Heart Murmurs in Dogs: Symptoms and Treatment

by: Laura McLain Madsen DVM

What is a heart murmur?

Heart murmurs in dogs are abnormal sounds that your veterinarian hears when listening to the heart with a stethoscope. The heart normally sounds like “dub-dub… dub-dub” but a murmur will make the heart sound more like “whoosh… whoosh.”

What does the murmur mean?

Heart murmurs in dogs indicate turbulent blood flow—rough and choppy flow like turbulence around an airplane. Think of water flowing through a garden hose. It doesn’t normally make a lot of noise as it flows, but if the hose is kinked you can hear a loud whoosh or hiss at the kink. The noise happens because the flow is turbulent, not smooth.

Various heart conditions can cause turbulent blood flow leading to a heart murmur. Some are acquired (developing in an adult dog) while others are congenital (birth defects which a puppy is born with). Specific conditions include:

1. Mitral valve disorder: The dog's mitral valve is a one-way valve in the left side of the heart, between the upper chamber (atrium) and lower chamber (ventricle). Normally, blood only goes from the atrium to the ventricle, but in some dogs this one-way valve starts to leak. When the valve leaks, blood squirts backwards up through the valve into the atrium, causing turbulence. The extra blood flow also stretches out the atrium over time; this stretching can pull the valve open even further, meaning even more blood can leak. Mitral valve problems are typically acquired, as the valve degenerates in older dogs.

2. Tricuspid valve disorder: The dog's tricuspid valve sits between the atrium and ventricle on the right side of the heart. It can be leaky, just like the mitral valve, in congenital problems (tricuspid dysplasia) or heartworm disease.

3. Pulmonic stenosis (PS): The dog's pulmonic valve is located at the point where blood leaves the heart to go to the lungs. Sometimes, puppies are born with this valve being formed too narrow (stenotic). The heart has to pump harder to squeeze blood through the narrow opening—just like trying to breathe through a straw. Blood spurts through the valve, leading to turbulence and a murmur. The high pressure of blood jetting through the valve and hitting the vessel wall on the other side can lead to dilation and weakness of the vessel wall.

4. Subaortic or aortic stenosis (SAS or AS): The dog's aortic valve is located at the point where blood leaves the heart to go to the rest of the body. Similar to pulmonic stenosis, the aortic valve is narrow and blood spurts through at high pressure.

5. Atrial septal defect (ASD) and ventricular septal defect (VSD): Both of these defects are essentially a hole in the heart—an abnormal window between the two atria (upper chambers) or the two ventricles (lower chambers). Blood “swishes” back and forth between the chambers.

6. Patent ductus arteriosus (PDA): The dog's ductus arteriosus is a vessel near the heart that is only active in the fetus. Normally as soon as a puppy is born and starts to breathe, the ductus closes up. In certain puppies it fails to close and a murmur is heard from the abnormal flow through the vessel.

Are heart murmurs in dogs serious?

Heart murmurs may or may not be serious. Veterinarians grade heart murmurs on a scale of 1-6 to indicate how loud they are. Grade 1 is the mildest, which sounds like “shh-dub… shh-dub,” and Grade 6 is the most severe, sounding like “WHOOSH…WHOOSH.”

The loudness of a murmur doesn’t necessarily correlate with how severe the heart problem is but can be suggestive. For example, if a murmur is heard on a routine physical exam your veterinarian will grade it. If your dog had a Grade 2 murmur last year but it is a Grade 4 this year, that might mean that a valve is getting progressively leakier or that the heart is enlarging.

Sometimes puppies will have a mild (Grade 1 or 2) murmur, called an “innocent” or “flow” murmur, that they outgrow as they mature. But if a puppy has either a loud murmur (Grade 3 or higher) or any murmur that persists past sixteen weeks of age, there is a significant concern for a congenital heart abnormality like SAS, PS or VSD. These puppy murmurs should definitely be evaluated by a veterinarian or veterinary cardiologist.

Which dogs are at risk?

Dachshund Dog with Heart MurmursOne of the most common causes of heart murmurs in dogs is a leaky mitral valve. With age, the mitral valve can degenerate, becoming thin and shaggy. This degeneration is most often seen in small-breed dogs, including Cavalier King Charles spaniel, Chihuahua, mini poodle, mini schnauzer, Dachshund, Yorkshire terrier, Maltese, Pomeranian and Shih-Tzu. Large dogs occasionally have mitral valve problems, particularly Great Danes and Dobermans.

Other causes of murmurs can be heritable in some breeds: tricuspid valve dysplasia in Labrador retrievers and other large breeds; subaortic stenosis in Newfoundlands, Rottweilers, Golden retrievers and Boxers; and PDA’s in German shepherds, Maltese, Collies, Shetland sheepdogs, Yorkshire terriers and cocker spaniels.

How are heart murmurs in dogs diagnosed?

When your veterinarian hears a heart murmur in your dog, she will grade it on the 1-6 scale. She will also note where it sounds the loudest (on which side of the dog, and where on the chest), and the type of murmur. Systolic murmurs are heard during the pumping phase of the heart beat while diastolic murmurs are heard during the filling stage. Some murmurs are heard with each distinct beat of the heart while others are continuous (called “machinery” murmurs). The loudness, location, and type of murmur will give your veterinarian a hint about the cause.

The best way to diagnose a heart murmur is with an echocardiogram (“echo”). This uses an ultrasound, a non-invasive means to visualize the heart. With the ultrasound machine, your veterinarian can measure the thickness of the heart muscle, look for a leaky valve or a hole in the heart, measure how much blood is passing through the valve or hole and how fast, and quantify how strongly the heart is pumping.

X-rays (radiographs) may also be recommended to make sure there is no fluid in the lungs (congestive heart failure) and to see the overall size of the heart.

Another test that may be of use is the NT-pro-BNP (brain natriuretic peptide) blood test. This blood test looks at an enzyme that is released by heart muscle under stress from stretching. An elevated level suggests heart disease. This test can be useful to determine whether an incidental murmur is anything to worry about.

How is it treated?

The treatment will depend on the cause of the murmur. If the murmur is low-grade and the dog has no symptoms, your veterinarian may just recommend monitoring. Some dogs can live their entire lives with a mild heart murmur that never causes any problems. However, if the murmur is louder or the dog has symptoms of heart problems (coughing, weakness, difficulty breathing) your veterinarian may recommend treatment. Medications for congestive heart failure may be necessary. In the case of congenital birth defects, surgery or balloon valvuloplasty (to stretch a narrowed valve) may be recommended.

What is the prognosis?

Like treatment, the prognosis for heart murmurs in dogs depends on the exact cause. A mildly leaky mitral valve in an older small-breed dog may never progress to the point of causing symptoms, while a severely malformed tricuspid valve in a young Labrador could lead to death within a year.

Anytime a heart murmur is diagnosed, it’s best to pursue an echocardiogram to clarify the exact nature of the murmur. After the echo, you and your veterinarian will be able to discuss your dog’s prognosis and any treatment needed.

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