Unless its progress is disrupted, blood moving through the heart and vessels flows quietly along a pressure gradient—that is, from an area of higher pressure to an area of lower pressure. Experts in fluid dynamics use the term “laminar flow” to describe this silent flow.
Just as a stream purls around a boulder in its course, irregularities in blood vessels or heart valves create turbulence in the laminar flow of blood, and turbulence generates noise. Noise within the cardiovascular system can be detected through a stethoscope (or an ear placed against the overlying skin). Noises heard in the region of the heart are known as “murmurs.”
Any alteration in the normally smooth surface or uniform contours of the circulatory system can lead to turbulent blood flow: narrowed or roughened heart valves, constricted arteries or veins, irregular surface contours of vessels—even higher-than-normal flows can create turbulent flows.
While a murmur may indicate pathology in a heart valve or a nearby major vessel, most murmurs are benign, or “functional”. Indeed, in children, approximately 60% of murmurs discovered on routine physical examination are innocent. (McCrindle BW, et al. Factors prompting referral for cardiology evaluation of heart murmurs in children [Letter]. Arch Pediatr Adolesc Med 1995;149:1277-9).
Common Causes of Heart Murmurs
- Innocent (“functional” or "physiologic"): These murmurs, theoretically caused by relatively high flow rates in normal hearts, are often heard in children. They are also commonly heard in individuals with a fever, due to increased blood flow. Women often exhibit murmurs during pregnancy as a result of increased blood volume and flow.
- Venous hum: A variant of a functional murmur, venous hums are heard in children as the result of venous blood returning to the thorax from the head and neck. A physician can diagnose these innocent murmurs with a few simple maneuvers in the office.
- Valvular pathology: Any of the heart valves—triscupid, pulmonic, mitral, or aortic—can be affected by congenital malformation or damaged by disease (e.g., rheumatic fever, endocarditis, myocardial infarction). Which valve(s) is affected can be determined through careful physical examination, including maneuvers like squatting, breath-holding, lying down, etc.; definitive diagnosis is usually made by echocardiography (ultrasound of the heart).
- Septal defects: The atria (upper chambers of the heart) and ventricles (lower chambers) are normally separated from each other by a wall (septum) of muscular tissue. During embryologic development, these walls may not completely fuse, and a hole remains between the chambers. With each heartbeat, blood is forced through the hole, creating turbulence that can be heard through a stethoscope.
- Hypertrophic cardiomyopathy: While this disorder is relatively rare in the general population, it is one of the most common causes of sudden death in young athletes. In this condition, a thickening of muscle beneath the aortic valve constricts the outflow of blood. This anatomical abnormality creates a murmur that, once again, can be characterized by specific maneuvers during a physical exam.
Heart murmurs are actually fairly common in the population. Not all murmurs indicate the presence of heart disease; many are incidentally found during examinations for other problems or during routine checkups.
Pathologic murmurs may necessitate the administration of antibiotics prior to dental work, or even corrective surgery. Health care professionals who are trained in cardiac evaluation can usually determine which murmurs require further evaluation.
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