Postural changes during anesthesia can lead to decreased cerebral blood flow and oxygenation, especially when moving from a supine to a prone position. This is particularly relevant during spinal surgery with controlled hypotension. Cerebral oximetry, monitored in the frontal cortex using an O3 sensor, is a noninvasive and continuous method to investigate the impact of anesthetic techniques on cerebral oxygenation in such scenarios.

All patients will be monitored with standard monitoring, SedLine sedation monitor, and O3 regional oximeter, Masimo Corp, Irvine, CA. Fentanyl, propofol, and cisatracurium will be administered for anesthesia induction. Anesthetic agents will be maintained with intravenous anesthetic agents in one group and sevoflurane in another group to obtain the PSI 25-50. Mean blood pressure will be maintained 20-30% below baseline. MAP, HR, SpO2, ETCO2, and SrO2 will be assessed at supine position without oxygen administration (T1), supine position with oxygen administration (T2-baseline), intubation in the supine position (T3), just after prone positioning (T4), 10 minutes after prone positioning (T5), at the end of surgery in the prone position (T6) and at the end of anesthesia in the supine position (T7).

Data will be expressed as mean and standard deviation. The mean differences will be analyzed using the Student's T test for quantitative variables in those cases in which those variables followed a normal distribution. Distribution will be evaluated by Kolmogorov-Smirnoff's Z test. Differences in proportion will be conducted with contingency analysis using Chi-square and Fisher's exact test when needed.

A P-value < 0.05 will be statistically significant. Statistical analysis will be performed using the SPSS-PC statistical software program (version 15.0; SPSS, Inc., Chicago, IL, USA).

Inclusion Criteria:

• American Society of Anesthesiologists (ASA) Physical Status I and II
• Scheduled for elective spinal surgery
• Prone position
• General anesthesia with controlled hypotension.

Exclusion Criteria:

• Hypertension
• Coronary artery disease
• Renal, hepatic, or cerebral insufficiency,
• Patients with coagulopathy or receiving drugs affecting coagulation
• Grossly anemic
• Hypovolemic patients
• Chronically diseased and debilitated patients,
• Bleeding more than 200 mL
• Hemodynamic instability (decrease of 25% of baseline mean arterial pressure [MAP] for three minutes)
• Patients with significant baseline bradycardia.

Fifty-four patients of both sexes between 18 and 68 years