The overall objective of this study is to describe the incidence of postoperative pulmonary complications in patients with a positive and negative AIR-test result. Second, the investigators wish to describe the incidence of a positive AIR-test and its association with the development of PPC. In addition, the investigators aim to describe whether mechanical ventilation strategy differs between patients with a positive and negative AIR-test.

The 'AIR-test', a method wherein inspiratory oxygen concentration is reduced to 21% and pulse-oximetry hemoglobin saturation (SpO2) is monitored, can be used to demonstrate the presence of an atelectasis-induced alveolar shunt. It remains uncertain whether an intraoperative positive AIR-test is associated with clinical outcomes. It is hypothesized that in a general surgical population, the incidence of postoperative pulmonary complications (PPC) is higher in patients with a positive AIR-test than in patients with a negative AIR-test. The aim of this study is to describe the incidence of PPC in patients with a positive and in patients with a negative AIR-test. Second, the investigators want to describe the incidence of a positive AIR-test and its association with the development of PPC. The investigators also want to evaluate whether mechanical ventilation strategy differs between patients with a positive and negative AIR-test.

Inclusion Criteria:

• Patients undergoing surgery under general anesthesia in the Amsterdam University Medical Centers, location Meibergdreef with an Endotracheal Tube;
• Patients > 18 years;
• Preoperative SpO2 of ≥97%;
• AIR-test performed after induction of anesthesia, in a hemodynamic and respiratory steady state.

Exclusion Criteria:

• Age < 18 years;
• No consent/objection;
• Patient participated in another conflicting interventional study on mechanical ventilation during general anesthesia

Adult patients undergoing surgery under general anesthesia in whom an AIR test is performed