Written by: Dr. Chip Coombs

Simply put, a heart murmur is an abnormal sound heard when listening to your pet’s heart with a stethoscope. This is not to be confused with an abnormal heart rate or rhythm. Most murmurs are detected when a dog or cat is very young and going to see their veterinarian for the first time at about 8 weeks of age. However, a murmur can also be detected for the first time in older pets, as the result of a form of heart disease that is causing structural changes to the heart or its valves.

As mentioned, a heart murmur is an abnormal sound that is caused by a change in the normal flow of blood as it courses through the various chambers and valve systems of the major blood vessels connected to the heart. There are 4 sets of valve systems involving both sides of the heart (mitral and tricuspid), as well as the two major arteries (aortic and pulmonary) attached to the heart. The murmur could be caused by any one of these valve systems not closing properly when the heart contracts. When listening with a stethoscope, instead of hearing a crisp “lub-dub” sound, one also hears a “whoosh”. For example, when a valve doesn’t close properly, blood can leak back into a heart chamber from which it just came and this disruption in blood flow causes the murmur (in this case it is referred to as a regurgitation murmur).

Murmurs can also be caused by the existence of a blood vessel, present after a pet is born, that would normally close at birth. It can also be caused by defects in the heart chamber walls. Although not common, if a pet were extremely anaemic, the lack of red blood cells in the blood changes the blood’s consistency and this “thinning” can also produce a murmur. In the case of some breeds of cats and dogs, as they age, they can develop conditions known as congestive heart failure or cardiomyopathy, both of which can change the normal structural shape and size of the heart muscle which can, in turn, change the ability of the valves to seat properly; causing murmurs. Finally, murmurs can also be produced by the narrowing (stenosis) of a major blood vessel leaving the heart which can readily disrupt blood flow.

Murmurs are graded in terms of how intense they are (l-Vl) and by the sounds they produce. This categorization is standardized to help assist in the monitoring and diagnosis of the most likely cause of the murmur, as well as communication between veterinarians about a particular patient.

Having a murmur is not always a bad thing. In fact, many murmurs in puppies and kittens are considered “innocent.” They can be present at birth, but disappear as the puppy matures. There are some murmurs, though, that have a characteristic sound that may concern your veterinarian enough to do further diagnostic tests to evaluate their significance. The most useful diagnostic test would be a colour flow echocardiogram which literally allows one to examine the inside of the heart and to follow the blood flows.

Like people, our pets can have a minor heart murmur and live a completely normal life. However, a murmur does need to be monitored as your pet ages, to ensure it doesn’t worsen in the vintage years leading to congestive heart failure. If your dog or cat is diagnosed with a heart murmur for the first time at an older age, then the concern would be whether or not they are developing congestive heart failure or cardiomyopathy. The treatment and diagnosis for these problems is the same as with people, but catching it early and initiating treatment early is the best means to control it. Ensuring your pet receives an annual physical exam is the best prevention for any potential disease, as well as the monitoring of any existing abnormality.