A heart murmur is an extra whooshing sound heard between heartbeats, caused by turbulent blood flow inside the heart or nearby blood vessels. Most murmurs are harmless. Between 20% and 80% of healthy children have one detected during a routine checkup, and many adults develop them temporarily during pregnancy, exercise, or illness. But some murmurs signal a structural problem with the heart that needs attention.
The causes fall into two broad categories: conditions that speed up or thin the blood flowing through a normal heart, and physical changes to the heart’s valves or walls that disrupt flow even at normal speeds.
How Blood Flow Creates the Sound
Blood normally moves through your heart in smooth, organized streams. A murmur appears when that flow becomes turbulent, meaning the blood swirls chaotically instead of flowing in one direction. The turbulence creates vibrations that travel through your chest wall and become audible through a stethoscope. Research from the American Heart Association confirmed a direct linear relationship between the energy of turbulent blood flow and the loudness of the murmur. The louder the murmur, the more turbulence is present.
Turbulence increases when blood moves faster, when it’s forced through a narrower opening, or when the blood itself becomes thinner (less viscous). Any of these changes raises what physicists call the Reynolds number, a measure of how likely a fluid is to become turbulent. That single principle explains nearly every cause of a heart murmur.
Innocent Murmurs: Temporary and Harmless
Innocent murmurs happen when blood flows faster than usual through a structurally normal heart. Nothing is damaged or malformed. The murmur typically disappears once the underlying condition resolves. Common triggers include:
- Fever. Your heart pumps faster to help regulate body temperature, increasing flow speed.
- Anemia. Fewer red blood cells make the blood thinner and less viscous, which promotes turbulence. Anemia also forces the heart to pump harder to deliver enough oxygen, further increasing flow speed.
- Pregnancy. Blood volume rises significantly during pregnancy, and the heart works harder to supply both parent and baby.
- Hyperthyroidism. An overactive thyroid ramps up your metabolic rate, which can push cardiac output 50% to 300% above normal. The thyroid hormones also increase heart rate, strengthen contractions, and expand blood volume, all of which drive faster blood flow.
- Exercise. Vigorous physical activity temporarily increases heart rate and output.
- Rapid growth in children and teens. During growth spurts, the heart may pump more vigorously relative to the size of the vessels, creating brief turbulence.
In children especially, innocent murmurs are extremely common and almost never a sign of trouble. They’re usually soft, occur only when the heart is pushing blood out (during systole), and don’t come with any other symptoms.
Heart Valve Problems
Your heart has four valves that open and close with each beat to keep blood moving in one direction. When a valve doesn’t open fully or doesn’t close completely, it creates turbulence that produces a murmur. These are the two main types of valve dysfunction:
Stenosis (Narrowed Valves)
A stenotic valve doesn’t open as wide as it should, forcing blood through a smaller opening at higher speed. The most common example is mitral stenosis, where the valve between the left atrium and left ventricle becomes stiff and narrow. The leading cause worldwide is rheumatic heart disease, a complication of untreated strep throat in which the immune response scars the valve tissue over time. Aortic stenosis, where the valve leading out of the heart stiffens, is more common in older adults and often results from years of calcium buildup on the valve leaflets.
Regurgitation (Leaky Valves)
A regurgitant valve doesn’t close tightly, allowing blood to leak backward. This backflow creates its own turbulent whoosh. Aortic regurgitation, for instance, can develop from long-standing high blood pressure that gradually widens the base of the aorta, preventing the valve from sealing. It can also appear suddenly if an infection damages the valve or if the aorta tears (aortic dissection). Mitral regurgitation, where blood leaks back into the left atrium, is one of the most frequently diagnosed valve problems.
Congenital Heart Defects
Some people are born with structural abnormalities that cause murmurs from infancy. The most common is a ventricular septal defect (VSD), a hole in the wall separating the heart’s two lower chambers. Blood flows through this hole from the higher-pressure left side to the lower-pressure right side, producing a distinct whooshing sound. A small VSD may cause a surprisingly loud murmur, while a very large one can sometimes be harder to hear because the pressure difference between the two chambers is smaller.
Atrial septal defects (holes in the wall between the upper chambers) and patent ductus arteriosus (a blood vessel that should close after birth but stays open) are other congenital causes. Many of these defects are detected in newborns or during early childhood checkups. Some close on their own as a child grows, while others eventually need repair.
Thickened Heart Muscle
Hypertrophic cardiomyopathy is a condition where the heart muscle, particularly the wall (septum) between the two lower chambers, grows abnormally thick. This thickened muscle bulges into the path blood takes as it exits the heart. To make things worse, the extra muscle also pulls the mitral valve out of position, dragging part of it into the outflow path during each heartbeat. The combination of a narrowed exit and a displaced valve creates significant obstruction.
This produces a loud murmur with a distinctive feature: it gets louder when you stand up from a squatting position or bear down (the Valsalva maneuver), because these actions reduce the volume of blood in the heart and make the obstruction worse. That behavior helps distinguish it from most other murmurs, which get quieter under the same conditions. Hypertrophic cardiomyopathy is often inherited and can appear in otherwise healthy young adults.
Infections That Damage the Heart
Endocarditis is an infection of the heart’s inner lining and valves. Bacteria, fungi, or other germs enter the bloodstream (sometimes through dental procedures, IV drug use, or skin infections) and attach to the heart, particularly to valves that are already slightly damaged. The germs form clumps called vegetations on the valve surfaces, warping them and preventing proper closure. A new murmur or a change in an existing murmur is one of the hallmark signs of endocarditis, along with persistent fever, fatigue, and sometimes small red spots on the skin.
Rheumatic fever, triggered by the body’s immune response to a strep infection, attacks heart valve tissue in a different way. Rather than direct infection, the immune system mistakenly targets the valve, causing inflammation and eventual scarring. This scarring can take years to become severe but ultimately leads to stenosis, regurgitation, or both. Rheumatic heart disease remains a major cause of valve-related murmurs in many parts of the world.
How Doctors Identify the Cause
When a provider hears a murmur through a stethoscope, they evaluate several characteristics to decide whether it’s innocent or worth investigating. These include the loudness (graded on a scale of 1 to 6), the pitch, where on the chest it’s heard, whether the sound spreads to the neck or back, and most importantly, the timing. A murmur that occurs while the heart is ejecting blood (systolic) is more likely to be innocent. One that occurs while the heart is filling (diastolic) or runs continuously almost always signals a structural problem.
Changes with body position or activity also provide clues. A murmur that gets louder when you stand or strain suggests hypertrophic cardiomyopathy, while one that softens with those maneuvers points toward other causes.
If there’s any concern, an echocardiogram is the primary diagnostic tool. This ultrasound of the heart shows the valves opening and closing in real time, reveals holes or thickened walls, and maps exactly how blood is flowing. A chest X-ray may also be taken to check whether the heart appears enlarged. Together, these tests can pinpoint the cause of virtually any murmur and determine whether treatment is needed.