Oxygen reserve index (ORI) measured by multiple wave pulse co-oximeter is a new technological development in medical science. Our aim in this study is to identify the correlation between arterial oxygen pressure (PaO2) and ORI, and to identify the specificity and sensitivity of ORI as a tool to prevent hypoxia and hyperoxia in abdominal surgery patients.

Patients aged between 20-55 years old,American Society of Anesthesiologists physical status 1-2 who will have hemicolectomy will be included to the study. If hemodynamic instability or a sudden decrease in oxygen saturation by pulse oximetry (SpO2) occurs, arterial blood gas analysis will be taken from the patient and ORI, SpO2, PaO2, and arterial oxygen saturation (SaO2) will be recorded. The primer endpoint of the study is to identify the correlation between PaO2 and ORI, seconder end-point is to identify the specificity and sensitivity of ORI as a detector of PaO2. Pearson test for correlation, regression analysis for sensitivity and specificity will be performed. The study will include 240 patients according to power analysis

Inclusion Criteria:

• Patients aged between 20-55 years old
• American Society of Anesthesiologists physical status I-II
• Patients undergoing hemicolectomy

Exclusion Criteria:

• Chronic obstructive and restrictive pulmonary disease
• Diabetes Mellitus
• Ischemic heart disease
• Peripheric artery disease.
• Operations longer than 200 minutes, requirement transfusion of more than 1 U erythrocyte suspension, and requirement of vasopressor or inotrope infusion will be excluded from the study