The human heart produces rhythmic sounds, often described as a “lub-dub,” which represent the mechanical activity of the cardiac cycle. The first heart sound, known as S1 or the “lub,” is the first of these two characteristic noises heard during a heartbeat. Listening to these sounds is a fundamental part of a physical examination, providing important information about the heart’s function and the condition of its valves.
The Mechanical Event: What Creates the Sound
The first heart sound is not simply the noise of valves snapping shut, but rather the result of complex fluid and tissue vibrations immediately following valve closure. S1 is generated by the rapid deceleration of blood and the resulting vibration of the valve leaflets and supporting structures. This occurs when the pressure within the ventricles quickly exceeds the pressure in the atria, forcing the atrioventricular (AV) valves closed.
S1 is primarily composed of two distinct, high-frequency components: M1 and T1. M1 is caused by the closure of the mitral valve, located between the left atrium and the left ventricle. This is typically the louder and dominant part of the sound because the left side of the heart operates under much higher pressure.
T1 follows M1 almost immediately and is caused by the closure of the tricuspid valve, situated between the right atrium and the right ventricle. The sound is due to the sudden tensing of the chordae tendineae—the fibrous cords that anchor the valve leaflets—and the reverberation of blood within the ventricular chambers. Since the timing difference between M1 and T1 is usually very short, the human ear typically perceives S1 as a single sound.
S1’s Role in the Cardiac Cycle
The first heart sound marks a precise moment in the heart’s pumping rhythm, signaling the transition from filling to contraction. S1 occurs at the beginning of ventricular systole, the period when the heart’s ventricles contract to push blood out to the body and lungs. Conversely, this sound simultaneously marks the end of diastole, the heart’s resting and filling phase.
The pressure changes that cause S1 initiate the isovolumetric contraction phase of systole, where the ventricles are sealed off and begin to build pressure rapidly. The closing of the AV valves ensures that when the ventricles contract, blood is ejected forward through the aortic and pulmonary valves, rather than backward into the atria.
This timing has a distinct relationship with the peripheral pulse, such as the carotid pulse. When an examiner listens to the heart and palpates the carotid artery, S1 occurs just before the palpable upstroke of the pulse. This correlation is a practical tool used to distinguish S1 from the second heart sound, S2, which follows the pulse.
Identifying S1: Pitch, Duration, and Location
S1 has specific acoustic properties that allow it to be identified through auscultation, the practice of listening to the internal sounds of the body, usually with a stethoscope. The sound is generally described as having a lower pitch and a longer duration than the second heart sound (S2), which is higher-pitched and sharper. This lower-pitched, slightly booming quality is often best heard using the bell of the stethoscope.
The sound’s intensity and clarity vary depending on where the stethoscope is placed on the chest, which helps isolate the two components. The mitral component (M1) is most clearly heard at the cardiac apex, located near the fifth intercostal space at the mid-clavicular line. This location corresponds anatomically to the position of the mitral valve.
The tricuspid component (T1) is best heard closer to the lower left sternal border, often in the fourth intercostal space. Due to the rapid succession of M1 and T1, the first heart sound is typically perceived as a single entity, especially over the apex. Recognizing these distinct auscultation areas provides a method to listen for changes in the heart’s mechanical function.