S1 (loud)

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A loud first heart sound occurs when the leaflets are wide open at the end of ventricular diastole and shut forcefully at the beginning of ventricular systole.

Possible causes of a loud first heart sound include:

  • late atrioventricular closure
    • this occurs when there is rapid flow at the end of diastole e.g. mitral stenosis, the short diastole of tachycardic states and left to right shunts
    • where the P-R interval is less than 0.12 s. This situation results in an opened valve that is not closed at the time of ventricular contraction.


  • atrial fibrillation
  • short diastole - tachycardia
  • atrial premature beat
  • mitral stenosis where high left atrial pressure delays mitral valve closure