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