The heart produces two primary sounds with each beat, often described using the familiar “lub-dub” cadence. The first sound is S1 (“lub”), and the second is S2 (“dub”). The S2 sound signals a major change in the heart’s pumping cycle. Heart sounds are not created by the valves physically slapping shut, but rather by the sudden deceleration of blood and the resulting vibration of the valves, vessel walls, and surrounding tissues immediately after valve closure.
The Mechanical Source of the Sound
The S2 sound is directly caused by the closure of the heart’s two semilunar valves: the aortic valve and the pulmonic valve. These valves separate the ventricles from the great arteries—the aorta and the pulmonary artery, respectively. As the ventricles complete contraction and begin to relax, the pressure inside them rapidly drops.
When the pressure in the left ventricle falls below the high pressure in the aorta, the aortic valve snaps shut to prevent backflow. The pulmonic valve closes similarly when the pressure in the right ventricle falls below the pressure in the pulmonary artery. This rapid closure stops the column of blood, causing it to slam against the closed leaflets.
This sudden stop generates turbulence and vibration within the blood and the arterial walls. These resulting vibrations, rather than the sound of the valve leaflets themselves, travel through the chest to be heard as the “dub” sound. Because systemic circulation pressures are much higher than pulmonary circulation pressures, the aortic valve closure (A2) is louder than the pulmonic valve closure (P2).
Placement in the Cardiac Cycle
The S2 sound serves as a precise marker in the timing of the cardiac cycle, signaling the transition between the two main phases of the heartbeat. It occurs at the end of ventricular systole, the period of contraction and blood ejection. During systole, the ventricles push blood out through the aortic and pulmonic valves.
The closure of the semilunar valves to produce S2 marks the beginning of ventricular diastole, the period of relaxation and filling. The heart then moves into a phase where the ventricles are filling with blood from the atria. The S2 sound is the acoustic event that separates the heart’s ejection phase from its filling phase.
Understanding Physiological Splitting
The second heart sound is composed of two separate components: the aortic valve closure (A2) and the pulmonic valve closure (P2). Because the left side of the heart operates under higher pressure and empties faster, A2 normally precedes P2. This slight timing difference is usually too short to hear as two distinct sounds during exhalation, making S2 sound singular.
However, during inhalation, the timing difference between A2 and P2 widens, creating physiological splitting. Inhalation causes a drop in chest cavity pressure, increasing the blood returning to the right side of the heart. This extra volume prolongs the right ventricle’s ejection time, causing P2 to close slightly later than usual.
The increased blood volume also temporarily delays the return of blood to the left side, allowing A2 to close slightly earlier. The combined effect of a delayed P2 and an earlier A2 is an audible separation of the two sounds, which is normal in healthy individuals. The splitting disappears during exhalation when A2 and P2 return to near-simultaneous closure.
When Splitting Becomes a Concern
While physiological splitting is normal, certain respiratory patterns of the S2 sound can indicate underlying cardiac issues. One pattern is fixed splitting, where A2 and P2 sounds are permanently separated and do not change timing with breathing. This suggests a constant volume load on the right ventricle, often seen when blood continuously shunts from the left side of the heart to the right side.
A different pattern is paradoxical splitting, which is the reverse of the normal physiological pattern. Here, the two sounds are split during exhalation but become a single sound during inhalation. Paradoxical splitting occurs when the aortic valve closure (A2) is significantly delayed, causing it to close after the pulmonic valve (P2).
This delay in A2 suggests a problem with the left side of the heart, such as a prolonged contraction time or a blockage to the left ventricular outflow. Any deviation from the normal inspiratory splitting pattern warrants further investigation. Auscultation of the S2 sound provides a non-invasive way to detect changes in cardiac function.