All patients are over 18 years old and give their inform consent after approval of the study protocol by an ethic committee. Anesthesia is standardized and all patients are intubated. Precise protocols are used to perform pressure and volume controlled ventilation in order to regulate End-Tidal CO2 between 33 and 35 mmHg. Primary goal is comparison of plateau pressures observed in the 2 ventilation modes after 45 minutes of pneumoperitoneum. Secondary goals are : comparison of PaO2 and PaCO2 after 45 minutes of pneumoperitoneum and after 2 hours spent in post-anesthesia care unit ; comparison of total respiratory resistances and peak flow generated between the 2 modes.