 Collapse of lung volume, atelectasis, is a common complication in sedated and mechanically ventilated patients. In adults the atelectasis occur mainly in the dependant areas of the lung. In this study the investigators examine whether these atelectasis can be opened by repeatedly sustaining the inspiratory pressure for 10 seconds when the patient has been positioned so that the atelectasis is located in a non-dependant position. Furthermore, the investigators examine how many times the inspiratory pressure needs to be sustained before the atelectasis is opened and at what pressure. 