your pet’s heart when they look up and mention, “Fluffy has a heart murmur.” You are immediately worried, anything to do with the heart sounds bad. You can feel your own heart rate picking up as you listen to what your veterinarian is discussing. You try your best to understand what they are saying, but you’re quickly losing focus. Is a heart murmur bad?
Let’s start by defining what a heart murmur is. All that a heart murmur indicates is that blood is not flowing smoothly between the chambers of the heart. Smooth blood flow is what produces the rhythmic normal heartbeat that can be heard when listening to the chest of patients — human and animal alike. Turbulent blood flow can occur for a variety of reasons, and not all of them are bad. We will review the most common reasons for a heart murmur in a dog here.
Hearing a heart murmur does not indicate that your pet will ever go on to develop any form of clinical heart disease or heart failure.
Both dogs and cats have the potential to develop a heart murmur in high-stress situations. So, when pets are very anxious or nervous in the clinic, we will sometimes auscultate (hear) a heart murmur that resolves when the pet is more relaxed.
It is difficult to differentiate between stress-induced murmurs and true ones; however, I often ask the owner to leave and come back on a completely new day to listen again first thing (i.e. before anything else has been done to the pet). The stress-related murmurs will often not be present at that time. Murmurs from stress occur more commonly in cats than in dogs in my experience.
Heart murmurs in dogs can also result from an alteration to one of the heart valves. There are four major valves in the heart that serve to separate the chambers from one another. There is a valve between the smaller atrium and the larger ventricle on each side of the heart. The left-sided heart valve is called the mitral valve (bicuspid valve) and the right-sided valve is the tricuspid valve. Then there are two valves separating the heart from the major blood vessels. There’s the aortic valve on the left and the pulmonic valve on the right.
Mitral valve murmurs make up a bulk of murmurs in dogs. There is a strong genetic predisposition to this valvular disease in small breed dogs like Chihuahuas, Pomeranians, and Pugs. Basically, the valve thickens and becomes less flexible and less capable of performing its job. This causes back-flow of blood from the ventricle into the atrium which creates a murmur that your veterinarian will hear.
Fortunately, there is a broad range of severity for mitral valve murmurs, and most progress slowly — sometimes never leading to true heart disease. So, hearing a heart murmur does not indicate that your pet will ever go on to develop any form of clinical heart disease or heart failure. It is important to work with your veterinarian to monitor a heart murmur and decide if additional testing is worthwhile.
The mitral valve on the left side of the heart is the most common culprit for heart murmurs in dogs, and the tricuspid valve on the right is the second most common cause of valvular heart murmurs. The difference is only in the location that your veterinarian will hear the murmur. It will be louder on the right side of the chest rather than the left. Like with mitral murmurs, the murmur tends to develop because of decreased flexibility in the tricuspid valve. The other valves that I mentioned can also create heart murmurs; although, they are not very common, so I will not discuss them here.
Now the next common cause of a heart murmur would be a structural disease of the heart called Dilated Cardiomyopathy (DCM). Large and giant breed dogs are over-represented in this disease, like Great Danes, Irish Wolfhounds, Dobermans, and Boxers. Contrast this to cats, who develop Hypertrophic Cardiomyopathy most commonly.
With DCM, the chambers, rather than the muscle, of the heart will dilate and enlarge. This dilation causes them to be flimsy and less efficient at contraction. The lack of efficient contraction requires the heart to work harder to perform its usual tasks.
This disease does not always create a heart murmur, but when it does, it is usually left-sided. The lack of a murmur makes diagnosis difficult, so in predisposed breeds, like the ones listed above, if a murmur is heard then diagnostics are usually indicated sooner rather than later. DCM is generally a condition that warrants more concern over a standard mitral valve heart murmur, as it is more likely to be problematic to the pet.
It is worth mentioning here that the FDA is currently investigating a link between DCM and dog foods marketed as, “Grain-Free.” Currently, most veterinarians are recommending patients avoid exclusively grain-free diets until the mechanism is better deciphered. However, not all pets that are fed grain-free go on to develop the disease, so it is a discussion worth having with your veterinarian.
With heart murmurs of any cause, the standard workup includes a blood panel to rule out other systemic illnesses, X-rays of the chest to assess overall heart size and assess for the presence of fluid in the lungs, and finally an echocardiogram. An echocardiogram, or echo for short, is the only way to truly assess where the heart murmur is coming from and what needs to be done about it. It is the definitive way to diagnose DCM and will be able to assess whether a valve-based disease requires treatment or not.
Canine heart murmurs can be scary to hear about for the first time, but they often do not indicate a true disease. All a heart murmur means is that there is blood flowing less smoothly through an area of the heart than it should be. You will have to work with your veterinarian to assess the severity of your own pet’s situation. I hope this article helps a little to demystify the heart murmur and recognize that hearing one does not translate directly to heart disease. Thanks for reading!