The heart generates distinct sounds with each beat, often described as a “lub-dub” rhythm. These sounds are generated by the closing of heart valves, which ensures blood flows in one direction through the heart’s chambers. The first sound, “lub,” marks the beginning of heart contraction, while the second sound, “dub,” signals the end of this phase and the start of relaxation.
The Second Heart Sound Defined
The second heart sound, S2, marks a specific point in the cardiac cycle. This sound occurs at the end of ventricular systole, the period when the heart’s ventricles contract and eject blood into the major arteries. It simultaneously signifies the beginning of ventricular diastole, the phase when the heart muscle relaxes and the ventricles start to fill with blood.
S2 is produced when specific heart valves close, creating vibrations within the heart and blood vessels. These vibrations result from the sudden deceleration of blood as the valves snap shut under pressure.
Valves Behind the Sound
The second heart sound originates from the abrupt closure of two heart valves: the aortic valve and the pulmonic valve. These are known as the semilunar valves. The aortic valve is positioned between the left ventricle and the aorta, the body’s main artery, while the pulmonic valve lies between the right ventricle and the pulmonary artery, which carries blood to the lungs.
As the ventricles finish ejecting blood, the pressure within them drops below the pressure in the aorta and pulmonary artery. This pressure difference causes the aortic and pulmonic valves to snap shut, preventing blood from flowing backward into the ventricles. The sudden closure of these valves generates the distinct S2 sound.
Understanding S2 Splitting
While S2 is often heard as a single sound, it comprises two distinct components: A2, from aortic valve closure, and P2, from pulmonic valve closure. Normally, the aortic valve closes slightly before the pulmonic valve. This slight timing difference, particularly noticeable during inspiration, leads to physiological splitting of S2.
During inspiration, changes in intrathoracic pressure cause more blood to return to the right side of the heart. This increased blood volume in the right ventricle prolongs its ejection time, delaying the closure of the pulmonic valve (P2). Simultaneously, a slight decrease in blood return to the left side causes the aortic valve (A2) to close slightly earlier. This increased time difference between A2 and P2 during inspiration results in a “split” sound, which typically merges back into a single sound during expiration.
Why the Second Heart Sound Matters
Listening to the second heart sound provides valuable insights into cardiovascular health. Healthcare professionals assess the intensity, timing, and presence of S2 splitting during physical examinations. Variations in these characteristics can offer important clues about the function of heart valves and the overall state of the circulatory system.
For example, an abnormally loud S2 can suggest high pressure in the pulmonary artery, known as pulmonary hypertension. Conversely, a diminished or absent S2 component might indicate issues with a specific valve, such as a narrowed aortic or pulmonic valve. Abnormal splitting patterns, such as a fixed split (where the split does not change with respiration) or paradoxical splitting (where the split occurs during expiration), can also signal underlying heart conditions. S2 serves as a diagnostic indicator for various cardiac conditions.