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)

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 can see that a dog or 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 (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 through, 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 (where in the heart beat it occurs), the location of the murmur, 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 ausculted 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 veterinarians fail to hear a congenital murmur in a puppy and the defect is only 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.

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