The rhythm of the heart produces distinct sounds that can be heard using a stethoscope, a process known as auscultation. These sounds provide valuable information about the mechanical function of the heart’s chambers and valves. The two most prominent sounds are traditionally described as “lub-dub,” corresponding to the first heart sound (S1) and the second heart sound (S2). S2 is the “dub” sound, marking the end of the heart’s contraction phase (systole) and the beginning of the relaxation phase (diastole).
Understanding the Heart’s Valves and Sounds
The S2 heart sound is generated by the rapid, almost simultaneous closure of the two semilunar valves: the aortic valve and the pulmonic valve. This mechanical event signifies the conclusion of ventricular systole, the period when the ventricles contract to eject blood. Once blood is ejected into the great arteries, the higher arterial pressure forces the semilunar valves shut. The vibrations caused by this abrupt closure create the S2 sound.
In contrast, the S1 sound is produced earlier by the closure of the atrioventricular valves (mitral and tricuspid) at the beginning of systole. Because the aortic and pulmonic valves are located at the base of the heart, S2 is typically louder than S1 in the upper region of the chest. The intensity of S2 reflects the pressure within the aorta and the pulmonary artery when the valves close.
Primary Listening Post: Where S2 is Heard Loudest
The S2 heart sound is consistently heard loudest at the base of the heart, which is the upper part of the chest where the great vessels exit the heart. This sound is composed of two primary components: the aortic component (A2), caused by the closure of the aortic valve, and the pulmonic component (P2), caused by the closure of the pulmonic valve. Auscultation of S2 is maximized at the two specific areas corresponding to the anatomical positions of these valves.
The first location is the Aortic Area, found at the second intercostal space immediately to the right of the sternum. The second location is the Pulmonic Area, situated at the second intercostal space immediately to the left of the sternum. These positions, collectively known as the base of the heart, are where the stethoscope is placed to capture the sound waves generated by the semilunar valves. The A2 sound, however, is generally louder and radiates more widely across the chest than P2, which is more localized to its specific area.
Why Acoustics Dictate the Loudness
The difference in loudness between the two components of S2 is primarily determined by the pressure within the systemic and pulmonary circulatory systems. The aortic component (A2) is usually significantly louder than the pulmonic component (P2) because the systemic arterial pressure is far greater than the pulmonary arterial pressure. For instance, the closing pressure in the aorta is typically around 80 mmHg, while the pressure in the pulmonary artery is much lower, often around 10 mmHg. The greater the pressure against the valve leaflets at the moment of closure, the more forceful the resulting vibration and sound transmission.
The anatomical relationship between the valves and the chest wall also plays a role in sound perception. While A2 is inherently louder and can be heard over the entire precordium, P2 is normally soft and confined to the Pulmonic Area. The base of the heart provides the most direct acoustic window for both the aorta and the pulmonary artery. This ensures the S2 sound is heard most clearly and intensely in this region.
The Phenomenon of S2 Splitting
A unique characteristic of S2 is its ability to “split,” meaning the two components, A2 and P2, can be heard separately. This physiological splitting is a normal finding that is influenced by the respiratory cycle, and it is best identified in the Pulmonic Area. During expiration, the aortic and pulmonic valves close very close together in time, often sounding like a single, unified “dub”.
When a person inhales, the negative pressure created in the chest cavity increases the flow of blood back into the right side of the heart. This extra volume of blood takes slightly longer for the right ventricle to eject, which in turn delays the closure of the pulmonic valve (P2). The aortic valve (A2) closure time remains relatively unchanged, so the delay in P2 closure causes the two sounds to separate and become audibly distinct.
The widening of the A2-P2 interval during inspiration is the definitive sign of normal physiological splitting. If the split remains wide and does not change between inspiration and expiration, it is termed fixed splitting, which may suggest a structural issue like an atrial septal defect. Conversely, paradoxical splitting occurs when the split is heard during expiration but disappears during inspiration, which can be an indicator of conditions that delay the closure of the aortic valve.