Heart murmur

Heart murmur is an abnormal sound that your veterinarian may hear when listening to your pet's heart with a stethoscope. Normally there are two distinct sounds audible, which are often described as "boo" and "put". So when you listen to the heart with a stethoscope you can hear something like this: Bu-tup, bu-tup, bu-tup.
A murmur is an additional sound that can sometimes override normal heart sounds. It occurs most frequently between "boo" and "tup" and can be described as "shhh" or "pfu".
A heart murmur found during a routine clinical examination by your veterinarian is usually the first symptom of heart disease. Heart murmur is only a symptom of a condition and not a final diagnosis. It is a reason for a more comprehensive investigation to reveal the cause and find a diagnosis. Knowing the diagnosis and severity of the disease is vital for your veterinary cardiologist to make a prognosis of the cardiac problems affecting your pet.
A heart murmur is not a reason to panic. Many dogs and cats live many years with a murmur and need no treatment. But the only way to be sure is to work with your veterinarian to find the cause and severity of the heart murmur.
Basically, heart murmurs are caused by turbulent blood flow. Blood normally flows through the heart smoothly and without interference, in what we call laminar flow. As with a river, narrowing or obstructions will cause a faster stream and disruption of laminar flow. A turbulent river is of course much louder than a steady, calm river, so turbulence in blood flow can be heard as a heart murmur.
There are many reasons for turbulent flow. To understand this, we need a brief lesson in heart anatomy:

 

In Figure 1. We can see that the heart has four sections - two atria and two ventricles (one atrium and ventricle on the right as well as the left side). Blood enters the heart in the right atrium. It then passes through the tricuspid valve into the right ventricle, which then by its contraction ejects blood through the pulmonary valve to the lungs to be oxygenated. Oxygenated blood is then introduced into the left atrium. From there it enters the left ventricle through the mitral valve. From the left ventricle it is expelled throughout the body through the aortic valve.

The purpose of the valves is to prevent backflow of blood. If there is an insufficiency of a valve, it disrupts the flow of blood to that location, causing the aforementioned turbulent flow. Then your veterinarian may hear the murmur. The most common murmur in dogs is caused by mitral valve insufficiency.

Another cause can be a "hole" in the heart between two chambers, or atria, which are separated by septum.
Additionally, murmurs may be caused by narrowing (stenosis) within the heart.  This forces the blood through at a higher pressure, much like water through a constricted garden hose.
Finally, murmurs can be heard when the blood is too thin (anemia), or if the patient is very excited and the heart beats much more rapidly and stronger than normal.
Some heart murmurs are called benign (or innocent or physiological). This is where no heart disease which would explain the murmur is found. These sounds are often seen in puppies and also in cats of any age, but are uncommon in adult dogs. Benign murmurs are usually soft and can be intermittent. Benign puppy murmurs usually disappear between 12 to 15 weeks of age. Murmurs associated with anemia or excitement are also considered benign.
A congenital murmur is one which is present from birth (or nearly from birth). Congenital murmurs are associated with heart defects at birth. However, some congenital murmurs in puppies and kittens are missed and only detected later in life.
Acquired murmur is a murmur that a pet acquires during their lifetime. They can be benign, but more often (especially in males) are associated with heart disease.

Your veterinarian uses six degrees of grades to describe murmur volume. The lower the grade, the lower the murmur. However, it is often easier to describe them as "weak", "medium" and "strong" murmurs. There are other expressions which veterinarian may use when describing a murmur. These facilitate communication between veterinarians about the type of murmur, as these are often associated with an exact heart disease.

The volume of the murmur is not always in direct proportion to the severity of the illness it causes.

Keep in mind that auscultation is subjective because it is based on how the murmur seems to the listener. It is also difficult to assess a murmur if the dog is too excited, and breath sounds can mimic heart sounds as well. Usually, only highly trained cardiologists can detect first grade murmurs. Fifth and sixth grade can be so strong that they are felt through the chest wall (sounding as if water was sprayed on a piece of cloth).

Dogs

In many cases, the veterinarian is already able to determine the cause of the murmur upon first discovering it and further investigation is not necessary. However, in order to be sure, it is often best to confirm the cause of the murmur and the severity of the condition causing the murmur. This will give you the best idea of what to expect in the future regarding your pet. If your pet is used for breeding, a murmur may indicate the presence of a hereditary disease transmissible to offspring.

Cats

The cause of heart murmurs in cats usually cannot be determined simply by auscultation. Many benign murmurs can sound just like murmurs in serious heart conditions.

With cats as well as dogs, your veterinarian may choose to take an x-ray of the thorax, order cardiac echocardiography, or decide to refer you to a specialist for these tests. It all depends on the individual case.

A heart murmur is not a disease in itself. The cause of the murmur may or may not be treatable, depending on the type and severity of the disease, and on other circumstances such as age, quality of life, treatment rate, etc. Your veterinarian is the best source for evaluation of all these circumstances.

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