Surgery for Mitral Regurgitation

I've heard of doctors/clinics that perform surgery on dogs with MR. Are there any criteria for determining whether surgery is applicable for individual cases? For example, is there any common knowledge like: "Patients who have experienced pulmonary edema aren't a candidate for surgery since the outcome aren't very good" or "Cases that are rather mild which can be classified as around grade 2 on the Levine scale give better results with medical therapy"? Any information would be helpful.