What Causes Older Dogs to Have Heart Murmurs?

Reviewed and updated for accuracy on May 23, 2019, by Dr. Jennifer Coates, DVM

Has your older, small breed dog been diagnosed with a new heart murmur? If so, myxomatous mitral valve degeneration (MMVD) is likely to blame.

As the American College of Veterinary Internal Medicine says, MMVD “is the most common acquired type of heart disease and [cause of] new murmurs in older dogs.”

The condition is sometimes also called endocardiosis, or degenerative mitral valve disease.

To understand the effect that this condition has on dogs and how it is best treated, you’ll need a basic understanding of heart anatomy and function.

Heart Valves and Heart Murmurs in Dogs

The mitral valve is one of four valves in the heart that keep blood flowing in the right direction. It is located between the heart’s left atrium and left ventricle.

The “lub-dub” sound that we associate with a healthy heart is the sound of heart valves closing; it should be all that a veterinarian hears when listening to a dog’s heart with a stethoscope.

Small breed dogs have a genetic tendency to develop pathological changes to their mitral valves. This is MMVD.

We don’t know exactly why or how it happens, but the valve leaflets that are normally thin become irregularly thickened, with bumps developing on the edges in many cases. These changes prevent the leaflets from closing as they should.

The valve begins to leak, which causes blood flow around it to become turbulent. The sound that this makes is called a heart murmur.

In the case of MMVD, the murmur occurs between the normal “lub” and “dub” heart sounds. Murmurs can be heard most clearly at a particular point on the left side of the dog’s chest.

Diagnosing the Cause of a Dog’s Heart Murmur

This condition is so common that when veterinarians hear a characteristic murmur in an older, small breed dog, it’s not unreasonable to assume that it’s caused by MMVD unless proven otherwise.

The diagnosis can be confirmed when an X-ray reveals an enlarged left atrium and no other potential causes for the murmur, but an echocardiogram (ultrasound of the heart) is sometimes necessary to reach a definitive diagnosis.

Coughing is usually the first symptom of MMVD in dogs. The left atrium enlarges as a result of being overfilled with blood that is “backwashing” out of the left ventricle through the leaky valve. The abnormally big left atrium presses on the dog’s airways, leading to compression, irritation and coughing.

MMVD is a progressive disease. The mitral valve becomes increasingly distorted and unable to perform its job, which causes a worsening cough and sometimes a progression to congestive heart failure.

Treating Myxomatous Mitral Valve Degeneration in Dogs

Dogs who have MMVD without congestive heart failure (CHF) can simply be monitored for a worsening of their condition. If necessary, a veterinarian will prescribe a cough suppressant. Studies haven’t shown a clear benefit to starting any other form of therapy before CHF is present.

Of course, you want to catch CHF as soon as possible, so schedule checkups with your veterinarian at least twice yearly, and make an appointment ASAP if your dog’s coughing worsens.

If CHF does develop, standard treatment (enalapril, furosemide and pimobendan, for example) for that condition should be started immediately.

Some dogs with MMVD rapidly progress to CHF; others never do.

A recent study may help veterinarians predict which dogs are at the greatest risk for CHF. These findings may help veterinarians determine which patients need the closest monitoring so we can improve the care of dogs with myxomatous mitral valve disease.

Featured Image: iStock.com/Wavetop