We conducted a double blind prospective randomized trial on 20 healthy volunteers. Each subject went through three modes of preoxygenation in random order: normal breathing (SP), breathing under inspiratory support 4 cmH2O/PEEP 4 cmH2O (PSM-4) and inspiratory support 6 cmH2O /PEEP 4 cmH2O (PSM-6). Each mode lasted for 3 min and parameters were recorded every 10 s. We evaluated subject's tolerance, leaks and easiness of each mode of preoxygenation according to a simplified (1 to 4) scale. Two-way ANOVA was used to compare FEO2 while simple ANOVA was used for time to reach FEO2=90%.