The sounds your heart makes, often described as a “lub-dub,” are the audible evidence of your heart valves closing as blood is pumped through the body. The first heart sound, known as S1, is the “lub,” produced by the closure of the mitral and tricuspid valves at the beginning of the heart’s contraction cycle. The second heart sound, or S2, is the “dub,” which marks the end of the contraction cycle and the beginning of the rest period. S2 is created by the closing of the aortic and pulmonic valves.
The Anatomy of the Second Heart Sound (S2)
The second heart sound, S2, is composed of two separate valve closures. These two components are the aortic valve closure, designated as A2, and the pulmonic valve closure, designated as P2. The aortic valve separates the left ventricle from the systemic circulation, while the pulmonic valve separates the right ventricle from the pulmonary circulation.
The left side of the heart, which is responsible for pumping blood to the entire body, operates under significantly higher pressure than the right side, which only pumps blood to the lungs. Because of this pressure difference, the aortic valve typically closes slightly earlier than the pulmonic valve. When the time difference between A2 and P2 is extremely small—less than 20 milliseconds—the two sounds merge into what is heard as a single “dub” sound. This simultaneous closure is the normal finding during expiration.
Understanding the Split (Physiological Splitting)
A “split S2” occurs when the A2 and P2 sounds separate enough in time to be heard as two distinct sounds, rather than one. This separation is often a normal event called physiological splitting, which is directly linked to the act of breathing. During inspiration, the negative pressure created in the chest cavity causes an increase in the volume of blood returning to the right side of the heart.
This increased blood volume in the right ventricle requires longer to eject, which delays the closure of the pulmonic valve (P2). Simultaneously, inspiration causes a slight reduction in blood flow returning to the left side of the heart, which causes the aortic valve (A2) to close slightly earlier. The combined effect of a slightly earlier A2 and a delayed P2 causes the gap between the two sounds to widen, making the split S2 audible. When a person breathes out, the blood flow returns to its balance, and the A2 and P2 components merge back together into a single sound.
Distinguishing Normal from Abnormal Splitting
While the physiological split is a benign finding, certain underlying heart conditions can cause the S2 sound to split abnormally. Physicians categorize these abnormal splits based on how they behave during the breathing cycle. One type is fixed splitting, where the A2 and P2 sounds are permanently separated and the interval between them does not change between inspiration and expiration. This pattern is a strong indicator of an atrial septal defect (ASD), a hole between the upper chambers of the heart, because the constant flow of blood through the defect eliminates the normal respiratory variation in right ventricular volume.
Another pattern is wide splitting, which means the split is present during both inspiration and expiration, but it widens further upon inspiration, exaggerating the normal pattern. This commonly occurs in conditions that delay the right ventricle’s ability to contract, such as a complete right bundle branch block (RBBB). The electrical delay in RBBB causes the right ventricle to contract later, pushing the P2 component back in time regardless of the breathing cycle.
The third type is paradoxical splitting, which is the reverse of the normal pattern; the split is heard during expiration and disappears during inspiration. This happens because the closure of the aortic valve (A2) is significantly delayed, causing it to occur after the pulmonic valve closure (P2). Conditions like a left bundle branch block (LBBB) or severe obstruction of the aortic valve can delay the left ventricle’s contraction, pushing A2 later than P2. When the patient breathes in, the normal delay of P2 moves it closer to the already delayed A2, causing the two sounds to temporarily merge.
Clinical Significance and Further Evaluation
A physician listening to a split S2 uses the pattern of the split as a diagnostic clue. The presence of a physiological split confirms that the heart’s electrical and mechanical timing is functioning normally. Conversely, hearing a fixed, wide, or paradoxical split immediately suggests an issue that requires further investigation.
The finding of an abnormal split S2 is not a final diagnosis, but an indication that structural or electrical problems may be present. To determine the precise cause, the physician orders non-invasive imaging tests. An echocardiogram (Echo) visualizes the heart’s structure, measures blood flow, and assesses valve function. An electrocardiogram (EKG) can identify electrical conduction delays, such as bundle branch blocks.