My Pet Has a Heart Murmur – What Does This Mean?

A heart murmur is one of several types of abnormal sounds your veterinarian can hear when listening to your pet’s heart with a stethoscope. Normally, two distinct sounds are heard when listening to the heart of a normal dog or cat. These are often described as “lub” and “dub." When listening with a stethoscope one hears: Lub-dub...Lub-dub...Lub-dub.

A murmur is an abnormal extra sound, which can sometimes drown out the normal sounds. Murmurs most commonly occur between the “lub” and the “dub” and have a “shooshing” or “whooshing” quality. (See below for audio files.)

Dog Heart Anatomy Graphic

Copyright Veterinary Information Network

What Causes a Heart Murmur?

The short answer to this question is turbulent blood flow. Like the water in a calm river or stream, normally blood flows through the heart in a quiet, smooth manner – what is called laminar flow. However, narrowing, rocks, or other obstacles create rapids in rivers or streams that disrupt this smooth flow, creating turbulence. The same happens with blood flow through the heart. In a river, the turbulent rapids emit sounds much louder and less tranquil than the calmer sections of river. In the heart, we hear this turbulence as a murmur.

Lots of things can cause turbulent flow. To understand what can, we need a brief lesson in heart anatomy and function.

In the illustration, we see that a dog and cat’s heart has four chambers – two atria and two ventricles (one of each on each side). Blood initially enters the heart in the right atrium. The blood then passes through the tricuspid valve into the right ventricle which pumps the blood through the pulmonic valve into the lungs to pick up oxygen and release carbon dioxide (among other things). The oxygenated blood then enters the left atrium. Blood in the left atrium passes through the mitral valve to reach the left ventricle, which then pumps the blood through the aortic valve out to the rest of the body.

The purpose of each of the valves (tricuspid, pulmonic, mitral, aortic) is to keep the blood flowing forward, not backward, through the circuit described above (RA->RV-> lung > LA>LV>body). If a valve malfunctions (e.g., it doesn’t open or close properly), it can disturb blood flowing through it enough to create turbulence and the result is that your veterinarian will hear a murmur. The most common murmurs in dogs are associated with leaky mitral valves.

In other cases, the turbulence develops because there is a hole in the heart between two chambers or two arteries that are not normally connected; these holes are mostly ventricular septal defects.

Another cause is a narrowing (stenosis) within a chamber or vessel through which the blood has to squeeze, like water through a pinched hose.

In other cases, structures within the heart can vibrate as blood flows past them, much like the strings of a guitar. These types of murmurs are common in children.

Finally, turbulence can be heard when the blood is too thin (anemia) or even when a patient is very excited, causing the heart to pump faster and harder than normal.

Are Different Types of Murmurs Associated With Specific Diseases or Conditions?

Sometimes. Murmurs are caused by different structures inside and outside the heart. Your veterinarian can use various clues about the murmur to try to determine what the likely origin is. Specifically, veterinarians will use the timing of the murmur (when in relation to the "lub" and "dub' of each heartbeat the murmur is heard), the location on the patient’s chest where the murmur can be heard, and, sometimes, the “quality” of the murmur. For example, murmurs on the lower left of an older dog’s chest are most likely associated with degenerative mitral valve disease. Murmurs associated with patent ductus arteriosus are continuous when auscultated in the left armpit. Murmurs of aortic stenosis can sometimes radiate up the carotid arteries and be heard over the neck. Murmurs associated with ventricular septal defects tend to be loudest on the right side (as are murmurs associated with leaky tricuspid valves).

However, in many cases, these clues are insufficient for the veterinarian to make a diagnosis. This is especially the case in cats, where most murmurs are heard near the sternum.

What is a Benign or “Innocent” Murmur?

Some heart murmurs are called benign (or innocent or physiological), meaning there is no apparent heart disease that explains the murmur. These murmurs are often seen in puppies and can occur in cats of any age. They are uncommon in adult dogs. Benign murmurs are usually soft (rather than loud) and can be intermittent. Benign puppy murmurs will generally disappear by 12 to 15 weeks of age. Murmurs associated with anemia or excitement are also considered benign murmurs.

What is a Congenital Murmur vs. an Acquired Murmur?

A congenital murmur is a murmur in a pet that is present from birth or near it. Congenital murmurs are associated with heart defects that the pet was born with. However, sometimes puppies aren't examined by a veterinarian, or a congenital murmur is overlooked in a puppy and the murmur and defect are first detected later in life.

An acquired murmur is a murmur that a pet acquires during their life. These can be benign, but more often (especially in dogs) are associated with developing heart disease.

My Pet’s Murmur Has a Grade. What Does This Mean?

Murmur grading is simply your veterinarian’s way of describing the loudness of a murmur. Most veterinarians grade murmurs on a scale from 1 to 6. The lower the grade, the quieter the murmur. However, it is often easiest to simply describe them as “soft,” “moderately,” “loud” or “palpable” (where you can feel the murmur like a vibration through the chest). There are other terms that a veterinarian will use to describe the character of a murmur – this helps communicate to other veterinarians the characteristics of the murmur as certain types of murmurs are more commonly associated with specific heart or valve diseases.

The grade or loudness of the murmur is only sometimes related to the severity of the heart abnormality causing it.

Bear in mind that grading is subjective because it is based on how it sounds to the listener. Also, it’s hard to tell if an animal has a heart murmur if the pet is excited or anxious because rapid breathing sounds can mimic a murmur.

The following audio files are examples of murmurs.  They are best listened to through headphones.

Normal heart with no murmur:

Soft murmur:

Moderate murmur:

Loud murmur:

(Audio files courtesy of Dr. Clarence Kvart)

What Should I Do if My Pet Has a Murmur?

Dogs
In many cases, a veterinarian will be confident diagnosing the origin of a murmur in a dog based on the age, breed, medical history, and physical exam findings, including how the murmur sounds and where it is heard loudest on the chest. In some cases, based on these findings and the desires of the patient’s owner, no additional testing will be deemed necessary at this time. However, to be certain, it is often best to work with your veterinarian to confirm the origin of the murmur as well as the severity of the condition causing the murmur. This will give you the best idea of what to expect in the future -- the prognosis for your pet. This might include a referral to a veterinary cardiologist or another imaging specialist.

In cases where a pet may be used for breeding, a murmur may indicate the presence of a hereditary defect that could be passed on to progeny. These pets should be thoroughly worked up to rule out congenital heart disease or neutered.

Cats
The cause of a cat’s murmur cannot usually be determined by listening alone. In many cats, benign murmurs can sound exactly like murmurs in a cat with serious heart disease, and only an echocardiogram can differentiate the two. 

In both dogs and cats, your veterinarian may elect to perform chest radiographs (x-rays), an echocardiogram (ultrasound of the heart), or other imaging studies, or to refer your pet to a specialist for these procedures. The tests that are performed depend on the individual case.

How is a Murmur Treated?

The murmur itself is not treated. Your veterinarian might advise treating the underlying cause of the murmur, depending on the cause, severity, and other circumstances (age, the well-being of the pet, cost of treatment, etc.). Your veterinarian is best suited to discuss specific treatment options with you.