A heart murmur is an extra or unusual sound heard during a heartbeat, distinct from the normal “lub-dub” rhythm caused by the valves closing. This sound is produced when blood flow through the heart becomes turbulent instead of maintaining a smooth, quiet pattern. The sound occurs when blood encounters a narrowed opening, a leaky valve, or an abnormal connection between heart chambers. For clinicians, the specific sound heard through a stethoscope is a primary diagnostic tool used to assess the heart’s function and structure.
The Basic Sound Quality
A heart murmur sounds like a “whoosh,” “swish,” or “rushing” noise that accompanies the heart’s regular rhythm. This acoustic effect is a direct result of disorganized blood movement within the heart or major blood vessels. Healthy blood flow is typically quiet and smooth, a pattern known as laminar flow, which is not audible. When the path is obstructed, or blood flows backward or too rapidly, the resulting vibration and chaotic motion create the characteristic murmur sound. The particular quality of the sound depends entirely on the location and nature of the underlying flow issue.
Classification by Timing
The most important factor in characterizing a heart murmur is its timing relative to the two normal heart sounds, S1 and S2. S1 marks the closure of the mitral and tricuspid valves, indicating the start of the ventricle contraction phase, known as systole. S2 marks the closure of the aortic and pulmonary valves, signaling the end of systole and the start of the relaxation and filling phase, called diastole.
Systolic murmurs are the most commonly encountered type, occurring during the contraction phase between S1 and S2. They often result from blood flowing forward through a narrowed valve (stenosis) or backward through a leaky valve (regurgitation). A holosystolic murmur is a specific type that lasts for the entire duration of systole, starting with S1 and ending with S2.
Diastolic murmurs occur during the heart’s relaxation and filling phase, heard between S2 and the next S1 sound. These murmurs often suggest a serious structural problem, typically involving blood flowing backward through the aortic or pulmonary valves, or forward through a narrowed mitral or tricuspid valve. Since they are less common and usually indicate an abnormality, any diastolic murmur warrants further evaluation.
A continuous murmur is distinctly heard throughout both the systolic and diastolic phases without interruption. This pattern is associated with blood flowing from a high-pressure area to a lower-pressure area throughout the cardiac cycle. An example is the murmur caused by a patent ductus arteriosus, which is often described as having a machine-like quality.
Describing the Sound’s Characteristics
Beyond timing, clinicians use several characteristics to grade the severity and source of the sound. Intensity, or loudness, is graded on a scale of 1 to 6, which helps quantify the sound’s strength. A Grade 1 murmur is the softest, barely audible even to an experienced listener.
A Grade 3 murmur is moderately loud but is not associated with a palpable vibration on the chest wall. Murmurs loud enough to be felt as a vibration, known as a thrill, are classified as Grade 4 or higher. The loudest classification, Grade 6, describes a murmur so intense it can be heard with the stethoscope entirely lifted off the chest.
Pitch provides diagnostic clues, ranging from high-pitched, blowing, or squeaky sounds to low-pitched, rumbling noises. High-velocity jets of blood through a small opening often create a higher pitch, while low-pitched sounds are generally heard better with the bell of the stethoscope. The shape or configuration describes how the loudness changes over its duration, such as a crescendo (getting louder), decrescendo (getting softer), or a plateau shape (remaining constant). The location where the sound is loudest and where it radiates across the chest or back helps pinpoint the specific valve or area of the heart causing the disturbance.
Differentiating Innocent and Pathological Murmurs
The acoustic qualities described help clinicians distinguish between a harmless, or innocent, murmur and one that signifies underlying heart disease, known as a pathological murmur. Innocent murmurs are common, especially in children and adolescents, and are considered physiological rather than structural. These are typically soft, low-grade (Grade 1-3), and almost always occur during systole. Innocent murmurs often arise from the heart temporarily pumping blood faster than normal, such as during a fever, exercise, or in cases of anemia, and they do not require treatment. They are sometimes referred to as flow murmurs.
In contrast, a pathological murmur indicates a structural abnormality in the heart or vessels, requiring further investigation. Any diastolic or continuous murmur is generally considered pathological, regardless of its intensity, because they almost always represent a structural issue. Systolic murmurs that are loud (Grade 4-6) or are associated with concerning symptoms, like shortness of breath or chest pain, are also classified as abnormal. Pathological murmurs point toward issues like valve failure, narrowing, or congenital defects.