S1

S1 refers to the first heart sound, which is the noise heard when the heart's ventricles (lower chambers) contract and close the valves between the heart and the arteries. This sound signals the beginning of a heartbeat cycle and is primarily caused by the closing of the mitral and tricuspid valves. S1 is typically heard as a "lub" during a heartbeat and is usually louder and longer than the second heart sound (S2). It helps in assessing heart function and detecting potential abnormalities in valve operation or heart rhythm.