Inclusion and exclusion criteria will be the same as the Balanced trial with participants limited to those sites described above. Survival data will be censored at date of accessing mortality database.  Inclusion  Age over 60 years and over ASA physical status 3 or 4 Surgery expected to last over 2 hours Hospital length of stay expected to be 2 nights or more General anaesthesia with or without major regional block Able to monitor BIS throughout anaesthesia  Exclusion  Unable to monitor BIS Unable to consent Surgery with 'wake up' test Propofol infusion for part or all of maintenance of anaesthesia Previous enrolment in Balanced study  Exclusion The intention-to-treat (ITT) population is defined as all randomised participants who met the inclusion and exclusion criteria who had surgery.  The per-protocol (PP) population is defined as all randomised participants in the study who meet all the inclusion/exclusion criteria for BALANCED with BIS group classified according to the actual median BIS value achieved irrespective of randomisation.  Participants were allocated to the BIS=50 group if the achieved median BIS is between 45 and 55 inclusive, and to the BIS=35 group if the achieved median BIS is between 30 and 40 inclusive. Participants who are not within these ranges will be excluded from these analyses.