Severe mitral regurgitation (MR) is typically characterized by significant left atrial dilation due to volume overload, and systolic flow reversal in the pulmonary veins caused by retrograde flow from the left ventricle into the left atrium during systole.

An eccentric mitral regurgitant jet is common in severe MR, often due to leaflet prolapse or flail, resulting in directed jets that hug the atrial wall.

Mitral regurgitant jet velocity, however, is usually significantly higher in severe MR due to the high pressure gradient between the left ventricle and left atrium during systole. A jet velocity less than 0.5 cm/sec is extremely low and inconsistent with severe MR. Typically, MR jet velocities are in the range of several meters per second.

Thus, a mitral regurgitant jet velocity less than 0.5 cm/sec is NOT associated with severe MR.

This is detailed in echocardiography literature discussing MR quantification and Doppler findings, emphasizing high-velocity regurgitant jets in severe MR and hemodynamic consequences seen on pulmonary vein flow and LA size .