To investigate the influence of xenon-anaesthesia on transpulmonary pressure and tidal volume Distribution, the investigators will enroll 20 patients - 10 with a Body mass index of less than 30 and 10 with a Body mass index of 30 or more - undergoing routine xenon-based anesthesia in the course of a prospective cohort study. Performance of anesthesia will be at the discretion of the attending anesthesiologist. Typically, xenon-based anesthesia is induced and initially maintained with propofol and an opioid and tracheal intubation is facilitated using a muscle relaxant. Initially, the patient is ventilated with 100 per cent oxygen in order to wash out nitrogen from the respirator and the lungs. When nitrogen wash-out is completed, 55-65 per cent xenon in oxygen is introduced.