The major goal of the pre-oxygenation is to increase the alveolar partial pressure of oxygen available in the end-expiratory pulmonary volume. This can be done by replacing the nitrogen in the alveolus by oxygen and by increasing the pulmonary stocks, the FRC. It has been demonstrated that the FRC after the induction of anesthesia is cut by half for the obese. This reduction is explained by a diminished thoracic compliance and an increase of the dependent lung regions' atelectasis because of a more cephalic position of the diaphragm.