The null hypothesis is that remifentanil lowers the concentration of sevoflurane dissolved in a mixture of 60% nitrous oxide and 40% oxygen to (1) tolerate a specific stimulation and to (2) obtain a certain electroencephalographic parameter value and vice versa. Additionally the results of this study will be compared with a former interaction study of the same study group that did not involve nitrous oxide, in order to depict the shift in the response surface evoked by nitrous oxide.