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Inclusion Criteria:

• - Older than 18 years.

• - American Society of Anesthesiologists physical status classification system (ASA) I and II.

• - Patients scheduled for elective craniotomy.

• - Patients with supratentorial mass.

Exclusion Criteria:

• - Hemoglobin concentration lower than 9 mg/dL.

• - Uncontrolled hypertension.

• - Congestive heart failure.

• - Severe chronic obstructive lung disease.

• - Cerebrovascular disease.

• - Pulmonary edema.

• - History of carotis surgery or stenosis of carotid artery.

• - Unstable hemodynamics.

• - Pregnancy.

• - Skin reaction to the NIRS sensor.

In this prospective single-blind study, American Society of Anesthesiologists physical status classification system (ASA) I and II patients aged older than 18 years undergoing craniotomy will be enrolled for the study. Patients will be divided into two groups with computer-based randomization technique. Written and verbal approval will be taken from all patients. Premedication will not be used. Patients will be monitorized by standard non-invasive monitorization (electrocardiography, non-invasive blood pressure measurement, peripheral oxygen saturation) in the operating room. Cerebral/somatic oximeter, near infra-red spectroscopy (NIRS) sensor will be placed on the frontotemporal area and regional cerebral oxygen saturation will be monitorized continuously. Remifentanil infusion with a dose of 0.25 mcg/kg/min will be started two minutes before the induction. After preoxygenation with 6 L/min oxygen for 5 minutes, general anesthesia induction for patients in both groups will be held with intravenous 2 mg/kg propofol. Rocuronium bromide (0.6 mg/kg) will be used for muscle relaxation. Endotracheal intubation will be performed. Controlled mechanical ventilation will be used during the surgery. Invasive arterial blood pressure monitorization will be done and a central venous catheter will be placed in both groups. In group low PEEP, the patients' settings of mechanical ventilation will be as tidal volume 6-8 mL/kg, inspired oxygen fraction (FiO2) 0.4, PEEP 5 cmH2O. Frequency will be set according to the end tidal carbon dioxide pressure (ETCO2) where it will be 30-32 mmHg. In group high PEEP, the patients' settings of mechanical ventilation will be as tidal volume 6-8 mL/kg, inspired oxygen fraction (FiO2) 0.4, PEEP 10 cmH2O. Frequency will be set according to the end tidal carbon dioxide pressure (ETCO2) where it will be 30-32 mmHg. Remifentanil with a dose of 0.05-2 mcg kg/min and propofol with a dose of 50-200 mcg/kg/min will be infused for general anesthesia maintenance in both groups. Dose of remifentanil infusion will be changed according to the blood pressure. If the mean arterial blood pressure and/or heart rate will decrease to 20% of the baseline mean arterial blood pressure and heart rate, the infusion dose will be lowered. If this decrease in heart rate will be over 25%, intravenous 0.5 mg atropin will be administered. If the mean arterial blood pressure will be under 55 mmHg, intravenous 5 mg ephedrine will be administered. If the mean arterial blood pressure will continue to decrease, PEEP will be lowered. If the peripheral oxygen saturation will be under 92%, the inspired oxygen fraction or PEEP will be increased. In these conditions, the patient will be out of study. 1 gr paracetamol will be given intravenously for pain relief. Sugammadex will be used with a dose of 4 mg/kg for extubation. Regional cerebral oxygen saturation measures will be recorded nine

• (9) times during the study; pre-induction (period 1), post-induction (period 2), skull pinning (period 3), before dura opening (period 4), after dura opening (period 5), surgical resection of the mass (period 6), dura closure (period 7), end of the surgery (period 8) and end of anesthesia (period 9).

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