Which of the following is NOT associated with a loud first heart sound – Correct answer: c) Prolonged PR interval
In mitral stenosis, abrupt closing of the stiff mitral valve at the onset of ventricular systolic causes the loud first heart sound. In tachycardia, shortening of diastole leaves the mitral valve fairly in the open position at the end of diastole so that there is a larger closing excursion. When there is no tachycardia, mitral leaflets gradually float back towards a semi closed position before the onset of systole. Same mechanism occurs in short PR interval causing loud first heart sound and long PR interval causing a soft first heart sound.