NPPB and mitral valve insufficiency: A retrospective cohort study based on preoperative NT-proBNP measurements in an unselected cohort including all patients undergoing first time surgery for coronary artery disease (CAD; n = 2226), aortic stenosis (AS; n = 406) or mitral regurgitation (MR; n = 346) from April 2010 to August 2016 in the southeast region of Sweden (n = 2978).