Blood samples are taken from the indwelling arterial line, the exact time being recorded for each sample. At the point of sampling the investigators note the BIS. If the infusion of Propofol 2 % is changed, the time and the BIS will be noted. Extra blood samples are taken at the moment of changing the infusion rate and 5 or 15 minutes after the change in infusion rate. All samples are collected in glass oxalate tubes and analysed within three months.