Just before leaving the post-anaesthesia care unit, the patient was asked to evaluate the preoxygenation method with the following questions: How do you evaluate the preoxygenation phase on the scale (cursor placed on a 100 mm visual analogue scale) between very comfortable (0 mm) to very uncomfortable (100 mm)? Do you want to have the same preoxygenation method for further anaesthesia (yes/no)? Primary endpoint : time for preoxygenation (average 180 s) calculated from face mask positioning to FEO2=90%.