Si is softer than S2 at the base (right and left 2nd interspaces).
Si is often but not always louder than S2 at the apex.
Si is accentuated in (1) tachycardia, rhythms with a short PR interval, and high cardiac output states (e.g., exercise, anemia, hyperthyroidism), and (2) mitral stenosis. In these conditions, the mitral valve is still open wide at the onset of ventricular systole, and then closes quickly.
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