The fourth heart sound, known as S4, is an abnormal extra noise heard during a cardiac examination, signaling an underlying issue with heart function. This low-pitched sound occurs just before the first normal heart sound (S1) in the cardiac cycle. When S4 is present, the combination of S4, S1, and the second heart sound (S2) creates a rhythm frequently described as a gallop. This distinct sound provides physicians with a clue about the stiffness and filling mechanics of the heart’s main pumping chamber, the ventricle.
Understanding Normal Heart Sounds
The rhythmic “lub-dub” heard when listening to the heart represents the two normal heart sounds, S1 and S2. The first sound, S1, or “lub,” marks the beginning of the heart’s contraction phase (systole) and is generated by the closure of the mitral and tricuspid valves. The second sound, S2, or “dub,” occurs at the end of systole and beginning of the relaxation phase (diastole). It is caused by the closure of the aortic and pulmonic valves as the ventricles finish pumping blood.
The Physiological Mechanism of the S4 Sound
The S4 sound is timed to the very end of diastole, the period when the heart is relaxed and filling with blood. During this late phase, the atria contract forcefully to push the final volume of blood into the ventricles, an action known as the atrial kick. In a healthy heart, this process is silent.
An S4 sound is produced when the atria are forced to contract against a stiff or non-compliant ventricle. This stiffness means the ventricle resists being stretched. The resistance causes the blood pushed in by the atrium to decelerate suddenly, creating vibrations in the ventricular wall. This turbulent flow and wall vibration are heard as the low-frequency S4 sound. The presence of S4 requires an effective atrial contraction, so it cannot be heard in conditions like atrial fibrillation.
Primary Medical Conditions Associated with an S4
The root cause of the S4 heart sound is the underlying condition that leads to decreased ventricular compliance or stiffness. The most common cause is chronic systemic hypertension (high blood pressure), which forces the left ventricle to pump against high resistance. This results in left ventricular hypertrophy (LVH), a thickening of the ventricular muscle that makes the chamber less flexible during filling.
Aortic stenosis, a narrowing of the aortic valve, similarly increases the pressure load on the left ventricle, also leading to LVH. Other conditions that impair the ventricle’s ability to relax and fill correctly can also produce an S4. These include ischemic heart disease, where a lack of blood flow impairs the heart muscle’s ability to relax, and hypertrophic cardiomyopathy, where extreme muscle thickening reduces the chamber’s flexibility.
Clinical Significance and Interpretation
The detection of an S4 heart sound during auscultation is a significant clinical finding. Unlike some extra heart sounds heard in healthy individuals, an S4 is pathological, indicating an issue with ventricular relaxation and filling. Its presence points directly to an underlying cardiac disease, such as uncontrolled hypertension or coronary artery disease. Physicians typically order diagnostic tests, such as an electrocardiogram (EKG) and an echocardiogram, to visualize the heart’s structure. These tests confirm the diagnosis of ventricular hypertrophy or other causes of decreased compliance, and treatment focuses on the underlying disease.