Several projects in high altitude research in environments with a low oxygen partial pressure (hypobaric hypoxia) leading to hypoxemia showed, that cerebral perfusion and cerebral performance could be improved by adding C02 (cabon dioxide).

The investigators hypothesize that adding 5% C02 to 02 (Oxygen) also under normobaric conditions increases the time until a significant cerebral hyopxia is measured by near infrared spectroscopy (NIRS) compared to the administration of 95% 02.

lf this hypothesis proves to be true, this approach might be used in situations in which individuals are prone to cerebral hypoxia. In bariatric surgery, patients that experience an apnea phase are more prone to (cerebral) hypoxia due to the fact, that they have a higher body mass index (BMI) leading to a reduced functional residual capacity (FRC), which serves as the oxygen reserve in the body.

On the day of surgery patients will be randomized. After induction of anesthesia and endotracheal intubation patients will be managed by the study team to perform the intervention and the measurements.

According to randomization, investigational medical product (IMP) or comparator will be administrated by ventilator 1/ventilator 2.

At time point 1 apnea will be performed by disconnecting the ventilator from the endotracheal tube until the NIRS value has dropped by 20% from baseline. After that, re-ventilation will be resumed immediately, until parameters have returned to baseline.

During apnea time NIRS and vital signs will be recorded permanently, blood samples will be drawn at definite time points.

Application of IMP (or comparator) will be performed in this cross-over study design in the same manner after the baseline level is reached again.

At the end of the study procedure, after the baseline level of end-expiratory CO2 is reached again, patient will be taken over by the clinical anesthesia team and surgeons.

Inclusion Criteria:

• Patients who are designated for bariatric surgery at the University Hospital Zurich (USZ)
• Informed Consent as documented by signature
• BMI >35 kg/m2

Exclusion Criteria:

• Severe end-organ damage: chronic obstuctive pulmonary disease (COPD) GOLD (cassification) III and IV, known hepatic insufficiency or elevated liver enzymes, renal creatinine clearance <30ml/min
• Severe cardiovascular disease (NYHA classification III and IV)
• Known pulmonary Hypertension
• Cerebrovascular disease
• Pregnancy and lactation
• Cardiac dysrhythmias
• acidosis, chronic pulmonary disease
• Known or suspected non-compliance, drug or alcohol abuse

Ist not planned to share data with other researchers

The sequence whether the IMP or the comparator is applied first will be randomly assigned using the web-based randomizer.