A heart murmur is caused by turbulent blood flow through or near the heart valves, producing a whooshing or blowing sound a doctor can hear with a stethoscope. Many murmurs are harmless, created by blood moving faster than usual through a structurally normal heart. Others signal a problem with a valve or the heart’s structure itself. The distinction between the two is what matters most.
How Normal Blood Flow Becomes Audible
Blood normally flows through the heart in smooth, orderly streams. When something disrupts that flow, whether a narrowed valve, a leaky seal, or simply a faster heart rate, the blood churns and creates vibrations. Those vibrations travel through the chest wall as sound. The louder and harsher the sound, the more turbulent the flow.
Doctors grade murmurs on a scale from 1 to 6. A grade 1 murmur is so faint it takes several seconds of careful listening to detect. A grade 6 is loud enough to hear without a stethoscope touching the chest. The grade alone doesn’t determine whether a murmur is dangerous, but louder murmurs (grade 3 and above) are more likely to reflect a structural problem that needs further evaluation.
Innocent Murmurs: Common and Harmless
More than half of all children have an innocent heart murmur at some point, and they occur in healthy adults too. These murmurs happen when blood flows a bit faster or more forcefully than usual through a heart that is completely normal. There’s no damaged valve, no hole, no defect.
Several everyday situations can temporarily speed up blood flow enough to create an audible murmur:
- Pregnancy: Blood volume increases by nearly 50% during pregnancy, pushing more blood through the heart with each beat.
- Anemia: When you have fewer red blood cells, the heart compensates by pumping faster, which can create turbulence.
- Overactive thyroid: Hyperthyroidism raises heart rate and cardiac output, making flow noisier.
- Fever or exercise: Anything that raises your heart rate forces blood through the valves more quickly.
Innocent murmurs typically disappear when the underlying condition resolves. They don’t cause symptoms, don’t need treatment, and don’t increase your risk of heart disease.
Valve Problems: Stenosis and Regurgitation
The heart has four valves that open and close with each heartbeat, directing blood in one direction. When a valve doesn’t work properly, it creates the turbulence behind most pathological murmurs. The two main types of valve dysfunction are stenosis and regurgitation.
Stenosis (Narrowing)
A stenotic valve doesn’t open fully, forcing blood through a smaller-than-normal opening. Think of it like partially covering a garden hose nozzle: the water speeds up and sprays unevenly. As we age, calcium deposits can build up on valve leaflets, making them stiff and narrow. The aortic and mitral valves are most commonly affected. Aortic stenosis is especially common in people over 65 and can progress to the point where the heart has to work significantly harder to push blood through the narrowed opening.
Regurgitation (Leaking)
A regurgitant valve doesn’t close completely, allowing blood to leak backward. This creates a distinctive whooshing sound as blood flows in the wrong direction. Mitral valve prolapse, where the valve leaflets bulge and don’t seal tightly, is one of the most common causes of regurgitation. A calcified valve can also fail to close properly, so some valves develop both stenosis and regurgitation at the same time.
Congenital Heart Defects
Some murmurs are caused by structural problems present from birth. The most common is a ventricular septal defect (VSD), a hole in the wall between the heart’s two lower chambers. This hole lets oxygen-rich blood flow back toward the lungs instead of out to the body, mixing oxygenated and deoxygenated blood. The abnormal flow through the hole produces a murmur that a doctor can hear in both babies and adults.
A small VSD may never cause symptoms and can close on its own during childhood. Larger defects force the heart and lungs to work harder and may eventually need repair. Other congenital defects that cause murmurs include holes between the upper chambers (atrial septal defects) and an abnormal connection between two major blood vessels near the heart (patent ductus arteriosus). All of these create extra pathways for blood to flow where it shouldn’t, generating turbulence.
Infections and Inflammatory Damage
Infections can damage heart valves and produce a new murmur or change an existing one. Two conditions stand out.
Rheumatic heart disease develops after one or more episodes of rheumatic fever, which is the immune system’s overreaction to a strep throat infection. Instead of only attacking the bacteria, the immune response inflames and scars the heart valves. Over time, the scarred valves become stiff or leaky, creating a persistent murmur. Rheumatic heart disease remains a leading cause of valve damage worldwide, particularly in lower-income countries where strep throat goes untreated.
Endocarditis is a bacterial infection of the heart’s inner lining, usually targeting the valves directly. Bacteria colonize the valve surface, forming growths that erode the tissue. A new murmur or a change in an existing murmur during a fever and unexplained illness is one of the hallmark warning signs of endocarditis.
Symptoms That Point to a Serious Cause
An innocent murmur produces no symptoms at all. A murmur caused by valve disease or a structural defect, on the other hand, may come with signs that the heart isn’t pumping efficiently. These include shortness of breath during activity or when lying flat, swelling in the legs or feet, unusual fatigue, dizziness or fainting, and a bluish tint to the skin or fingertips (particularly in infants with congenital defects).
A murmur discovered during a routine physical in someone who feels fine is often innocent. A murmur paired with any of those symptoms warrants further testing. A change in how an existing murmur sounds, whether it becomes louder or takes on a different character, is also a reason for re-evaluation.
How Murmurs Are Evaluated
The first step is a careful physical exam. Where on the chest the murmur is loudest, when it occurs during the heartbeat cycle, and how it responds to changes in body position all give clues about the cause. But physical exam alone can’t definitively distinguish an innocent murmur from a pathological one in many cases.
An echocardiogram is the standard next step for any murmur that raises concern. This ultrasound of the heart shows the valves opening and closing in real time and measures how blood moves through each chamber. It can reveal whether a valve is narrowed, leaking, or structurally abnormal, and it quantifies how severe the problem is. For example, the speed of blood jetting through a narrowed aortic valve directly indicates the degree of stenosis: a peak velocity under 2 meters per second is normal, while 4 meters per second or higher indicates severe narrowing.
The echocardiogram also measures the heart’s pumping strength and whether chambers have enlarged from working too hard. These details determine whether the murmur can simply be monitored over time or whether it needs active treatment. For significant valve disease, guidelines recommend evaluation by a specialized heart valve team to weigh the options, which can range from regular monitoring to valve repair or replacement depending on severity and symptoms.