The use of continuous positive pressure (CPAP/PEEP) during pre-oxygenation and mask-ventilation in patients undergoing anesthesia induction is increasing. Continuous positive pressures have several respiratory benefits with well-documented data on both spontaneously breathing patients and intubated patients. However the effects of PEEP on anesthetized patients during mask-ventilation is poorly investigated. In a previous trial the that the investigators conducted, a PEEP-level of 10 cmH2O during mask ventilation after anesthesia induction surprisingly delayed time to alveolar ventilation compared to ZEEP. In order to investigate this phenomenon, four groups of patients will be compared during preoxygenation and mask ventilation after anesthesia induction.