What is the ‘first heart sound’ in Mitral Stenosis?

First heart sound (S1) is typically loud in MS.

Mechanism of Loud S1 in MS

Because of high pressure in the left atrium, the left ventricle [LV] needs to reach a higher pressure before it can close the mitral valve and hence LV pressure has more time to accelerate
wide closing excursion of the leaflets – The wide closing excursion of the leaflets happens because the transmitral gradient keeps the valve wide open at the end of diastole rather than allowing the valve to go to a semi-closed position as happens normally without mitral stenosis.

Intensity of the first heart sound is reduced – if the valve is thickened and calcified.
Presence of a loud S1 suggests – that the patient may be a likely candidate for balloon mitral valvuloplasty.
Variable intensity of S1 – With the development of atrial fibrillation – intensity of the S1 will vary depending upon the RR intervals with a louder S1 audible in shorter cycles.