What would be the immediate effect when, for some reason, mitral valve fail to close completely?

The abrupt onset of mitral regurgitation promptly leads to a reduction in cardiac output and a rise in pulmonary vascular pressure. As a consequence, a portion of the stroke volume is expelled from the ventricle into the left atrium, resulting in a diminished forward cardiac output. This leads to elevated pressures in the left atrium, subsequently transmitted to the pulmonary circulation.