A commissurotomy is a type of heart surgery that repairs a mitral valve that is too narrow as a result from mitral valve stenosis. The mitral valve is located between the left ventricle and the left atrium of the heart. Mitral valve stenosis causes this valve to become stiff or develop scar tissue. The valve becomes too narrow and sometimes the valve flaps fuse together. During a commissurotomy scar tissue and calcium are removed from the valve.