Variable importance of the final XGBoost-23 model. Ao.Arch.Procedure: operation on the aortic arch; BMI: basal metabolic index; CardiacRhythm: preoperative cardiac rhythm; CPS: critical pre-operative state; CrCl: creatinine clearance; HospCode: hospital identification code; LVF2: left ventricular function using same criteria as EuroSCOREII; mechanicalSupport: need for pre-operative mechanical support. NYHA: New York Heart Association Score; PrevCABG: previous coronary artery bypass grafts; PrevMI: number of previous myocardial infarctions; PrevOp: any previous cardiac surgery; PrevValve: previous valve surgery; PVD: peripheral vascular disease; Stroke: pre-operative stroke.