caused by the abrupt halt of the closing movement of the mitral valve (M1) which is followed by the softer sound caused by the closing of the tricuspid (T1)
the first heart sound is initiated by the contraction of the left ventricle (LV) followed by the right ventricle (RV)
the differential contraction of the LV and the RV causes a splitting of the first heart sound
an abnormally large splitting of the first heart sound is heard when closure of the tricuspid is delayed as in right bundle branch block, pacing from an electrode on the LV, or with LV ectopics
intensity of the first heart sound is closely related to the timing of the final halt of the closing atrioventricular valves (mitral mainly) in relation to the ventricular pressure pulse:
soft first heart sound is produced if closure occurs during the initial slow rise of LV pressure
a loud first heart sound is produced if valve closure occurs on the steep part of the LV pressure pulse
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