Propofol is a common intravenous anaesthetic drug used for anaesthesia induction and for permanent infusion to maintain anaesthesia (total intravenous anaesthesia). Different to the volatile anaesthetics (feedback concentrations in exhaled breathing gas) piloting of an total intravenous anaesthesia with propofol is done only by experience of the anaesthetist or according to pharmacokinetic models. In this study we want to investigate if the exhaled fraction of propofol is a potential measurement parameter for piloting anaesthesia. Exhaled propofol will be measured with an electrochemical sensor developed by "Drägerwerk AG & Co. KGaA". Exhaled propofol levels will be compared with the corresponding arterial serum concentrations of propofol. Furthermore we investigate the correlation of exhaled propofol levels with the Bispectral Index (BIS).