The familiar “lub-dub” sound of a heartbeat consists of the first heart sound (S1) and the second heart sound (S2), which mark the closing of the heart’s valves. The S1 sound (“lub”) occurs when the mitral and tricuspid valves close at the beginning of ventricular contraction (systole). The S2 sound (“dub”) follows when the aortic and pulmonary valves close as the ventricles relax (diastole). The third heart sound (S3) is an additional, low-frequency sound heard shortly after S2. Its presence can signal either a normal finding in a young heart or an indicator of cardiac health in an older adult.
Timing and Physiological Origin of the S3 Sound
The S3 sound occurs in the early phase of diastole, immediately following the S2 sound. This timing places it during the rapid ventricular filling phase, when blood rushes from the atria into the relaxed ventricles. The sound is generated by mechanical vibrations caused by the sudden deceleration of blood as it meets the limits of the ventricular chamber. This sudden halt causes the ventricular walls to vibrate, producing the low-frequency S3 sound. Because of its low pitch and intensity, a healthcare provider must use the bell of the stethoscope to detect the S3 sound best.
When the S3 Sound is Normal (Physiological Finding)
The presence of an S3 sound is not always a sign of disease and can be a benign finding, particularly in younger individuals. This normal variant is often referred to as a “physiological S3” because it reflects a healthy, compliant heart capable of rapid filling. It is most commonly heard in children and young adults, typically those under the age of 40.
The physiological S3 is also sometimes detected in healthy, well-trained athletes and in individuals experiencing high-output states, such as during the third trimester of pregnancy. In these cases, the sound is likely due to an increased volume of blood returning to the heart, causing a more forceful filling of the ventricles. The sound usually disappears when the young adult reaches middle age or when the individual changes position.
S3 as an Indicator of Cardiac Dysfunction
When an S3 sound is heard in an adult over the age of 40, it is usually considered a pathological finding, signaling underlying cardiac dysfunction. This pathological S3 is a sensitive indicator of ventricular dysfunction, reflecting an issue with the ventricle’s ability to handle incoming blood volume. The presence of this sound, along with S1 and S2, creates a triple rhythm known as a “ventricular gallop,” which has a cadence likened to the word “Kentucky.”
The mechanism involves either volume overload or systolic dysfunction, where the ventricle is weak and cannot pump effectively. In heart failure, the ventricle is often dilated and fails to empty completely during contraction. When the next rush of blood enters this already overfilled ventricle, the sudden impact on the ventricular wall produces the S3 sound.
The most common condition associated with a pathological S3 is congestive heart failure, particularly involving the left ventricle. The sound indicates the heart is struggling to meet the body’s demands, resulting in elevated ventricular filling pressures. Other conditions that significantly increase the volume of blood entering the ventricle can also produce this sound.
These conditions include severe mitral regurgitation, which causes blood to flow backward into the atrium and increases the volume rushing into the ventricle. High-output states like severe anemia or thyrotoxicosis can also increase the speed and volume of blood flow, leading to the S3 sound. Unlike the physiological variant, the pathological S3 often persists regardless of the patient’s position and prompts further medical evaluation.