The rhythmic “lub-dub” heard when listening to the chest is the sound of the heart’s valves opening and closing as it pumps blood. These two distinct sounds, S1 and S2, are fundamental acoustic markers of cardiac activity. They provide insight into the heart’s mechanical function and the sequence of a normal heartbeat. These sounds are created by vibrations set up when blood rapidly decelerates and the valve leaflets snap shut, preventing backflow. The timing and character of S1 and S2 serve as the baseline against which medical professionals assess cardiac health.
S1: The “Lub” Sound and Its Origin
The first heart sound, S1, is the “lub” component of the heartbeat and marks the beginning of the heart’s contraction phase, known as systole. It is generated by the nearly simultaneous closure of the two atrioventricular (AV) valves: the mitral valve and the tricuspid valve. These valves control blood flow between the atria and the ventricles.
Closure occurs when the pressure within the ventricles quickly exceeds the pressure in the atria. This pressure increase forces the valve leaflets shut to prevent blood from flowing backward into the atria. The mitral valve component (M1) is typically louder and closes marginally before the tricuspid valve component (T1) due to the higher pressures in the left side of the heart. S1 is generally lower in pitch and longer in duration compared to the second heart sound.
S1 is best heard at the apex of the heart, where the mitral valve’s closure is most audible. The intensity of S1 is influenced by the position of the AV valve leaflets before contraction and the force of the ventricular muscle contraction. Conditions that keep the valves wide open at the onset of contraction can result in a louder S1.
S2: The “Dub” Sound and Its Origin
The second heart sound, S2, is the characteristic “dub” and represents the end of ventricular contraction. This sound is produced by the closure of the semilunar valves: the aortic valve and the pulmonic valve. These valves prevent blood ejected into the major arteries from flowing back into the heart.
Closure occurs when the ventricles finish contracting and begin to relax, causing the pressure within the great arteries to exceed the pressure inside the ventricles. The aortic valve component (A2) typically closes before the pulmonic valve component (P2), forming the two parts of S2. This slight delay is normal because the left side of the heart empties more quickly than the right side due to higher pressures.
The S2 sound is typically shorter and sharper than S1 and is best heard at the base of the heart, near the second intercostal space. The two components of S2 often merge into a single sound during exhalation. During inhalation, increased blood return to the right side slightly delays the closure of the pulmonic valve, causing a brief and audible separation of A2 and P2, known as physiological splitting.
Defining the Heartbeat: Systole and Diastole
The alternating sequence of S1 and S2 defines the two distinct phases of the cardiac cycle: systole and diastole. Systole is the contraction phase, during which the ventricles actively pump blood out of the heart. This phase begins immediately with the S1 sound and ends with the S2 sound.
The time interval between S1 and S2 is the period of ventricular ejection. Following S2, the heart enters diastole, the relaxation and filling phase, which lasts until the next S1 sound. During diastole, the ventricles relax and the AV valves open, allowing them to passively fill with blood from the atria.
At a normal heart rate, the diastolic period is longer than the systolic period, creating the familiar rhythm of a shorter interval (systole) and a longer interval (diastole) between the sounds. This S1-S2-pause pattern illustrates the continuous cycle of contraction and rest.
When Sounds Go Wrong: The Significance of Extra Heart Sounds
Changes in the intensity of S1 and S2 or the presence of additional sounds can signal underlying cardiac issues. Listening to heart sounds is a foundational method for detecting deviations from normal function. For example, a weakened S1 can suggest a problem with AV valve closure or reduced ventricular contraction strength.
Extra heart sounds, such as S3 and S4, represent deviations from the standard “lub-dub” pattern. S3 occurs early in diastole, caused by vibrations from rapid ventricular filling when the heart muscle is overly compliant or volume overloaded. S4 is a late diastolic sound, often associated with the atria forcing blood into a stiff ventricle.
Heart murmurs are distinct from these discrete sounds and are caused by turbulent blood flow, often due to a narrowing (stenosis) or leaking (regurgitation) of one of the heart’s four valves. The timing of a murmur, such as between S1 and S2 (systolic) or between S2 and S1 (diastolic), provides important clues about which valve is affected and the nature of the problem. Analyzing the characteristics of S1, S2, and any additional sounds helps guide further diagnostic testing.