A heart murmur is a sound heard during the heartbeat cycle that is distinct from the normal “lub-dub” sounds, resulting from turbulent or rapid blood flow through the heart valves or vessels. This sound is not a disease but a physical sign that blood flow is moving in a non-laminar, “whooshing” pattern. The intensity of this sound can change dramatically based on physiological factors, including body position. A murmur often becomes louder when an individual lies down, which is related to temporary changes in how blood circulates back to the heart.
How Body Position Affects Blood Flow
Lying down, or moving to a supine position, causes a predictable change in the body’s hemodynamics. This change is driven by the elimination of gravity’s effect on blood pooling in the lower extremities. When standing, gravity works against the return of blood from the legs and abdomen to the heart.
The supine position allows this volume of blood to rapidly shift back toward the central circulation, significantly increasing the amount of blood returning to the heart, a measurement known as preload. This temporary increase in volume leads to a more forceful filling of the heart chambers just before they contract. A larger volume of blood moving through the heart’s structures increases the speed and turbulence of the flow.
This heightened turbulence is what makes the murmur sound louder when a person lies flat. The increased blood flow accentuates the turbulent sound, making it more easily audible through a stethoscope. This change is a normal, physiological response to position and does not inherently indicate a serious heart problem. However, the degree to which a murmur changes intensity is diagnostic information used by physicians.
Differentiating Innocent and Pathologic Murmurs
The most important distinction in evaluating a heart murmur is determining whether it is “innocent” (harmless, not caused by structural disease) or “pathologic” (a sign of an underlying structural issue). Innocent murmurs, also called functional or physiologic murmurs, are common, especially in children, and are caused by blood flowing quickly through a structurally normal heart. These murmurs often change significantly in intensity with body position, frequently becoming louder when lying down due to increased blood flow.
Pathologic murmurs are caused by structural abnormalities like defective heart valves (stenosis or regurgitation) or congenital defects. While these murmurs may also change with position, they are typically louder, harsher, and associated with other concerning characteristics. Any murmur heard during the heart’s resting phase, known as a diastolic murmur, is considered pathologic until proven otherwise.
Physicians use several characteristics to distinguish between the two types, including the timing of the sound within the cardiac cycle. Innocent murmurs are almost always systolic, occurring when the heart muscle contracts. Pathologic murmurs may be pansystolic, continuous, or diastolic. Innocent murmurs are often soft, localized, and do not radiate to other parts of the body, unlike many pathologic murmurs.
The intensity of a murmur is graded on a scale of 1 to 6. Innocent murmurs rarely exceed a grade 2 or 3. Louder murmurs, especially those accompanied by a palpable vibration known as a thrill, are more concerning for a structural problem. The dynamic change in sound with position is a key finding that helps categorize the murmur as likely innocent.
Medical Assessment and Diagnostic Testing
When a physician detects a heart murmur, the evaluation begins with a detailed medical history and a comprehensive physical examination. The clinician listens to the heart with a stethoscope while the patient is in several different positions, including sitting, standing, and lying down. They may also ask the patient to perform maneuvers like breath-holding or squatting. Observing how the murmur’s intensity changes with these actions provides significant clues about its underlying mechanism.
If the murmur has the characteristics of an innocent flow sound—soft, systolic, and highly responsive to positional changes—no further testing is typically necessary. If the physical exam raises any suspicion of a structural abnormality, the next step is usually a non-invasive imaging test.
The primary diagnostic tool for evaluating the heart’s structure and function is the echocardiogram, which uses sound waves to create moving pictures of the heart. The echocardiogram allows the physician to visualize the heart valves, chambers, and blood flow in detail, confirming or ruling out structural defects, such as a leaky valve or a septal defect, that would cause a pathologic murmur.
Other complementary tests, such as an electrocardiogram (ECG) or a chest X-ray, may also be used. Once a murmur is confirmed to be innocent by a physician, no specific follow-up or treatment is generally required.