This study investigates under controlled conditions the variation of poorly ventilated lung units (silent spaces) in obese patients scheduled for laparoscopic bariatric surgery in the perioperative phase.

Eligible patients with written informed consent, scheduled for laparoscopic bariatric surgery. As for standard of care no premedication will be given for bariatric patients. Patients will be monitored according the local standard.

Thoracic electrical impedance tomography measurements (each measurement will last 1 min) will be performed at the following time points: before induction of the anaesthesia; before the surgical procedure when the induction is terminated and recruitment manoeuvers have been performed; after the termination of the surgical procedure; before transport to the Post anaesthesia Care Unit (PACU); before the discharge from the PACU after 2 hours of monitoring. An additional thoracic electrical impedance tomography measurement (duration 1 min) will be performed before discharge at home. For this, resulting potential differences are measured, and impedance distribution sampled at 30 Hz will be calculated by an automated linearized Newton-Raphson reconstruction algorithm. Relative change in poorly ventilated lung regions (silent spaces) and end-expiratory lung impedance (EELI) and measures of ventilation inhomogeneity such as the global inhomogeneity index will be calculated.

Inclusion Criteria:

• BMI >40
• ASA physical status 1 to 4
• Over 18 years of age providing written informed consent

Exclusion Criteria:

• BMI <40
• Known severe pulmonary hypertension,
• Need of home oxygen therapy
• Known heart failure
• Suspected, or known recent pulmonal infections

The investigators will include obese patients exceeding body mass index (BMI) 40, scheduled for laparoscopic bariatric surgery at the University Hospital - Inselspital in Bern