The influence of xenon anaesthesia on transpulmonary pressure and tidal volume distribution will be investigated in a prospective cohort study in 10 patients of normal weight and 10 obese patients undergoing routine xenon-based, general anaesthesia for surgery.

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.

5 minutes after anaesthesia induction, as well as 5 minutes after completion of Xenon wash-in, transpulmonary pressure will be measured from inspiratory plateau pressure and pleural pressure, which will be estimated from esophageal pressure. Esophageal pressure measurements will be facilitated via esophageal balloon catheters (Adult Esophageal Balloon Catheter Set 5 French, CooperSurgical, Trumbull, USA).

Tidal volume distribution will be assessed with the use of Electrical Impedance Tomography (EIT). Images will be acquired in the supine Position 5 minutes before anesthesia induction, 5 minutes after induction as well as 5 minutes completion of xenon wash-in. EIT measurements will be made for a period of one minute and functional EIT images of changes in tidal impedance will be generated. The Center of Ventilation Index and Global Inhomogeneity Index will be calculated for each time point.

Analysis Plan:

The primary analysis will include all recruited patients, irrespective of their Body mass index. Changes in transpulmonary pressure before and after Xenon wash-in will be assessed with the Wilcoxon signed-rank test for matched pairs. Center of Ventilation Index and Global Inhomogeneity Index, which will be assessed at three time points, will be compared with the use of repeated measures ANOVA with the post-hoc Sidak test.

Secondary subgroup analyses will consist of separate analyses for patients with a BMI over or equal to and a BMI of less than 30. A type I error will be accepted with a maximum probability of 0.05.

Inclusion Criteria:

• Age over 18 years
• Xenon-based, general anaesthesia for elective surgery independent of this investigation

Exclusion Criteria:

• Inability to provide informed consent
• Known pregnancy
• American Society of Anesthesiologists status >III
• Restrictive or obstructive pulmonary disease
• History of esophageal or gastric operation
• Esophageal varices
• Liver cirrhosis Child-Pugh B or C
• Therapeutic anticoagulation
• Cardiac pacemaker or internal automatic defibrillator