What causes a heart murmur?

Suzanne Berman, M.D.

What is a heart murmur?

When we listen to your child's heart with a stethoscope, we usually hear just the normal two sounds of the heartbeat.

lub dub. lub dub. lub dub. lub dub. lub dub.

Doctors call the first sound (or "lub") S1 and second sound (or "dub") S2. These two sounds are caused by healthy heart valves slapping shut.

But once in a while, we hear an extra sound, called a murmur. It could sound like this:

lub [click] dub. lub [click] dub. lub [click] dub. lub [click] dub. lub [click] dub.

Or like this:

lub dubshhh. lub dubshhh. lub dubshhh lub dubshhh lub dubshhh.

Or even like this:

lhonk! dub. lhonk! dub. lhonk! dub. lhonk! dub.

What causes the extra sound?

In both innocent and concerning heart murmurs, the murmur is caused by turbulence in the flow of blood. When blood flows smoothly through a straight vessel, its motion is quiet. When blood hits an obstacle, it starts making noise moving past that obstacle. Think of a water faucet. When you turn it on, water flows out quietly. But if you put your thumb under the spigot, the water starts spraying out around your hand, making lots of noise.

Could a murmur be something bad?

Nearly all children have a heart murmur at some point in their lives, but less than 1% of children have a heart defect. This means that 99% of murmurs in kids are "innocent" -- that is, harmless and not caused by a heart defect. See our page on innocent heart murmurs for more details. Fortunately, most murmurs which indicate a serious problem are discovered within the first few days after birth.

Why are some murmurs bad and others aren't?

It depends where the turbulence is coming from. Turbulence occuring inside the heart is what causes most "bad" murmurs; turbulence occuring in the large vessels outside the heart is usually harmless. Note that even if the turbulence comes from a place outside the heart, we still call it a "heart murmur."

Here's a diagram of how blood flows through a healthy heart.

Blue blood (without oxygen in it) returns to the heart from the body.
The heart pushes it out to large blood vessels which take the blood to the lungs.
The lungs put oxygen back into the blood.
The red blood (with oxygen) returns to the heart.
The blood moves through the other side of the heart and out of the heart through the aorta.
The aorta branches and divides, supplying blood to the body where the tissues can use the oxygen.

Now let's look at two examples of how turbulence causes heart murmurs. The double arrows represent turbulence.

Here's an example of a non-reassuring murmur caused by a hole between two chambers of the heart. Instead of red and blue blood staying separate, a hole lets blood mix through the middle. Blood bounces around back and forth through the hole. This creates a lot of turbulence and makes a harsh "grinding" sound. Some small holes in the heart can close up on their own, and nothing needs to be done. Large holes in the heart can be bad because, with time, the blood flowing improperly in this way can cause the heart to become misshapen.

Here's an example of an innocent murmur caused by a curve in a blood vessel. When the blood exits the heart, it's going at high speed. As it hits a sharp curve in a narrow blood vessel, it makes a "whooshing" sound as it passes. It's noisy, all right, but harmless, because the blood has no problem getting to where it needs to go. And, of course, there's nothing wrong with the heart itself.