The familiar “lub-dub” rhythm of a heartbeat represents the two normal sounds, S1 and S2, which correspond to the closing of the heart’s valves. S1 marks the beginning of the heart’s contraction phase (systole), while S2 signals the start of the relaxation phase (diastole). The Fourth Heart Sound (S4) is an additional, low-frequency sound that occurs during the heart’s filling phase, just before S1. Its presence indicates an extra event within the cardiac cycle and prompts a closer examination of heart function.
The Origin and Timing of the Fourth Heart Sound
The S4 sound is generated during the final moments of diastole, when the heart’s ventricles are filling with blood. It occurs immediately before S1, positioning it late in the relaxation phase, corresponding to the P wave on an electrocardiogram (EKG) tracing. This sound results from the atria contracting forcefully to push the last volume of blood into the ventricles.
This forceful contraction, termed the “atrial kick,” is usually silent in a healthy heart with compliant ventricles. S4 arises when the ventricle (typically the left ventricle) is stiff or non-compliant, offering high resistance to incoming blood flow. When blood is rapidly decelerated against the thickened, rigid ventricular wall, it creates low-frequency vibrations (20 to 30 Hz) audible as S4.
The presence of S4 depends on an effective atrial contraction to generate this “kick” into the ventricle. Therefore, S4 cannot be heard in patients with atrial fibrillation, a condition where the atria merely quiver instead of contracting properly. The sound’s timing is sometimes described using a three-syllable cadence, compared to the rhythm of the word “Tennessee,” where the first syllable represents S4.
Primary Location for Optimal Auscultation
Because S4 is a low-frequency vibration, it requires a specific technique and location for clear auscultation. The low pitch means it is best heard using the bell of the stethoscope, which is designed to pick up lower frequencies. The bell must be placed lightly against the skin, as pressing too firmly can filter out the low-pitched sound.
The most effective location to listen for S4 is over the cardiac apex, corresponding to the mitral valve area. This location is typically found in the fifth intercostal space at the midclavicular line. Hearing a left-sided S4 is enhanced when the patient is positioned in the left lateral decubitus position, which brings the heart closer to the chest wall.
This specific positioning and the light application of the bell maximize the transmission of vibrations from the stiffened left ventricle. If the sound originates from the right ventricle (which is less common), it is heard best at the lower left sternal border. The technique is tailored to ensure optimal detection of this soft, low-pitched sound.
Clinical Significance: When S4 Indicates Pathology
The presence of S4 indicates reduced ventricular compliance, a condition where the heart muscle is abnormally stiff. This stiffness means the ventricle is less able to relax and stretch during diastole, resulting in higher pressure at the end of the filling phase. S4, therefore, serves as an indirect sign of diastolic dysfunction.
The most frequent cause of left ventricular stiffness is chronic, uncontrolled systemic hypertension, which forces the heart to pump against greater resistance. This leads to left ventricular hypertrophy (thickening of the heart muscle walls) that reduces the chamber’s ability to relax and fill. Conditions like severe aortic stenosis (a narrowing of the aortic valve) also increase the heart’s workload and result in this muscle thickening.
Other underlying pathologies include ischemic heart disease, such as a prior myocardial infarction (heart attack), which causes scar tissue formation within the ventricular wall. This rigid scar tissue impairs the ventricle’s ability to stretch, leading to a persistent S4. Hypertrophic cardiomyopathy, a genetic condition causing excessive heart muscle thickness, is also associated with a prominent S4.
While S4 suggests underlying heart disease, it can occasionally be detected in individuals without overt pathology, such as older adults whose ventricles have naturally become less compliant with age. It is also noted in highly trained athletes due to physiological cardiac adaptations, though this finding is less common than the S3 sound in this population. The finding of S4 in a clinical setting prompts further evaluation to rule out serious conditions that compromise heart health.