Auscultation, the practice of listening to heart sounds, is a fundamental technique for assessing cardiac function. The rhythmic “lub-dub” heard through a stethoscope consists of two primary sounds, S1 and S2, generated by the mechanical action of the heart valves opening and closing during the cardiac cycle. S1, often described as the “lub,” marks the beginning of the heart’s contraction phase (systole). S2, the “dub,” corresponds to the start of the relaxation phase (diastole). The intensity of these sounds changes dramatically depending on stethoscope placement.
The Origin of the Heart Sounds (S1 and S2)
The first heart sound, S1, arises from the near-simultaneous closure of the two atrioventricular valves: the mitral valve and the tricuspid valve. This closure occurs as the ventricles begin to contract, marking the beginning of the systolic phase of the cardiac cycle. Because the left side of the heart operates under much higher pressure, the mitral component (M1) is typically the louder portion of S1, although the two components are usually heard as a single sound.
The second heart sound, S2, is produced by the closure of the two semilunar valves: the aortic valve and the pulmonic valve. This event signals the end of systole and the onset of the diastole phase. The aortic component (A2) is normally louder than the pulmonic component (P2) due to the greater pressure in the systemic circulation. This difference in pressure and timing can sometimes allow S2 to be heard as two distinct sounds, known as physiologic splitting, particularly during inhalation.
Identifying the Point of Equal Intensity
The specific location where S1 and S2 are heard with comparable intensity is known as Erb’s Point. This auscultation site is not named for a specific heart valve, but rather for its unique acoustic properties. It is anatomically located at the third intercostal space, immediately to the left of the sternum. This spot serves as a neutral zone because of its central position on the chest wall, acting as a balanced acoustic window. Sounds originating from all four valves are transmitted here, ensuring both S1 and S2 are heard with a similar degree of loudness and are clearly distinguishable from one another.
How Intensity Varies Across the Chest
Beyond Erb’s Point, the intensity of S1 and S2 shifts dramatically across the traditional auscultation areas, reflecting the proximity of the stethoscope to specific valves.
At the upper right sternal border, known as the Aortic Area, the S2 sound is louder than S1 because the sound from the aortic valve closure is transmitted most directly to this spot. Similarly, at the upper left sternal border, the Pulmonic Area, S2 is also dominant, as this location is best for hearing the pulmonic valve component.
Conversely, S1 intensity increases as the stethoscope moves toward the apex of the heart. The Tricuspid Area, located at the lower left sternal border, is where the S1 sound becomes more pronounced, mainly due to the acoustic transmission of the tricuspid valve closure. Moving further to the Mitral Area, or the cardiac apex, S1 is clearly louder than S2. This area sits at the fifth intercostal space in the mid-clavicular line and is the site where the closure of the mitral valve is heard most clearly. This systematic variation in sound dominance allows healthcare providers to isolate and evaluate the function of each individual valve.