This study will compare an oxygenation test based on arterial blood gas analysis with computed tomography for estimating the extent of atelectasis.

Computed tomography of the lungs is the gold standard for investigating the extent of atelectasis, which is common in the perioperative setting. Atelectasis increase the risk of postoperative pulmonary complications and result in pulmonary shunt, which is one of the two major causes of impaired oxygenation. The other cause is admixture of venous blood from poorly ventilated regions of the lungs. The investigators have developed an oxygenation test utilizing arterial blood gas analysis at different inspired fractions of oxygen, which may be used to distinguish between these two causes. This observational study aims to establish the agreement between computed tomography and the oxygenation test for estimating the extent of atelectasis.

Inclusion Criteria:

• American Society of Anesthesiology (ASA) class I-II.
• Non-abdominal day case surgery under general anesthesia.
• Patients enrolled in study registered in ClinicalTrials with NCT ID.

Exclusion Criteria:

• Arterial oxygen saturation (SpO2) <90 % with 40% Venturi mask.
• Unstable postoperative condition needing immediate treatment.

Patients fulfilling inclusion and exclusion criteria while attending the day case department of Köping county hospital in Region Västmanland.

De-identified individual participant data for all primary and secondary outcome will be available.

Data will be available within 6 months of study completion.

Data access requests will be reviewed by the board of the local clinical research center in Region Vastmanland. Requestors will be required to sign a Data Access Agreement.