 40 patients undergoing robot assisted prostatectomy should be enrolled in a study that consists of two phases. In the first phase 20 patients are ventilated with constant PEEP 10mbar and in ten patients each the Vt is set to 5 or 12mlkg ideal body weight. In the second phase in further 20 patients the Vt is set to 8ml kg ideal bodyweight and the PEEP is 3 or 12mbar in one half of the patients. Under the different ventilation settings the NSS and DSS, the oxygenation index and lung compliance will be investigated. 