Variables
Annual mean volume for surgeon and hospitals were determined by dividing the total number of procedures by the number of years for which the individual surgeon or hospital had data collected. Surgeon and hospital annual procedural volume were then categorised as ≤ 5, 5.1-10, 10.1-20, and 20.1-45, for surgeons and ≤ 10, 10.1-20, 20.1-30, and 30.1-75, for hospitals. We chose these points rather than quartiles as these gave arbitrary cut-off points with minimal clinical applicability.
Regarding procedural variables, we defined ‘CABG’ as any concomitant coronary artery bypass, irrespective of number of grafts. We defined any ‘major concomitant cardiac surgery’ as any other valve surgery, left ventricular reconstruction, ventricular septal defect repair, left ventricular outflow tract myectomy, pericardiectomy, pulmonary thromboendarterectomy or embolectomy, resection of cardiac tumour, any aortic procedure and any other surgery for congenital cardiac disease (excluding atrial septal defect). A third category of ‘atrial arrhythmia surgery’ was utilised; the specific lesion set was not captured. Excluded from these three categories of concomitant procedures, and therefore not defined as variables for logistic regression analysis, were atrial septal defect closures, minor cardiac procedures such as insertion of a left ventricular pacing lead, and exclusion/excision of the left atrial appendage, as well as thoracic operations. These were not deemed to add significant risk or alteration to surgical decision-making with respect to the mitral valve surgery. Procedural urgency was defined as per the ANZSCTS Database as elective, urgent or emergency; there were no salvage procedures in this cohort [11].