A functional heart murmur is an extra sound heard through a stethoscope that occurs in a structurally normal heart. It’s caused by blood flowing faster or more forcefully than usual, creating mild turbulence that a doctor can hear. Unlike murmurs tied to heart defects or valve problems, a functional murmur doesn’t signal disease and typically requires no treatment. You’ll also hear it called an “innocent murmur” or “flow murmur,” and it’s remarkably common: up to 72% of children will have one at some point during childhood or adolescence.
Why a Normal Heart Makes Extra Sounds
Blood flowing through the heart and its valves is normally quiet enough that a stethoscope picks up only two distinct sounds: the familiar “lub-dub” of valves closing. But when the volume or speed of blood increases, it can become turbulent, much like water rushing through a garden hose when you turn up the faucet. That turbulence vibrates the surrounding tissue and produces an audible whooshing or humming sound between the normal heartbeats.
The key distinction is that nothing is wrong with the heart itself. The valves open and close properly, the chambers are the right size, and the walls have no holes. The sound is purely a byproduct of flow dynamics, not a structural problem. This is why the term “innocent” is used: the murmur is harmless and has no effect on how the heart functions.
Common Triggers
Anything that makes the heart pump harder or increases blood volume can produce or intensify a functional murmur. The most common triggers include:
- Fever: raises heart rate and cardiac output
- Anemia: the heart compensates for fewer oxygen-carrying red blood cells by pumping faster
- Overactive thyroid (hyperthyroidism): speeds up metabolism and heart rate
- Pregnancy: blood volume increases substantially, and murmurs often appear between 12 and 20 weeks of gestation
- Exercise or physical activity: temporarily boosts blood flow
- Growth spurts: rapid development during adolescence can change flow patterns
Once the trigger resolves, the murmur often fades or disappears entirely. Pregnancy-related flow murmurs, for instance, typically vanish about a week after delivery.
Functional Murmurs in Children
If your child’s pediatrician mentions a heart murmur at a routine checkup, the odds are strongly in favor of it being innocent. The majority of childhood murmurs fall into this category, and most resolve on their own as the child grows. Some persist into adulthood without ever causing problems.
Children’s hearts are smaller and the chest wall is thinner, which makes flow sounds easier to hear with a stethoscope. A growth spurt, a mild illness with fever, or even excitement during an exam can be enough to make a functional murmur temporarily louder. None of these scenarios require treatment.
Athletes and Increased Stroke Volume
Endurance athletes frequently have functional murmurs, and the reason is straightforward. Prolonged exercise training enlarges the heart’s chambers, allowing each beat to push out a larger volume of blood. This increased stroke volume, both at rest and during exercise, creates enough flow turbulence to produce an audible murmur. It’s a sign of cardiovascular adaptation, not disease, and is sometimes grouped under the broader concept of “athlete’s heart.”
What a Functional Murmur Sounds Like
Doctors distinguish functional murmurs from worrisome ones based on a set of acoustic characteristics sometimes summarized as the “Seven S’s.” An innocent murmur is systolic (occurring when the heart squeezes), soft (grade 3 or less on a 6-point scale), small (doesn’t radiate to the neck or back), short (doesn’t last the entire squeezing phase), single (no extra clicks or gallops alongside it), sweet or musical in quality (not harsh or blowing), and sensitive to position changes.
That last feature is particularly useful. A functional murmur often gets louder when someone lies flat and softer when they sit or stand up. This happens because lying down increases the amount of blood returning to the heart, which increases flow turbulence. Pathologic murmurs caused by structural defects tend to behave differently. They may blur or replace normal heart sounds, last throughout the entire contraction, sound harsh or blowing, or radiate strongly to other parts of the chest.
The Grading Scale
Murmurs are rated on the Levine scale from 1 to 6 based on loudness. Grade 1 is barely audible, requiring concentration in a quiet room. Grade 2 is soft but clearly heard. Grade 3 is easily audible but still can’t be felt as a vibration on the chest wall. Grades 4 through 6 are progressively louder, with grade 6 audible even without the stethoscope touching the chest.
Functional murmurs are grade 3 or less. If a murmur reaches grade 4 or higher, or if a vibration (called a “thrill”) can be felt on the chest, it’s considered more likely to reflect a structural issue and warrants further evaluation.
When Testing Is Needed
Most functional murmurs don’t require any testing beyond the physical exam. According to appropriate-use criteria from the American College of Cardiology, an echocardiogram (an ultrasound of the heart) is rarely appropriate when a murmur sounds innocent and the person has no symptoms, no abnormal findings on exam, and no family history of heart disease.
An echocardiogram becomes appropriate when a murmur has characteristics that don’t fit the innocent pattern: harsh quality, high intensity, unusual timing, or association with symptoms like chest pain, fainting, shortness of breath, or poor exercise tolerance. It’s also warranted when the doctor simply can’t tell from listening alone whether the murmur is innocent or pathologic. The test itself is painless and takes about 30 to 45 minutes, using sound waves to create a real-time image of the heart’s structure and blood flow.
Living With a Functional Murmur
A functional murmur requires no medication, no activity restrictions, and no ongoing monitoring beyond routine checkups. You or your child can exercise, play sports, and live without limitations. The murmur may come and go depending on heart rate, hydration, and other day-to-day variables, but it has no impact on heart health or life expectancy.
If you’ve been told you have an innocent murmur and later see a new doctor, it’s worth mentioning it upfront. This saves the new provider from hearing the sound, not knowing its history, and ordering unnecessary tests. Some people carry a note in their medical record confirming the murmur was evaluated and deemed functional, which streamlines future visits.