The heart produces distinct, repetitive sounds generated by the mechanical action of its four valves as they open and close to control blood flow. These acoustic events are described as the first heart sound (S1) and the second heart sound (S2), forming the familiar “lub-dub” rhythm. The medical practice of listening to these internal body sounds is known as auscultation. This procedure allows healthcare providers to assess the heart’s function and the integrity of its valves.
The Purpose of Listening to Heart Sounds
Listening to the heart provides an immediate, real-time assessment of cardiac function during a physical examination. The primary goal is to evaluate the regularity and speed of the heart’s rhythm and rate. Changes in the timing between the sounds can indicate a disruption in the normal electrical or muscular sequence of the heart’s cycle.
Providers also focus on the quality and intensity of the heart sounds to determine the status of the four cardiac valves. The sounds are generated by the sudden halting of blood flow as the valves snap shut, and any abnormality in the valve structure can alter the resulting noise. The procedure helps identify turbulent blood flow, which may manifest as extra sounds or murmurs, indicating potential issues with valve opening or closing.
Mapping the Four Primary Listening Posts
The heart’s four valves are best heard not directly over their anatomical location, but rather at specific spots on the chest wall where their sounds project most clearly, known as auscultatory areas. This occurs because sound waves travel along the path of blood flow, projecting to distant areas on the surface of the body. Providers systematically move across the chest to isolate the acoustic signature of each valve.
The first area is the Aortic post, located in the second intercostal space just to the right of the sternal border. Moving across the sternum to the second intercostal space on the left side locates the Pulmonic post. These two upper areas are often referred to as the base of the heart.
The third area, the Tricuspid post, is found along the lower left sternal border, typically in the fourth or fifth intercostal space. This location is closer to the midline and is associated with the right side of the heart.
The Mitral post, also called the apical area, is situated further down and to the left, located in the fifth intercostal space along the mid-clavicular line. This is the point where the heartbeat’s maximal impulse is often felt and is considered the apex of the heart.
Distinguishing the Lub-Dub Sounds
The “lub” sound, or S1, marks the beginning of the heart’s contraction phase, known as systole. This sound is generated by the simultaneous closure of the two atrioventricular (AV) valves: the mitral and the tricuspid valves. S1 is generally heard as a single sound because the closures of the mitral valve and the tricuspid valve occur nearly instantaneously.
Following S1 is the “dub” sound, or S2, which signals the start of the relaxation phase, diastole. S2 is caused by the closure of the two semilunar valves, the aortic and the pulmonic valves, which sit at the exits of the ventricles. Because pressure is higher in the left side of the heart, the aortic valve typically closes slightly earlier than the pulmonic valve, resulting in a subtle, physiologic split of S2 that can sometimes be heard.
S1 is typically louder and more pronounced when listening over the Mitral and Tricuspid areas, which are closer to the AV valves. Conversely, S2 is loudest over the upper Aortic and Pulmonic areas, reflecting the higher pressure associated with the closure of the semilunar valves at the heart’s base.