Investigators are going to enroll 1500 patients undergoing elective major abdominal surgery, in a period of 15 days. Investigator decided to enroll all the patients in a limited period of time (as in LAS VEGAS [for more information...see www.euroanaesthesia.org., European Society of Anesthesiology (ESA) Clinical Trial Network]) and, if the sample size will not be sufficient, it will be possible to add 15 extra days. On the basis of works published by Squadrone et. al.[11] (16%), Lawrence et.al. [17] (4,5%), Hall et. al. [18] (15%), Mitchell et.al. [19] (11%) and Canet et. al. (7,2%) [10], the hypothesis was that the incidence of primary outcome (PaO2 /FiO2≤ 300) will be 7%.