During early rapid filling, when the mitral valve opens at the onset of diastole, the pressure gradient between the left atrium (LA) and left ventricle (LV) is at its peak, allowing blood to flow into the ventricle. As filling progresses during this phase, the left ventricular diastolic pressure rises rapidly and quickly approaches and equalizes with left atrial pressure.

The equalization of pressures is critical to facilitate ventricular filling and is reflected in the mitral inflow Doppler pattern, where the E-wave corresponds to early rapid filling. Diastasis is the mid-diastolic slow filling phase where pressures are nearly equal and little flow occurs. Atrial contraction is the late filling phase, adding a small volume to the ventricle.

This physiological timing is detailed in the 'Textbook of Clinical Echocardiography, 6e', Chapter on Diastolic Function and Hemodynamics, with emphasis on pressure changes during the cardiac cycle20:210-215Textbook of Clinical Echocardiography.