Patients undergoing general anesthesia require mechanical ventilation (artificial delivery of air and oxygen to their lungs). It is well known that during mechanical ventilation, so-called atelectasis formation occurs. This is a condition characterized by partial or complete collapse of lung tissue that can result in a reduction in oxygen uptake through the lung. A known risk factor for atelectasis formation during mechanical ventilation is the utilization of high oxygen concentration, as the oxygen molecules are absorbed in the lung, which then can lead to collapse of the tissue. Despite the proven association, standard operating procedure at the end of anesthesia still requires utilization of 100% oxygen. Its justification is the goal to ensure sufficient oxygenation throughout the extubation phase. However, clinical observation doesn't show a lack of oxygenation in this phase, but the patient is still exposed to the risk of atelectasis formation.

This study aims to investigate the hypothesis of whether the utilization of reduced inspiratory oxygen concentration before extubation (70% or 40% compared to 100%) reduces atelectasis formation. The study was originally planned to randomize 24 patients to either 70% or 100% inspiratory oxygen concentration at the end of anaesthesia. After completion of this first phase, the study was amended to enroll another 24 patients randomized to 40% or 100% inspiratory oxygen concentration at the end of anaesthesia.

Of note, both concentrations are still higher than when breathing room air in, which has of 21% oxygen. During the intervention, parameters such as the oxygen content in the blood (oxygen saturation, SpO₂), heart rate, and blood pressure are recorded, and atelectasis formation is measured using a technique called electrical impedance tomography (EIT). EIT measurements are performed at designated time points during the procedure. Anesthesia care providers are asked to document procedural, patient, and ventilator data in a questionnaire. Secondary outcomes are the homogeneity and distribution of air measured with EIT, as well as some clinical outcomes including post-extubation desaturation (<90% SpO₂), incidence of re-intubation or non-invasive ventilation, and the Post-anesthesia Care Unit (PACU) length of stay.

This study is a single blinded randomized controlled trial with and aims to assess whether the use of 100% oxygen before extubation favors postoperative atelectasis formation compared to the use of lower oxygen concentrations whilst still ensuring patient safety. The study was originally planned to randomize 24 patients to either 70% or 100% inspiratory oxygen concentration at the end of anaesthesia. After completion of this first phase, the study was amended to enroll another 24 patients randomized to 40% or 100% inspiratory oxygen concentration at the end of anaesthesia. Patients are randomized to either the control group or the intervention group.

The randomization scheme is developed by a non-blinded statistician using the Statistical Analysis Software (SAS) and uploaded to the Research Electronic Data Capture (REDCap) randomization module, enabling delegated team members to generate sequential allocations for enrolled subjects.

Screening of potential study subjects is performed by the PI. The patient is asked to participate in the study by the PI or co-investigator and the study design and procedures are explained to the patient, while they are still in the holding area on the day of the procedure.

The patients in both groups (control and intervention) are equipped with the EIT belt as well as an additional SpO₂ recording device before induction and intubation. After onset of mechanical ventilation as per institutional standards, an inspiratory oxygen concentration of 50% is aimed. Actual oxygen concentration as well as any administration of drugs remain at the discretion of the anesthesia care provider. At the end of the procedure, before the initiation of volatile anesthetic washout, fresh gas flow is increased to 15 liters per minute (or the highest fresh gas flow allowed at the respective anesthesia machine if 15 liters per minute cannot be achieved), and a fraction of inspired oxygen (FiO₂) of either 70% (intervention group) or 100% (control group) for the first 24 randomized patients, and of either 40% (intervention group) or 100% for the subsequent 24 randomized patients is applied.

EIT measurements take place at 6 designated time points before, during and after the procedure. These time points are: Pre-induction, after intubation, just before washout, pre-extubation, one minute after extubation and 60 minutes after extubation in the PACU.

The primary outcome for the study is the Center of Ventilation (CoV), measured through electrical impedance tomography (EIT) compared at one minute after extubation between the control group and intervention groups.

Secondary outcomes include the homogeneity index, and the distribution of lung aeration measured by EIT. In addition, we measure the peripheral oxygen concentration during the whole interventional period and during the PACU stay to evaluate any occurrence of desaturation (SpO₂ <90%) within the first 60 minutes after extubation in the PACU. The incidence of re-intubation or the need for non-invasive ventilation in the next 7 days and an unplanned admission, to either ICU, Intermediate Care Unit (IMC) or the normal ward, as well as the length of stay in the PACU are other clinical secondary outcomes compared between the groups. Subsequent analyses will assess differences between the 70% and 40% oxygen subgroups within the intervention arm to evaluate potential dose-dependency.

Inclusion Criteria:

• Age ≥ 18 years
• Elective surgery with general anesthesia and endotracheal intubation
• Expected duration of surgery 1-5 hours
• Consent obtained from patient

Exclusion Criteria:

• Patients with inability to give written informed consent
• American Society of Anesthesiologists (ASA) physical status> IV
• Morbid obesity BMI > 40
• Suspected pregnancy and lactation
• Cardiac or thoracic surgery
• Patients with thoracic epidural catheters
• Patients with active implantable devices, such as pacemakers, cardioverter defibrillators, or neurostimulators
• Compromised airways
• Impaired oxygenation at baseline or during surgery