What Causes Heart Murmurs? 

A heart murmur is not a disease, but rather a sound heard through a stethoscope that indicates turbulent blood flow within or near the heart. While the word ‘murmur’ can sound alarming, many are entirely harmless and do not require treatment. In the UK, clinicians categorise these sounds based on their timing, pitch, and location to determine if they are ‘innocent’ caused by blood moving quickly through a healthy heart or ‘abnormal’ indicating a potential structural issue. Understanding what causes these sounds is essential for navigating cardiac health and knowing when a murmur requires further investigation. This article provides a comprehensive overview of the causes of heart murmurs, the factors that trigger them, and the clinical steps taken in the NHS to ensure your heart is functioning safely and efficiently. 

What We’ll Discuss in This Article 

• The clinical definition of a heart murmur and how it is detected 

• The difference between ‘innocent’ murmurs and ‘abnormal’ murmurs 

• How heart valve issues like stenosis and regurgitation create turbulence 

• Common non-cardiac causes of murmurs, such as pregnancy and anaemia 

• Identifying symptoms that may accompany a pathological heart murmur 

• The role of echocardiograms and stethoscopes in the diagnostic pathway 

• Emergency guidance for severe cardiovascular symptoms 

Why Heart Murmurs Occur? 

Heart murmurs are caused by turbulent blood flow that creates a ‘whooshing’ or ‘swishing’ sound, much like water moving through a narrowed or leaky pipe. Innocent murmurs are often caused by blood flowing faster than usual due to fever, exercise, or pregnancy in an otherwise healthy heart. Abnormal murmurs are typically caused by structural issues, such as heart valves that are too narrow (stenosis) or that leak (regurgitation). 

In a healthy heart, blood flows smoothly and silently. A murmur occurs when something disrupts this laminar flow. In children, innocent murmurs are incredibly common because their hearts are closer to the chest wall and their blood often moves more vigorously. In adults, however, a new murmur is often investigated to rule out damage to the heart valves or holes in the heart walls (septal defects) that allow blood to move between chambers in a chaotic fashion. 

Causes: Innocent vs. Abnormal Murmurs 

Clinicians differentiate murmurs based on whether they are a result of physiological state or structural damage. 

• Innocent Murmurs (Physiological): These occur when the heart is structurally perfect, but the blood is moving with extra force. Common causes include physical activity, pregnancy, anaemia (low red blood cells), or an overactive thyroid. 

• Valvular Stenosis: This is the narrowing of a heart valve opening. It forces blood to squeeze through a smaller space at high pressure, creating a loud, harsh sound. 

• Valvular Regurgitation: This occurs when a valve fails to close tightly, allowing blood to leak backward. This ‘wrong-way’ flow creates a softer, blowing sound. 

• Congenital Heart Defects: Some people are born with ‘holes’ in the heart, such as an Atrial Septal Defect (ASD). These allow blood to bypass the normal route, creating turbulence as it crosses between chambers. 

• Hypertrophic Cardiomyopathy: A thickening of the heart muscle can physically obstruct the path of exiting blood, creating a specific type of murmur that changes with body position. 

Triggers: Factors That Make a Murmur More Audible 

A murmur may be present but ‘quiet’ until certain physiological triggers increase the speed or volume of blood flow. 

Trigger	Why It Causes Turbulence	Clinical Suggestion
Exercise	Heart rate and output increase, moving blood faster.	Murmurs heard only during exercise are often innocent.
Pregnancy	Total blood volume increases by up to 50% in the UK.	Most pregnancy-related murmurs settle after birth.
Fever/Infection	Increases metabolic demand and heart rate.	Murmurs often disappear once the fever is treated.
Anaemia	Thinner blood (low viscosity) flows more turbulently.	Treating iron deficiency can often ‘cure’ the murmur.
Stress/Anxiety	Adrenaline causes the heart to contract more forcefully.	Managing stress can reduce the loudness of ‘flow’ murmurs.

Differentiation: How Doctors Grade Murmurs 

Not all murmurs sound the same, and UK doctors use a specific grading scale (Levine scale) to communicate the severity. 

Grading the Loudness 

Murmurs are graded from 1 to 6. A Grade 1 murmur is so quiet it is barely audible even to an expert, while a Grade 6 murmur is loud enough to be heard with the stethoscope slightly off the chest. Generally, the louder the murmur, the more likely it is to be associated with a structural problem, though this is not always the case. 

Timing: Systolic vs. Diastolic 

A ‘systolic’ murmur happens when the heart is squeezing blood out. These are very common and often innocent. A ‘diastolic’ murmur happens when the heart is relaxing and filling with blood. In the UK, a diastolic murmur is almost always considered abnormal and requires an echocardiogram to determine the underlying cause, as it usually suggests a leaky aortic or pulmonary valve. 

Conclusion 

Heart murmurs are common clinical findings that reflect the physics of blood flow within the cardiovascular system. While many are innocent and triggered by temporary states like pregnancy or exercise, others serve as important indicators of underlying structural issues like valve disease or congenital defects. In the UK, the diagnostic process utilising stethoscopes and echocardiograms is highly effective at distinguishing between harmless flow sounds and those requiring medical intervention. By understanding the causes and triggers of these sounds, you can work collaboratively with your clinical team to ensure your heart health is monitored with accuracy and care within the NHS framework. 

If you experience severe, sudden, or worsening symptoms, such as crushing chest pain, fainting (loss of consciousness), or severe breathlessness, call 999 immediately. 

Authority Snapshot 

This article was reviewed by Dr. Stefan Petrov, a UK-trained physician with an MBBS and postgraduate certifications in Advanced Cardiac Life Support (ACLS) and Basic Life Support (BLS). Dr. Stefan Petrov has extensive clinical experience in emergency care, intensive care units, and surgery, where he regularly performs cardiac assessments and diagnostic procedures. This guide explains the physiological origins of heart murmurs, distinguishing between benign flow sounds and structural abnormalities according to UK medical standards.