The rhythmic “lub-dub” sound of a heartbeat, known medically as S1 and S2, provides immediate, audible insight into cardiac function. These sounds are produced by the rapid closing of the heart’s four valves as blood is pumped through the chambers. Health professionals listen to S1 and S2 to evaluate the basic rhythm and function of the heart, forming a foundation for assessing overall cardiac health.
The Physiology Behind S1 and S2
The two distinct heart sounds, S1 and S2, correspond precisely to events in the cardiac cycle. S1, commonly described as the “lub,” signals the beginning of ventricular contraction (systole). This sound is generated by the simultaneous closing of the two atrioventricular (AV) valves: the mitral valve and the tricuspid valve. Pressure increases as the ventricles contract, forcing these valves shut to prevent blood from flowing backward into the atria.
The second heart sound, S2, or the “dub,” marks the end of systole and the start of ventricular relaxation (diastole). This sound is caused by the closure of the two semilunar valves: the aortic valve and the pulmonic valve. As the ventricles relax, pressure drops, causing the backflow of blood to snap these valves shut. The sequence of S1 followed by S2 represents one complete heartbeat cycle.
Mapping the Heart Sounds: Auscultation Points
Listening to the heart sounds, or auscultation, relies on placing a stethoscope over specific points on the chest wall. These locations are not directly over the anatomical position of the valves but rather where the sound waves project most clearly onto the chest surface. Identifying these spots requires using anatomical landmarks like the ribs and the sternum.
The four primary auscultation points are:
- Aortic Area: Located at the second intercostal space, just to the right of the sternal border. This is where the aortic valve closure (a component of S2) is best heard.
- Pulmonic Area: Located at the second intercostal space, just to the left of the sternal border. This site is best suited for hearing the pulmonic valve component of S2.
- Tricuspid Area: Found at the fourth or fifth intercostal space near the left sternal border. The sound produced by the tricuspid valve closure (a component of S1) is most audible here.
- Mitral Area (Apex): Located in the fifth intercostal space on the left side, along the midclavicular line. This area provides the clearest sound from the mitral valve closure, the loudest part of S1.
What Each Listening Area Reveals
Listening at these four specific areas is crucial because the sounds from the four valves project differently across the chest. Listening at the Mitral Area allows for the clearest evaluation of the mitral valve’s function, while the Aortic Area provides the best acoustic window into the aortic valve. The sound heard at a specific point is the loudest projection of that particular valve’s activity.
This location-specific listening helps distinguish between the two components of the S2 sound: the aortic (A2) and pulmonic (P2) valve closures. At the Pulmonic Area, it is sometimes possible to hear a “splitting” of S2, where the two components are heard separately. This physiological splitting occurs because the aortic valve normally closes slightly before the pulmonic valve, a difference that often widens during inspiration.
By focusing on each area, a listener can evaluate the intensity, pitch, and timing of each valve’s contribution to the S1 and S2 sounds. An unusually loud or soft sound at the Aortic Area could suggest an issue with the aortic valve, such as stiffening or incomplete closure. Isolating these sounds provides valuable, early information about the mechanical health of the heart.