Study of Atelectasis by Electrical Impedance Tomography

The goal of this interventional study is to learn about the effects of different positions on the occurrence, regional distribution and reversibility of atelectasis by using a PulmoVista 500 monitor (Dräger. 23542 Lübeck), during general anesthesia with mechanical ventilation. The main questions it aims to answer are: * The increase in atelectasis after induction of general anesthesia in supine and Trendelenburg position compared with baseline (spontaneous breathing) * The eventual possibility of reversal of atelectasis caused by general anesthesia and Trendelenburg position by reverse Trendelenburg position and recruitment manœuvre. Participants will be adult, autonomous and able to express their will, undergoing elective gynecological or urological surgery in the extended Trendelenburg position.