A flow murmur is an extra heart sound caused by blood moving faster or more forcefully than usual through a structurally normal heart. It is not a sign of heart disease. Flow murmurs are extremely common, especially in children: up to 72% of kids will have one at some point during childhood or adolescence. Most disappear on their own over time.
If a doctor told you or your child has a flow murmur, you’re hearing the same reassurance that millions of families get every year. Of all new murmurs referred to a pediatric cardiologist, less than 1% turn out to be caused by a congenital heart defect.
How Blood Flow Creates the Sound
Your heart pumps blood through valves and chambers with every beat. Normally, this flow is smooth and silent enough that a stethoscope picks up only the standard “lub-dub” of the valves closing. But when blood speeds up beyond a certain velocity, it shifts from smooth (laminar) flow to turbulent flow. That turbulence vibrates the surrounding tissue just enough to create an audible whooshing or humming sound.
The key point is that nothing is wrong with the heart itself. The valves are intact, the chambers are the right size, and the walls are healthy. The sound comes purely from how fast the blood is moving. Think of it like water flowing quietly through a garden hose until you partially cover the nozzle and hear a rush. The hose isn’t broken; the water is just moving faster.
Common Causes of Increased Flow
Anything that raises cardiac output (the volume of blood your heart pumps per minute) can trigger a flow murmur. In children, the most common reason is simply being a child. Their hearts sit closer to the chest wall, and normal growth spurts can temporarily increase blood flow velocity. The sound typically appears between ages 5 and 14 and is most common in adolescents.
In adults, several conditions can produce a flow murmur:
- Pregnancy: Blood volume increases significantly, pushing more blood through the heart with each beat.
- Anemia: When your red blood cell count drops, your blood becomes less viscous (thinner), which makes turbulence more likely at normal flow rates.
- Fever or infection: Your heart rate and cardiac output rise to meet increased metabolic demands.
- Hyperthyroidism: An overactive thyroid can double or even triple cardiac output. The thyroid hormone relaxes blood vessels throughout the body, dropping resistance and forcing the heart to compensate with a much larger stroke volume and faster heart rate.
- Athletic conditioning: Endurance athletes develop enlarged hearts with thicker walls and greater stroke volume. This is a normal adaptation, but the increased volume of blood ejected with each beat can produce an audible flow murmur at the left side of the breastbone.
In each of these situations, the murmur resolves once the underlying condition is treated or the temporary state passes. A pregnant woman’s flow murmur disappears after delivery. An anemic patient’s murmur fades once iron levels recover.
What a Flow Murmur Sounds Like
Flow murmurs have a distinct sound profile that helps doctors identify them. They occur during systole, the phase when the heart contracts and ejects blood. This makes them “mid-systolic,” meaning the sound appears in the middle of the heartbeat rather than at the very beginning or end.
The sound itself is soft, often described as a gentle whoosh or musical hum. On the Levine grading scale, which rates murmur intensity from 1 (barely audible) to 6 (audible without a stethoscope even touching the chest), flow murmurs typically fall at grade 1 or 2. They don’t produce a “thrill,” which is a vibration you can feel by placing your hand on the chest. Any murmur at grade 4 or above, where a thrill is present, warrants closer investigation.
Flow murmurs also change with body position. They often get louder when you’re lying down (because more blood returns to the heart) and quieter when you stand up. This positional change is one of the clues doctors use to confirm the murmur is innocent.
How Doctors Tell It Apart From a Problem
The distinction between a harmless flow murmur and one that signals structural heart disease comes down to a handful of characteristics. A flow murmur is soft, mid-systolic, and doesn’t radiate widely across the chest. It changes with position and has no accompanying symptoms. A problematic murmur tends to be louder, harsher in quality, and may sound like a churning or cooing noise. It can be heard throughout systole or even during diastole (when the heart is filling), and it may radiate to the neck, back, or armpit.
Doctors also look for symptoms that would never accompany a simple flow murmur:
- Blue or gray lips or fingernails
- Shortness of breath during normal activity
- Chest pain
- Fainting
- Unexplained fatigue that worsens over time
- Swelling in the legs, liver, or neck veins
- Poor growth in infants
If none of these are present and the murmur has the typical innocent sound profile, many doctors will simply monitor it over a few visits. For children past infancy, serial exams are often all that’s needed when there are no concerning findings. If there’s any uncertainty, an echocardiogram (an ultrasound of the heart) can confirm normal structure and function within minutes. It’s painless, involves no radiation, and gives a definitive answer.
What to Expect Over Time
Flow murmurs in children frequently disappear as the child grows. The chest wall thickens, the heart’s position shifts slightly, and normal developmental changes reduce the turbulence that created the sound. Some murmurs persist into adulthood but remain completely harmless.
A flow murmur does not increase your risk of heart disease, does not require medication, and does not limit physical activity. Children with flow murmurs can play sports, run, and exercise without restriction. Adults with flow murmurs tied to pregnancy, anemia, or thyroid issues can expect the sound to resolve once the underlying condition is addressed. Athletes with flow murmurs from cardiac remodeling carry no additional risk either; the murmur is simply a byproduct of a well-conditioned heart.
The one thing worth remembering is that a murmur’s character can change. If a previously soft murmur becomes noticeably louder, or if new symptoms like breathlessness or chest discomfort appear, that’s worth a follow-up visit. The overwhelmingly likely outcome, though, is that a flow murmur stays exactly what it is: a harmless sound from a healthy heart working hard.