Lung function, upper and lower airway resistance measurements are performed in patients in prone horizontal or head tilted down (40°) position. Half of the patients with head down position will have surgery in laparoscopic technique (including gas insufflation to achieve a pneumoperitoneum). Measurements are performed on the day prior to surgery, on the morning of surgery, 30 - 45 minutes, 2 hours, and 24 hours after extubation.