 - At the end of the study drug infusion, and after a preoxygenation period of three minutes, anaesthesia will be induced with sufentanil 0.2μg kg-1 and propofol, using a Target Controlled Infusion TCI system Base Primea, Fresenius-Vial, Brezins, France and the pharmacokinetic model of Schnider et al Schnider et al, 1999. The initial effect site concentration will be 4 μg ml-1. 