Heart murmurs are caused by turbulent or unusually rapid blood flow through the heart or the major vessels attached to it. Normally, blood flows through the chambers and valves in a relatively smooth pattern, but changes in velocity, direction, pressure or the physical structure through which blood travels can produce vibrations that become audible through a stethoscope. The cause can therefore be completely harmless, as in an innocent flow murmur, or it can result from structural heart disease. The distinction is clinically important because treating a murmur itself is unnecessary; clinicians instead determine whether there is an underlying condition that needs monitoring or treatment.
Innocent murmurs commonly occur when the heart pumps blood faster than normal through an otherwise healthy cardiovascular system. Fever increases heart rate and cardiac output, while exercise increases the amount of blood circulating through the heart. Pregnancy increases circulating blood volume, creating greater blood flow through the heart. Anaemia can produce a high-output state because the blood carries less oxygen and the cardiovascular system compensates by increasing cardiac output. Hyperthyroidism can also increase heart rate and circulation. In children, growth and the normal characteristics of a small, rapidly beating heart can make normal blood flow audible. These mechanisms can produce a murmur without any structural defect.
Heart valve abnormalities are among the most important causes of pathological murmurs. Stenosis means that a valve has become narrowed and does not open adequately, forcing blood through a restricted opening and increasing flow velocity across it. Regurgitation or insufficiency means that a valve does not close properly, allowing blood to leak backwards when it should be moving forwards. Both processes disturb normal blood flow and can produce characteristic murmurs. Valve disease can affect the aortic, mitral, pulmonary or tricuspid valves. Causes include congenital valve abnormalities, ageing-related degeneration, rheumatic fever, infection such as infective endocarditis, cardiomyopathy and damage to the heart muscle following a heart attack.
Congenital heart defects are another important cause, particularly in babies and children. A structural defect may create an abnormal pathway between chambers or alter the normal route of blood through the heart and great vessels. A ventricular septal defect, for example, creates an opening between the ventricles and can permit abnormal blood flow across the septum. Some congenital valve abnormalities produce turbulence because the valve has an unusual structure or does not open or close normally. Not every congenital defect produces a murmur, and not every murmur indicates a congenital defect; the clinical context and, where necessary, echocardiography establish whether structural disease is present.
Infective and inflammatory conditions can also be associated with murmurs. Infective endocarditis is an infection involving the inner lining of the heart and commonly the heart valves; damage to a valve can interfere with normal blood flow and produce a new or altered murmur. Rheumatic fever can cause lasting inflammatory damage to the heart valves, particularly the mitral valve, and can subsequently produce valve stenosis or regurgitation. These causes are less common than innocent flow murmurs in children but are clinically important because untreated structural or infectious heart disease can cause progressive cardiovascular complications.
The characteristics of a murmur provide clues about its cause. Clinicians consider whether it is systolic, diastolic or continuous; where it is loudest; whether it radiates to another area; its intensity and quality; and whether it changes with posture, respiration or exercise. A murmur that changes with physiological conditions can be consistent with altered blood flow, whereas particular patterns may prompt investigation for valve disease or a structural defect. These acoustic characteristics are not interpreted in isolation: the clinician also considers age, symptoms, blood pressure, pulses, oxygenation, family history and the remainder of the cardiovascular examination.
Some conditions that increase cardiac workload can reveal a murmur that was previously difficult to hear. A child may have a murmur that becomes more noticeable during a fever because the heart is beating faster. A person with anaemia may develop a murmur because the circulation compensates for reduced oxygen-carrying capacity. Pregnancy can produce a temporary flow murmur because blood volume increases substantially. These examples illustrate why a murmur may appear, disappear or vary in intensity without representing a new structural abnormality. Nevertheless, a newly detected murmur should be assessed in context rather than automatically attributed to a benign cause.
The presence of a murmur cannot establish its cause without clinical assessment. A common misconception is that every murmur means a hole in the heart, heart failure or serious valve disease; another is that a loud murmur must necessarily represent a more dangerous condition. Neither assumption is reliable. Most murmurs in healthy children are innocent, while abnormal murmurs can arise from several different structural disorders. If a clinician suspects an abnormal murmur, echocardiography and other investigations can identify the underlying anatomy and physiology. The appropriate treatment therefore depends entirely on the cause rather than on the mere presence of an audible murmur.
