S1: Definition, Clinical Significance, and Overview
S1 is the first heart sound heard during cardiac auscultation. It is a normal physical-exam finding that marks the start of ventricular systole. S1 is generated mainly by closure of the atrioventricular (AV) valves (mitral and tricuspid). Clinicians use S1 when timing murmurs and assessing valve and conduction physiology.