In mitral stenosis, the narrowed mitral valve acts as a fixed obstruction between the left atrium (LA) and left ventricle (LV).

During diastole, blood can enter the LV only if LA pressure is higher than LV pressure. Because the stenotic valve opening is small and rigid, normal flow cannot pass through it easily. Blood therefore accumulates in the LA, raising LA pressure until it is sufficient to force blood across the narrowed valve.

So:

[ \text{Transmitral pressure gradient} = P_{LA} - P_{LV} ]

This is a diastolic pressure gradient. It persists while blood flows from LA to LV, and becomes greater when:

The elevated LA pressure is transmitted backward to pulmonary veins and capillaries, producing pulmonary congestion and eventually pulmonary hypertension. The LV pressure itself is usually normal or low in mitral stenosis because the problem is impaired filling of the LV, not LV outflow obstruction.

Doppler echocardiography estimates the instantaneous gradient from transmitral flow velocity:

[ \Delta P = 4V^2 ]

where (V) is the velocity of blood through the stenotic mitral valve. This is why higher velocity across a stenotic valve indicates a larger pressure difference. The medical text describes MS as incomplete mitral-valve opening during diastole that limits forward flow and produces a sustained LA-to-LV diastolic gradient. Textbook of Clinical Echocardiography