The familiar “lub-dub” rhythm represents the two main heart sounds, S1 and S2, heard during auscultation. The first sound, S1 (“lub”), is caused by the closing of the atrioventricular valves at the beginning of ventricular contraction. The second sound, S2 (“dub”), is generated by the closing of the semilunar valves, marking the end of contraction and the beginning of the heart’s resting phase. Locating where S2 is heard most clearly helps determine the health and timing of the heart’s mechanical cycle.
The Source of the Second Heart Sound
The second heart sound, S2, signals the end of the ventricles’ active pumping phase, known as systole. This sound is generated by the near-simultaneous closure of the two semilunar valves: the aortic valve (A2 component) and the pulmonic valve (P2 component).
These valves shut abruptly when the pressure in the great arteries exceeds the pressure within the relaxing ventricles. This sudden deceleration of the blood column against the closed valve leaflets creates the acoustic vibrations heard as S2. Because the systemic circulation has higher pressure, the aortic valve usually closes fractionally earlier than the pulmonic valve. The A2 component is louder than P2 and radiates widely over the chest due to the high closing pressure in the aorta.
Primary Auscultation Points for S2
To hear S2 clearly, listeners focus on the base of the heart, the upper chest area where the great vessels exit. The sound is not heard best directly over the valves, but over specific chest wall areas where vibrations are maximally transmitted. These locations, known as auscultation points, allow for the separate evaluation of the A2 and P2 components.
The Aortic Area is the primary location for assessing the aortic component, A2. It is situated at the second intercostal space just to the right of the sternum. Since A2 is louder and transmits widely, it is loudest and clearest at this right upper sternal border location.
The Pulmonic Area is the designated site for evaluating the pulmonic component, P2. It is found at the second intercostal space, directly to the left of the sternum. Although A2 is audible here, P2 is dominant and most intense at this spot. Because P2 is normally softer than A2, it is usually only distinctly heard in this specific region.
Erb’s Point is located at the third intercostal space along the left sternal border. This spot is considered ideal for assessing S2 because both the A2 and P2 components are well-heard here. This position makes it the optimal location for detecting subtle timing differences between the two components of the second heart sound.
Understanding Normal Physiological S2 Splitting
Under most circumstances, the A2 and P2 components occur so close together that they are heard as a single, unified sound. However, physiological splitting of S2 is a normal phenomenon that occurs during inspiration, allowing the two components to be heard separately. This splitting is best heard over the pulmonic area or Erb’s Point.
Inspiration causes the chest wall to expand, lowering the pressure within the chest cavity. This results in increased blood return to the right side of the heart, filling the right ventricle with greater volume. The increased volume takes longer to eject, delaying the closure of the pulmonic valve (P2). Simultaneously, the negative chest pressure reduces blood return to the left side, allowing the aortic valve (A2) to close slightly earlier.
The delayed P2 and earlier A2 widen the time interval, causing the “dub” sound to split into two distinct, audible sounds. During expiration, intrathoracic pressure increases, blood volume on the right side decreases, and the timing of A2 and P2 closure returns to near-simultaneous. This respiratory variation is a hallmark of a healthy heart and is referred to as normal physiological splitting.