<qa>  What is the difference in tidal variations of airway pressure subtracted byClassically lung elastance and transpulmonary pressure are measured from the difference in tidal variations of airway pressure subtracted by tidal variations i esophagus pressure divided by the tidal volume. This requires the presence of a esophageal balloon catheter which is cumbersome and costly. In this study values obtained as described above are compared to values obtained with a new method in which a stepwise increase in positive endexpiratory pressure PEEP is performed with a size which corresponds to the tidal volume which the patient is ventilated with. These measurements are performed in anesthetized patients prior to major surgery. 