 Modified prone position group: RM in modified prone position 120 degrees, left side up. EIT, ABG and lung compliance measurements in supine position before RM 30 degrees elevation, and then immediately after RM when turned back to supine position 30 degrees elevation, and 30 minutes after RM. Finally, measurements 30 minutes after extubation. FiO2 0.5 at all ABG measuring points. 