 The investigators aim at assessing the benefits and potential adverse effects of high-inspired oxygen fraction in two groups of children receiving either 30 FiO2 or 80 FiO2 during maintenance of anesthesia. Nitrogen multiple breath washout technique and forced oscillation technique will be used to measure end-expiratory lung volume EELV and airway resistance Rrs and respiratory elastance Xrs respectively. These measurements will be obtained before the general anesthesia, one hour and one day after the procedure. 