Where is S1 best auscultated?

• A. Second intercostal space, left sternal border, PMI
• B. Second intercostal space, right and left sternal borders
• C. Fifth intercostal space, left sternal border, PMI
• D. Fifth intercostal space, right sternal border, PMI

Answer: (C)

S1 is the first heart sound produced by the closure of the atrioventricular valves at the start of systole, and it is heard best at the apex of the heart. The apex sits in the left chest at the fifth intercostal space along the midclavicular line, where the point of maximal impulse (PMI) is felt. Because this is the area closest to the left ventricle and where the mitral valve closure vibrations are most prominent, S1 is loudest there. In contrast, S2 tends to be louder at the base of the heart (second intercostal spaces near the sternal borders) where the aortic and pulmonic valves close. So the optimal location for auscultating S1 is the left fifth intercostal space at the midclavicular line, corresponding to the apex/PMI.