Yes, an innocent heart murmur can change, and that changeability is actually one of its defining features. Innocent murmurs routinely get louder, softer, or disappear entirely depending on body position, activity level, and overall health. As the American Heart Association has noted, because these murmurs are caused by normal blood flow rather than a structural problem, whether the murmur disappears or changes is of no consequence to heart health.
Why Innocent Murmurs Change So Easily
An innocent murmur is simply the sound of blood moving through a normal heart. Anything that speeds up blood flow or changes how much blood the heart is pumping will change the sound. This is fundamentally different from a pathological murmur caused by a damaged valve, which tends to stay consistent regardless of what the body is doing.
That responsiveness to the body’s conditions is actually one of the ways doctors distinguish innocent murmurs from worrisome ones. Pathological murmurs, especially those from valve problems, do not change in intensity with physical maneuvers. Innocent murmurs often do change significantly with position.
What Makes the Sound Get Louder or Softer
Several everyday situations temporarily increase blood flow through the heart, which can make an innocent murmur louder or cause one to appear that wasn’t heard before:
- Fever: Your heart pumps faster to help regulate body temperature, increasing flow.
- Exercise: Higher cardiac output means more turbulence and a more audible murmur.
- Anemia: When red blood cell counts are low, the heart compensates by pumping harder.
- Pregnancy: Blood volume increases significantly, and the heart works harder to supply both parent and baby.
- Overactive thyroid: Hyperthyroidism raises the metabolic rate, pushing the heart to pump more blood per minute.
- Growth spurts in children: Rapid physical changes shift chest dimensions and blood flow patterns.
Once the temporary condition resolves (the fever breaks, the anemia is treated, the pregnancy ends), the murmur typically quiets down or vanishes.
How Body Position Affects the Sound
One of the most immediate ways an innocent murmur changes is when you shift between lying down and sitting or standing up. Different types of innocent murmurs respond to position in specific ways.
The Still’s murmur, the most common type in children, is a soft, musical, vibratory sound heard over the left side of the chest. It’s loudest when a child is lying flat on their back, because more blood returns to the heart in that position. Sitting or standing up reduces venous return, and the murmur gets quieter.
A venous hum, another common innocent finding, works in the opposite direction. It’s a soft, whirling sound heard near the collarbone area when a child is sitting upright. It disappears completely when they lie down or turn their head and extend their neck to one side. This happens because the position change alters blood flow through the large veins in the neck.
A mammary souffle, sometimes heard in pregnant or breastfeeding women, is a soft blowing sound over the breast area caused by increased blood supply. It gets softer when the person stands up or when the stethoscope is pressed more firmly against the skin.
Changes Over Months and Years
Beyond day-to-day fluctuations, innocent murmurs also change over longer timescales. In children, the murmur may come and go over months or years as the chest grows and the relationship between the heart and chest wall shifts. A murmur that was clearly heard at a checkup six months ago might be absent at the next visit, then reappear later. This is normal and expected.
Most childhood innocent murmurs fade as kids get older. As the chest wall thickens and the heart’s proportions relative to the body stabilize, there’s less turbulence for the stethoscope to pick up. Many innocent murmurs that are prominent in early childhood are no longer detectable by the teenage years, though some persist into adulthood without ever causing problems.
When a Change Warrants Attention
An innocent murmur doesn’t “become” a pathological one. The heart is structurally normal, and that doesn’t change. However, if a murmur that was previously identified as innocent starts sounding different to your doctor (harsher, louder in a new location, or accompanied by new symptoms), it’s worth re-evaluating. The concern in that scenario isn’t that the innocent murmur worsened. It’s that a second, separate issue may have developed, or the original assessment may need a closer look.
Characteristics that distinguish innocent murmurs from concerning ones include their quality and behavior. Innocent murmurs are soft, blowing or musical, and low-pitched. They occur during the pumping phase of the heartbeat (systole), not during the filling phase (diastole). A murmur heard during diastole is not considered innocent. Similarly, a murmur that is harsh, very loud, or accompanied by a vibration you can feel through the chest wall (called a thrill) goes beyond what’s typical for a flow murmur.
Symptoms alongside the murmur also matter. An innocent murmur by itself doesn’t cause chest pain, shortness of breath, fainting, or poor growth in children. If those symptoms appear, the murmur needs re-evaluation regardless of what it was called before. In most cases, a referral to a cardiologist and an echocardiogram (an ultrasound of the heart) can confirm whether the heart’s structure is truly normal and put the question to rest.
The Reassuring Bottom Line
The fact that an innocent murmur changes is not a warning sign. It’s the opposite. Flow-related murmurs are supposed to respond to what the body is doing at any given moment. A murmur that shifts with position, gets louder during a fever, or disappears for a while and comes back is behaving exactly the way an innocent murmur should. The heart underneath is normal, and no treatment or activity restrictions are needed.