The heart produces a continuous rhythm of sounds, often described simply as “lub-dub.” These sounds are created by the mechanical action of the heart valves opening and closing, causing vibrations within the heart chambers and great vessels. The first heart sound (S1) is the “lub,” and the second heart sound (S2) is the “dub.” Analyzing the timing and quality of these sounds is a fundamental method used to assess the mechanical health and function of the heart. S2 offers significant information about the pressures and valve function within the heart’s outflow tracts.
The Mechanism Behind the “Dub” Sound
S2 marks the end of systole (contraction) and signals the beginning of diastole (relaxation). This sound is generated by the simultaneous closure of the two semilunar valves: the aortic valve and the pulmonic valve, which are positioned at the exits of the ventricles.
When the ventricles finish ejecting blood and relax, pressure within them falls rapidly. This pressure drop causes blood in the great arteries to momentarily flow backward toward the ventricles, forcing the semilunar valves to snap shut abruptly. The resulting deceleration of the blood column creates the distinctive S2 sound.
The two components of S2 are designated A2 (aortic valve closure) and P2 (pulmonic valve closure). A2 is typically louder and occurs slightly before P2 due to the higher pressure within the systemic circulation compared to the pulmonary circulation.
Locating the Loudest Point of S2
The second heart sound is heard with the greatest intensity over the area known as the “base” of the heart. This region is located at the top of the heart, along the upper part of the breastbone, where the great vessels exit. The loudness is directly related to the proximity of the stethoscope to the closing semilunar valves and the high-pressure environment they operate within.
To hear S2 most clearly, clinicians focus on two specific auscultation points along the second intercostal space. The Aortic Area is located on the right side of the breastbone, and the Pulmonic Area is located on the left side. These two points lie directly over the anatomical positions of the aortic and pulmonic valves.
The aortic component (A2) is the loudest element of S2 in most healthy adults. This is because the aortic pressure, which causes the valve to close, is significantly higher than the pulmonary pressure. Consequently, S2 is often heard most loudly at the Aortic Area, on the upper right side of the chest.
The Phenomenon of Normal S2 Splitting
Under certain conditions, the single “dub” sound of S2 can be heard as two distinct sounds, referred to as S2 splitting. This is a normal physiological phenomenon that occurs exclusively during inspiration. During expiration, the two components, A2 and P2, merge closely together, sounding like a single sound.
The mechanism involves the respiratory cycle’s effect on blood flow. When a person inhales, the negative pressure in the chest cavity increases the volume of blood returning to the right side of the heart. This increased volume extends the time required for the right ventricle to eject its blood, causing the pulmonic valve (P2) to close slightly later than the aortic valve (A2).
This temporary delay separates the two components enough to be perceived as a distinct “A2-P2” sequence. Physiological splitting is best detected over the Pulmonic Area, on the left side of the upper breastbone.
Basic Clinical Context of S2
The assessment of the second heart sound provides important information about the pressures in the major arteries and the flexibility of the semilunar valves. Changes in the intensity of S2 can suggest underlying circulatory or structural issues.
For instance, an unusually loud S2, particularly an accentuated P2 component, may suggest high pressure in the pulmonary artery, a condition known as pulmonary hypertension. Conversely, a diminished or soft S2 may indicate that a valve is stiffened or immobile, such as in severe aortic stenosis.
The timing of the sound also holds diagnostic value. If the S2 sound remains split during both inspiration and expiration, it may point toward a structural issue, such as a volume overload on the right side of the heart. Abnormal splitting patterns or variations in the intensity of the S2 components serve as important clues for medical professionals.