S1 – The first heart sound occurs right after ventricular systole, when both AV valves shut. Although only one sound is perceptible, two components are present, relative to the closure of each AV valve,and are known as T1 for the tricuspid valve and M1 for the mitral valve. As the left ventricle contracts first, the M1 component will occur earlier. This delay between M1-T1 has a major clinical importance and is considered physiological if under 30 ms. Above this value it may indicate a pathological split of S1. S1 has a typical duration of 100-200 ms and its frequency spectrum is located mainly between 10-200 Hz