A heart murmur is the sound made by the vibration of turbulent blood flow as it flows through the heart incorrectly. This can be due to abnormal blood flow in the heart or through the major vessels leaving the heart.
How are heart murmurs found?
A heart murmur is found by listening to your pet’s heart with a stethoscope during an examination.
In a normal heart there are two distinct sounds and a pause. “Lub-dub….Lub-dub…” However, if there is a murmur there is an added abnormal “swishing” sound.
Hearing a heart murmur is not a diagnosis, it is a symptom of an underlying problem with the heart’s function or structure. If your pet has a heart murmur, your veterinarian may recommend further tests to understand what is causing the sound.
What causes a heart murmur?
A heart murmur is simply “turbulent flow.” Similar to a river the blood flow in the heart is supposed to go in one direction. When the water flows quietly down the river this is called laminar flow. Rapids are turbulent flow due to a narrow area or rocks obstructing the flow.
In the heart, this turbulence can be caused by:
- Narrowing in the blood vessels
- Back flow from a leaky valve
- A connection between the high pressure on the left side of the heart and the low pressure on the right
When a river (or blood vessel) narrows or becomes rapids (turbulent flow) the velocity of the flow increases and produces a louder sound your veterinarian can hear.
In the figure above, 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.
- The right ventricle pumps the blood through the pulmonic valve.
- The pulmonic valves sends the blood into the lungs to drop off CO2 and 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.
- Then, the left ventricle pumps the blood through the aortic valve out to the rest of the body.
The purpose of each of the valves (tricuspid, pulmonic, mitral, and aortic) is to keep the blood flowing forward, not backward, through the circuit described above. If a valve is not working correctly, blood can leak back through in the wrong direction. This disturbs the blood flow causing turbulence.
In other cases the turbulent blood flow is due to a “hole in the heart” connecting two chambers narrowing the outflow from the chamber at or near the valve. A third reason is narrowing of a chamber of vessel known as stenosis.
Are All Heart Murmurs Bad?
Some heart murmurs are considered benign, simply meaning there is no structural abnormality that causes the murmur and the murmur causes no significant abnormality to the flow of blood.These are often found in puppies and kittens and they grow out of them. They are uncommon in adult dogs and cats.
These murmurs are usually softer (grade 1-2) and disappear by 12 to 15 weeks of age. Benign murmurs can also be caused by anemia or excitement simply because the heart is pumping harder. These types of murmurs are different than congenital murmurs or acquired murmurs.
Congenital murmurs are ones your pet had from birth due to a heart defect. Acquired murmurs are murmurs that a pet gains during their life, and are more often associated with developing heart or valve disease. Because your veterinarian can not tell from listening if a murmur is benign or not, it is a good idea to pursue the cause with further testing.
Heart murmurs are graded based on their loudness. This is a way for veterinarians to describe the severity of the murmur to each other, or compare it to past visits. Murmurs are graded on a level of 1 to 6:
Grade 1: Difficult to hear and only present over a very limited area.
Grade 2: Audible over a wider area.
Grade 3: Increased volume over a wide area.
Grade 4: Very loud over the entire chest wall.
Grade 5: This grade is palpable, it can actually be felt over a small area.
Grade 6: The most significant murmur is palpable over much of the chest wall.
Murmur grades are not always indicative of the severity of the problem causing it. However, if the murmur grade continues to increase it is a sign that the problem is getting worse not better.
How are murmurs treated?
Keep in mind that the murmur is a symptom and not a disease, meaning that the murmur is not treated, the underlying cause is treated. This is why our veterinarians work to find the cause of the murmur through further testing.
Only in determining the cause can the heart be stabilized and the pet can continue to have a normal life. In many cases heart disease can be managed with the use of drugs, but in some situations surgery is required.