Usually, a low EF indicates there is some heart muscle damaged, and that often occurs as a result of lack of blood/oxygen to the deficit area...the underlying cause can be occluded coronary arteries.
Sometimes opening the blocked vessel provides a blood/oxygen source enough to revitalize the cells for normal functionality. If the cells can't be revitalized, medication can reduced the heart's workload and maintain the heart's ability to adequately pump enough blood to meet system demand. The EF can increase, stay the same or decrease, and it is difficult to know without more information, general health, life style, diet, etc.