The heart produces distinct sounds (vibrations created by closing valves and moving blood) that provide healthcare professionals with information about its function. The fourth heart sound, S4, is an extra sound that occurs late in the cardiac cycle, during the heart’s relaxation and filling phase, known as late diastole. Finding this extra sound often indicates the heart is working against abnormal resistance.
The Context of Basic Heart Sounds
The normal heartbeat is characterized by two primary sounds, often described as “lub-dub.” The first sound, S1, is the “lub,” which marks the beginning of the heart’s contraction phase, or systole. This sound is produced by the closure of the mitral and tricuspid valves, which separate the upper chambers (atria) from the lower chambers (ventricles).
The second sound, S2, is the “dub,” signaling the end of systole and the start of diastole. S2 is generated by the closure of the aortic and pulmonary valves. The S4 sound occurs during the final moments of diastole, just before the next S1 sound begins, placing it immediately preceding the main pumping action of the ventricles.
The Physiological Mechanism Behind S4
The S4 heart sound is generated by the forceful contraction of the atria, often called the “atrial kick.” This contraction is the final effort to push blood into the ventricles. The sound only occurs when the ventricles have become stiff or non-compliant, a condition known as diastolic dysfunction.
When the atrial kick forces blood into a ventricle that cannot easily expand, the blood suddenly decelerates as it hits the rigid ventricular wall. This sudden deceleration creates the audible vibration known as the S4 sound. For S4 to occur, the heart must be in a normal rhythm, as an effective atrial contraction is necessary. Because S4 is a low-frequency sound, it can be difficult to hear and is best detected using the bell of a stethoscope.
Clinical Meaning and Associated Conditions
The presence of an S4 heart sound in an adult is considered an abnormal finding, indicating an underlying cardiac issue. It signals decreased ventricular compliance, meaning the muscle is stiff and unable to relax properly to accommodate incoming blood. This sound is sometimes referred to as a “presystolic gallop” or “atrial gallop” because of its position just before systole.
A common cause of this ventricular stiffness is uncontrolled high blood pressure (hypertension). Hypertension forces the heart to pump against greater resistance, leading to a thickening of the heart muscle known as left ventricular hypertrophy. This thickened, less elastic muscle creates the stiffness necessary to generate the S4 sound.
Other conditions that cause the ventricle to become non-compliant can also lead to an S4, such as aortic stenosis, where the aortic valve narrows, increasing the workload on the left ventricle. Acute myocardial ischemia (a lack of blood flow to the heart muscle) can also cause the ventricular muscle to become temporarily stiff and produce an S4. While S4 can sometimes be heard in healthy older adults due to age-related stiffening, or in highly trained athletes, its detection almost always prompts a search for a treatable disease. Treating the underlying condition, such as managing high blood pressure, is the focus rather than treating the S4 sound itself.