Pet Vet: How a dog’s heart murmur is detected and evaluated

October 19, 2015

Today’s article centers around cardiology and dogs with heart murmurs. We will use Waldo, a 6 year old cockapoo from Green Bay, Wis., as our case study, as he has recently been found to have a heart murmur. His caretaker Kim is understandably concerned. She was told by her veterinarian that Waldo’s heart murmur was apparently causing him no problems at the moment.

An important point: a murmur, detected by listening to the heart of a particular patient with a stethoscope, needs to be characterized. This means the patient needs to have his heart evaluated to try to determine to what extent the murmur might be causing problems. A murmur is simply a sound created within the heart as a result of abnormal blood flow or turbulence. It must be determined what is causing the turbulence — and thus the audible murmur — and whether it is a problem or potential problem for the patient. Some heart murmurs are inconsequential to the patient while others may indicate potentially major problems.

We are going to have to the assumption that Waldo’s heart murmur is a mitral murmur as opposed to another type, as this information was not shared in Kim’s letter. Mitral murmurs are the most common type of heart murmur in dogs. To understand the term "heart murmur," we need to understand a bit about the anatomy of the heart. The heart, in the simplest of descriptions, is a one-way pump consisting of four chambers, one pair, the left and right atria, on top of another pair, the left and right ventricles, separated from one another by one-way "flapper" valves.

Blood flows through it in one direction as the chambers contract in rhythm pushing the valve flaps or leaflets, as they are properly termed, open to allow the blood to flow from each atrium into each ventricle. Then the ventricles contract, closing the valves back to each atrium and opening the valves to allow blood to flow from each ventricle. The right ventricle pumps blood to and through the lungs and back to the heart into the left atrium. From the left atrium the blood flows through the mitral valve into the left ventricle, then from there through the aortic valve out to the body returning into the right atrium then through the tricuspid valve into the right ventricle to start the trip all over again. Have you got all that?

If Waldo has a mitral murmur that means there is turbulence in the blood flow through his mitral heart valve. That is the valve between his right atrium and his right ventricle. We grade heart murmurs on a scale from one to six, six being the loudest. We were not given a number designation for Waldo’s murmur. Generally speaking, the louder the mitral murmur, the greater the potential for problems.

I recommend that Waldo have his heart evaluated. This should involve a few different studies, including a chest radiograph, an electrocardiogram (ECG), blood pressure determination and an echocardiogram (ultrasound of the heart). Using these studies we can try to find out what is causing the murmur and then decide whether disease currently is associated with it versus the potential for future disease.

A mitral murmur usually results from a mitral valve that does not seal properly when it shuts, allowing blood to flow back into the left atrium instead of entirely forward out of the left ventricle to the body. This backflow causes the audible turbulence heard as a murmur and, if significant enough in volume, can lead to congestive heart failure. Knowing that the potential for heart failure is there, evaluation of a heart murmur is always warranted.



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