The heart’s rhythm is typically described by the classic “lub-dub” sounds, which correspond to the normal closure of the heart valves (S1 and S2). The third heart sound (S3) is an extra noise that occurs shortly after S2. Identifying this faint, low-frequency sound provides significant information about the pressure and volume conditions within the heart’s chambers. Auscultating for S3 requires specific techniques and a focused approach, as the sound itself can be subtle and easily missed.
Defining S3 and Its Primary Location
The S3 heart sound is a low-pitched, brief vibration that takes place early in diastole, right after the S2 sound. It is generated by the sudden deceleration of blood rushing from the atria into a ventricle that is already filled or has reduced compliance. This phenomenon is sometimes referred to as a “ventricular gallop” due to the three-sound cadence, which can mimic the rhythm of a galloping horse.
To hear this sound, the stethoscope must be placed over the area where the left ventricle is closest to the chest wall. The primary location for auscultation of a left-sided S3 is the cardiac apex, corresponding to the mitral area. This point is typically found in the fifth intercostal space, along the mid-clavicular line.
Because S3 is a low-frequency sound, the bell of the stethoscope must be used, as it is designed to capture lower-pitched vibrations. The bell should be applied very lightly to the skin over the apex. Pressing down too hard stretches the skin and turns the bell into a diaphragm, filtering out the low-frequency sounds. This delicate technique is crucial for isolating the subtle sound of the S3.
Techniques for Optimal Auscultation
Since the S3 sound is quiet and difficult to hear, specific positioning and maneuvers are employed to enhance its audibility. The most effective position for bringing the heart closer to the chest wall is the left lateral decubitus position. In this maneuver, the patient lies on their left side, which shifts the heart and concentrates the sound at the apex.
Controlling the patient’s breathing pattern eliminates interfering lung sounds. The patient should exhale completely and momentarily hold their breath. This maneuver moves the lungs away from the heart, maximizing the transmission of vibrations to the chest wall.
The listening environment must be quiet, free from background noise that could mask the low-pitched sound. The examiner should focus intensely on the early diastolic period, listening for the sound immediately following S2. Simultaneous palpation of the carotid pulse helps time the heart sounds accurately, as S1 occurs just before the pulse, and S2 and S3 follow.
Interpreting the Presence of S3
Once S3 is identified, its clinical context determines if it is a normal finding or a sign of underlying cardiac dysfunction. In children, young adults under 40, and highly trained athletes, an S3 can be physiological. In these cases, the heart muscle is supple and elastic, and the sound reflects the rapid filling of a healthy ventricle.
In adults over 40, the presence of an S3 is considered pathological and points to a problem with the heart’s ability to handle blood volume. A pathological S3 is a strong indicator of left ventricular failure, particularly systolic heart failure, where the heart cannot effectively pump blood.
The sound is a manifestation of increased ventricular filling pressures or significant volume overload, such as that caused by severe mitral regurgitation. Pathological S3 is associated with conditions like ischemic heart disease or cardiomyopathy. If the S3 disappears when the patient stands up, it is likely physiological; a pathological S3 will persist.