During artificial ventilation the cyclic opening and closing of alveoli during artificial ventilation results in patients with acute lung injury (ALI) in a higher mortality. A positive endexpiratory pressure (PEEP) is applied with the intention to avoid these cyclic opening and closing of alveoli. The distribution of ventilation in patients with ALI is extremely inhomogeneous, consequently there are different regional opening pressures needed to open up the alveoli and avoid closing. Finding the best level of PEEP in patients with injured lungs to avoid atelectasis and alveolar strain induced by inadequately high or low PEEP levels is a challenge.