Excessive oxygen administration is known to cause oxidative stress, and absorption atelectasis. Hyperoxia is very common in general anesthesia settings. Even though there are concerns in using excessive oxygen during general anesthesia, the optimal fraction of inspired oxygen (FiO2) for general anesthesia is not well studied.

The oxygen reserve index (ORI) is a parameter which can evaluate partial pressure of oxygen (PaO2) rating from 0 to 1. There are growing evidences in ORI that it might be helpful to reduce hyperoxia in critically ill patients in the intensive care unit, as well as in general anesthesia.

The aim of this study is to evaluate efficacy of oxygen reserve index to reduce hyperoxemia in major abdominal surgery.

Inclusion Criteria:

• Patients scheduled for elective major abdominal surgery expected to last more than 2hours.
• Patients who are scheduled for invasive arterial cannulation during surgery.

Exclusion Criteria:

• Abnormal pulmonary function test
• Pregnancy
• Saturation of oxygen (SpO2) is below 92% in room air