A heart murmur is an extra or unusual sound heard during the cardiac cycle, distinct from the familiar “lub-dub” of a normal heartbeat. The normal sounds, known as S1 and S2, are created by the rhythmic closing of the heart’s four valves. The “lub” (S1) occurs when the mitral and tricuspid valves shut at the beginning of the heart’s contraction phase. The “dub” (S2) happens when the aortic and pulmonary valves close. A heart murmur introduces an abnormal sound, often described as a whooshing or swishing, that occurs between these two beats.
The Mechanism of the Sound
The presence of a murmur signals that blood flow within the heart has become rapid and choppy, a state known as turbulent flow. Normally, blood moves smoothly and silently through the heart in a streamlined fashion (laminar flow). When blood encounters an obstruction or an abnormal pathway, this smooth pattern is disrupted, creating vibrations that are loud enough to be detected by a stethoscope. These vibrations are the sound of the murmur itself.
Turbulence results from several physiological issues affecting the heart’s structure. One common cause is a narrowed valve, or stenosis, which forces blood to squeeze through a restricted opening, increasing its velocity and creating a loud sound. Another mechanism is a leaky valve, known as regurgitation, which allows chaotic blood flow backward into the previous chamber. Structural defects, such as a septal defect (a hole in the wall), also cause a murmur by allowing blood to shunt from one side to the other. Conditions that increase the overall volume or speed of blood, like anemia, fever, or pregnancy, can similarly cause a temporary increase in turbulence.
Describing the Murmur Sound
Physicians analyze several auditory characteristics to classify a heart murmur and determine its origin. The timing of the sound within the cardiac cycle is a primary characteristic. Systolic murmurs occur between S1 (“lub”) and S2 (“dub”), while diastolic murmurs happen after S2 and before the next S1. Systolic murmurs are more common and can be described as a crescendo-decrescendo pattern, where the sound builds up and then fades, or a uniform, plateau-like sound.
The quality of the sound provides an important clue, with descriptions including blowing, harsh, rough, rumbling, or musical. A musical or vibratory quality is often associated with the benign Still’s murmur, while a harsh quality may suggest a more significant structural issue. Pitch, referring to the frequency of the sound, is also noted. High-pitched murmurs are typically heard best with the diaphragm of the stethoscope, while low-pitched, rumbling murmurs are better detected with the bell.
Intensity is measured using a standardized six-grade scale. Grade I is the softest sound, audible only by an expert in a quiet room. Grade III is a loud murmur that is easy to hear but does not cause a palpable vibration on the chest wall, known as a thrill. The loudest murmurs, Grades IV through VI, are accompanied by a thrill, with Grade VI being audible even when the stethoscope is lifted slightly off the chest. These detailed classifications help pinpoint the specific valve or area causing the turbulent flow.
Distinguishing Harmless vs. Concerning Murmurs
Heart murmurs are broadly categorized as either innocent (functional) or pathological. Innocent murmurs are common, especially in children and adolescents, and are not caused by underlying heart disease. They signify a normal heart where blood is flowing more rapidly than usual, often due to fever, excitement, or temporary increases in blood volume.
Innocent murmurs are almost always soft (Grade I or II), short, and typically heard during systole. They often disappear when the person changes position. They do not require medical treatment or lifestyle changes, and many naturally resolve as a child grows. However, a medical evaluation is necessary to confirm the innocent nature of the sound.
Pathological, or abnormal, murmurs are those caused by an underlying structural heart issue, such as congenital defects, damaged valves, or acquired heart disease. Features that raise suspicion include diastolic timing, a loud intensity (Grade III or higher), a harsh quality, or a sound that radiates to other areas like the neck or armpit. Pathological murmurs signal the need for further diagnostic testing, such as an echocardiogram, to determine the extent of the heart issue. A medical professional must always perform the comprehensive evaluation.