A heart murmur is an extra or unusual sound your doctor hears when listening to your heartbeat, caused by blood flowing in a turbulent (rough, choppy) way instead of smoothly through the heart and its valves.

What Causes a Heart Murmur?

1. The Basic Mechanism

When blood is forced through a narrowed, leaky, or unusually shaped part of the heart or nearby vessels, it becomes turbulent, and that turbulence creates the sound called a murmur.

Murmurs are usually grouped into:

  • Systolic murmurs: happen when the heart is contracting and pumping blood out.
  • Diastolic murmurs: happen when the heart relaxes and fills with blood.
  • Continuous murmurs: start in systole and continue into diastole.

These patterns help doctors guess what’s causing the murmur.

2. “Innocent” (Harmless) Causes

Not all murmurs mean heart disease. So‑called “innocent” or “physiologic” murmurs occur when the heart and valves are structurally normal, but blood is moving faster than usual.

Common innocent causes include:

  • Pregnancy (increased blood volume and flow).
  • Fever (heart beats faster, flow speeds up).
  • Anemia (low red blood cells, heart pumps harder to deliver oxygen).
  • Exercise or intense physical training.
  • Growth spurts in children and teens.

These murmurs often:

  • Come and go over time.
  • Don’t cause symptoms like chest pain or fainting.
  • Need no treatment, just periodic checkups.

3. Structural Heart Problems Present from Birth

Some murmurs come from congenital (present at birth) changes in the heart’s structure that affect how blood flows.

Key congenital causes:

  • Holes in the heart walls
    • Atrial septal defect (ASD): hole between the upper chambers.
* Ventricular septal defect (VSD): hole between the lower chambers.
  • Cardiac shunts
    • Abnormal channels between chambers or major vessels that let blood flow the “wrong” way.
  • Patent ductus arteriosus (PDA)
    • A blood vessel that should close shortly after birth stays open, causing continuous flow between the aorta and pulmonary artery.
  • Other congenital defects
    • Coarctation of the aorta (narrowed main body artery), anomalous pulmonary veins, and valve malformations can all produce murmurs.

Some of these defects close on their own; others may need surgery or catheter- based procedures.

4. Heart Valve Problems (Very Common Cause)

Because valves control one‑way flow, anything that makes them too tight or leaky can generate a murmur.

Main valve-related causes:

  • Valve stenosis (narrowing)
    • Aortic stenosis or mitral stenosis: valves become stiff or narrowed, often from age-related calcification, congenital malformation, or past infections such as rheumatic fever.
  • Valve regurgitation (leakiness)
    • Mitral or aortic regurgitation: blood leaks backward through a valve that does not close properly.
  • Valve calcification and sclerosis
    • Calcium deposits and scarring make valves thick and stiff, which can eventually narrow them and change blood flow.

These conditions may cause symptoms like shortness of breath, chest discomfort, fatigue, or swelling in legs if they become significant.

5. Infections and Inflammation

Infections or inflammatory diseases that affect the heart lining or valves can damage their structure and create or worsen murmurs.

Important causes:

  • Endocarditis
    • Infection of the inner lining of the heart and valves, often from bacteria entering the bloodstream; it can destroy or perforate valves, leading to new or changing murmurs.
  • Rheumatic fever (a complication of untreated strep throat)
    • Can scar the valves, especially the mitral valve, causing stenosis, regurgitation, or both.
  • Autoimmune diseases
    • Conditions like lupus or rheumatoid arthritis can inflame or thicken valves and nearby tissue.

These causes are less common in high‑income countries today but still important worldwide.

6. Other Medical Conditions That Change Blood Flow

Some non‑structural conditions make the heart pump harder or change blood properties, which can create a murmur even if the valves are mostly normal.

These include:

  • Anemia (low red blood cell count).
  • Overactive thyroid (hyperthyroidism).
  • High blood pressure, especially in the lungs (pulmonary hypertension).
  • Cardiomyopathy (weakened or abnormally thick heart muscle).
  • Certain tumors such as carcinoid syndrome that release chemicals affecting heart valves.
  • Blood disorders with very high white cell counts (e.g., hypereosinophilic syndromes) that can damage heart tissue.

These conditions can either cause a murmur on their own or worsen one from an existing valve issue.

7. When a Heart Murmur Is More Likely to Be “Serious”

A murmur is more worrisome when it is linked to symptoms or known structural disease.

Red‑flag signs that need timely medical evaluation:

  • Chest pain, pressure, or tightness.
  • Shortness of breath at rest or with minimal activity.
  • Fainting or near‑fainting episodes.
  • Swelling in legs, ankles, or abdomen.
  • Bluish lips or fingers (in babies/children).
  • Fast breathing, poor feeding, or poor growth in infants and young children.

Doctors use a stethoscope exam plus tests like echocardiograms (ultrasound of the heart) to identify the exact cause.

If you or someone you know has been told they have a heart murmur and has any of the symptoms above, it’s important to see a clinician promptly for proper diagnosis and management.

8. Mini FAQ: Quick Scoop

Is every heart murmur dangerous?
No. Many murmurs, especially in children and pregnant adults, are “innocent,” meaning the heart is structurally normal and no treatment is needed.

What’s the single most common structural cause?
Valve problems (narrow or leaky valves), particularly aortic and mitral valve disease, are among the most frequent structural causes in adults.

Can a murmur go away?
Yes. Innocent murmurs from fever, anemia, or pregnancy often disappear once the underlying condition is treated or the situation (like pregnancy or a growth spurt) passes.

How do they figure out the exact cause?
A clinician listens to where in the chest the murmur is loudest, its timing (systolic, diastolic, continuous), and quality, then may order an echocardiogram or other imaging to see the valves and blood flow.

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A heart murmur is an extra heart sound caused by turbulent blood flow. Learn what causes a heart murmur, from harmless flow changes to valve disease and congenital heart defects. Bottom note: Information gathered from public forums or data available on the internet and portrayed here.