A heart murmur occurs when there is a turbulence to the normal flow of blood through the chambers of the heart. Sometimes it is so subtle as to be barely detectable with a stethoscope, while other times it is so loud that it can actually be felt by touching the chest wall. Although some murmurs are innocent, especially in young dogs, most indicate some problem.
The two main causes of a murmur are severe anemia and incomplete closing of the heart valves. The anemia related murmur is not as common as the heart valve problem.
The valve related murmur is found most often in young animals with a congenital heart problem, or in older pets with incomplete valve closure due to thickening of the heart valves. Sometimes the valve have a problem because they have a bacterial infection. One common source of this bacteria infection is dental disease.
Click on the link below if you would like to hear a moderately loud murmur. A normal heart with normal valves opening and closing has a lub-dub sound. The murmur is a whooshing sound, similar to a washing machine: