The first heart sound is heard just after the QRS on an electrocardiogram.The heart sound is caused by

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The first heart sound, also known as S1, is caused by turbulence created by the closure of the atrioventricular (AV) valves. The sound occurs just after the QRS complex on an electrocardiogram (ECG) because it coincides with the contraction of the ventricles, which causes the AV valves to close. Blood filling the cups of the semilunar valves (A) is not associated with the first heart sound, and neither is the pulling of the AV valve cusps into the correct position by chordae tendineae (D). The closing of the semilunar valves (C) produces the second heart sound, not the first.

The First Heart Sound Is Heard Just After the QRS