The first heart sound (S1) is the initial noise in the familiar “lub-dub” rhythm of the heartbeat. S1 marks the precise moment when the lower chambers of the heart, the ventricles, begin to contract, signaling the start of systole. This sound is a fundamental reference point for understanding the cardiac cycle and provides a non-invasive indicator of heart function.
Understanding How S1 is Produced
The S1 sound results from the abrupt closure of the two atrioventricular (AV) valves: the mitral valve (left side) and the tricuspid valve (right side). The mechanical event is not the gentle meeting of the valve leaflets, but rather the sudden vibration of the entire valve apparatus and the blood when flow is abruptly blocked.
As the ventricles begin to contract and pressure inside them rapidly increases, it exceeds the pressure in the atria above. This pressure differential forces the mitral and tricuspid valves to snap shut, preventing blood from flowing backward. The resulting vibrations and blood turbulence create the audible S1 sound, which is typically described as being lower in pitch than the subsequent S2 sound.
The S1 sound is composed of two distinct, high-frequency components: the closure of the mitral valve (M1) and the closure of the tricuspid valve (T1). Because the left side of the heart operates under much higher pressure, the mitral valve usually closes a fraction of a second before the tricuspid valve. The M1 component is almost always louder and dominates the overall S1 sound.
The Best Spots to Hear S1
To find where S1 is heard best, one must locate the anatomical areas on the chest wall where the vibrations from the closing valves are most clearly transmitted. The sound does not stay isolated to the precise location of the valve itself but is projected across the chest along the path of blood flow or where the vibrating structures come closest to the surface. S1 is generally loudest and most intense at the apex of the heart.
The Mitral Area (Cardiac Apex)
The Mitral Area, also called the cardiac apex, is the overall location where S1 is most prominent. This spot corresponds to the fifth intercostal space—the gap between the fifth and sixth ribs—and is positioned along the midclavicular line. Since the mitral component (M1) is the louder of the two S1 sounds, it is heard here with the greatest amplitude, reflecting the strong contraction of the left ventricle.
The Tricuspid Area
The Tricuspid Area is the secondary location where the S1 sound is specifically best heard. This area is found along the left lower sternal border, typically around the fourth or fifth intercostal space. Concentrating on this spot helps the listener better isolate the tricuspid component (T1), the quieter portion of the S1 sound.
Understanding the relative loudness of heart sounds across the chest is helpful for identifying S1. At the apex, S1 is normally louder than the second heart sound (S2). Conversely, at the base of the heart (the upper chest near the sternum), S2 is typically louder. This difference in intensity between S1 and S2 at the apex provides a reliable guide to confirming the correct heart sound.
Tips for Successful Listening
When attempting to hear S1 clearly, using the correct technique and equipment is necessary. Since the S1 sound is composed of high-frequency vibrations from the snapping shut of the valves, the diaphragm of the stethoscope should be used. The diaphragm is designed to filter out lower-frequency sounds, making the high-pitched S1 sound more distinct.
The stethoscope should be pressed firmly against the skin at the chosen listening site to ensure optimal transmission of these high-frequency vibrations. A loose fit will allow sound to leak, decreasing the clarity of S1. It is also beneficial to listen systematically, moving the stethoscope from the apex up the sternal border to ensure the entire region where S1 is dominant is covered.
Patient positioning can significantly enhance the clarity of the heart sounds, especially the mitral component. Asking the patient to turn slightly onto their left side, known as the left lateral decubitus position, shifts the heart closer to the chest wall. This maneuver brings the apex nearer to the stethoscope, which often intensifies the sound of the mitral valve closure.
Another useful technique is to synchronize the heart sound with the patient’s pulse. S1 occurs at the very beginning of ventricular contraction and coincides precisely with the palpable upstroke of the carotid artery pulse in the neck. By simultaneously feeling the carotid pulse while listening, one can confirm which sound in the “lub-dub” sequence is the S1.