The human heart typically makes two distinct sounds, often described as “lub-dub,” which represent the normal closing of heart valves. These sounds, known as the first heart sound (S1) and the second heart sound (S2), mark the start and end of ventricular contraction, respectively. However, sometimes an extra noise is heard, and one such sound is the fourth heart sound, or S4. The S4 heart sound is a low-frequency noise that occurs just before the normal S1 and S2 rhythm, creating a pattern often referred to as an atrial or pre-systolic gallop. Unlike S1 and S2, the presence of an S4 in adults is almost always an indication of an underlying issue within the heart.
Understanding the Cardiac Cycle and S4 Timing
The normal heartbeat is divided into two phases: systole, when the ventricles contract and pump blood out, and diastole, when the ventricles relax and fill with blood. The S1 sound begins systole with the closure of the mitral and tricuspid valves, while the S2 sound marks the end of systole and the start of diastole with the closure of the aortic and pulmonic valves. Diastole is the longer phase, during which the ventricles passively fill with blood from the atria.
The S4 sound occurs late in the diastolic phase, right before the S1 sound of the next beat. This timing is significant because it coincides with the moment the atria contract forcefully to push the final volume of blood into the ventricles, which is why it is described as a pre-systolic gallop.
The sound is low-pitched, meaning it is best heard using the bell of a stethoscope applied lightly to the chest. The characteristic rhythm produced by S4, followed by S1 and S2, is often compared to the cadence of the word “Tennessee” (S4-S1-S2). The low frequency and quiet nature of the S4 mean that it can sometimes be difficult to detect during a standard physical examination.
The Physiological Cause of the S4 Sound
The S4 sound results from the collision of blood against a stiff or non-compliant ventricular wall. This stiffness means the ventricle is unable to relax properly during diastole, a condition known as diastolic dysfunction. When the atria contract—the “atrial kick”—they force blood into a ventricle that is abnormally resistant to the extra volume.
This forceful entry of blood causes the ventricular wall and surrounding structures to vibrate, creating the low-frequency sound audible as S4. The more rigid the ventricular wall, the more forceful the atrial contraction must be, leading to a louder S4 sound.
The stiffening is typically the result of ventricular muscle thickening, known as hypertrophy, which develops as the heart works harder against increased pressure. The mechanism requires a functioning atrial contraction, which is why S4 is absent in patients with atrial fibrillation, a condition where the atria quiver ineffectively. The S4 sound reflects the heart’s attempt to overcome an issue with ventricular relaxation and filling.
Clinical Significance and Associated Conditions
The presence of an S4 heart sound in an adult is a significant clinical finding, signaling increased resistance to ventricular filling due to decreased compliance or stiffness of the ventricular wall. This finding should prompt a healthcare provider to investigate the root cause that led to the ventricular changes.
Chronic high blood pressure (hypertension) is a common cause, as persistent high pressure causes the left ventricle to thicken and become stiff over time. Hypertrophic cardiomyopathy, a genetic condition involving abnormal thickening of the heart muscle, is also frequently associated with S4. In both cases, the thickened muscle impairs the ventricle’s ability to fully relax and fill with blood.
Associated Conditions
Conditions that obstruct blood flow leaving the heart, such as aortic stenosis, increase the workload on the ventricle, leading to hypertrophy and S4. Acute coronary artery disease, which can lead to a heart attack, may cause temporary stiffness in the affected ventricular wall, resulting in a transient S4 sound. While an S4 may occasionally be heard in older individuals due to age-related changes in ventricular stiffness, it is still considered a marker of an underlying issue that requires evaluation. The discovery of an S4 often guides doctors to order additional diagnostic tests, such as an echocardiogram, to visualize the ventricle and confirm the degree of stiffness and the underlying disease process.