## 2 Materials and Methods  
Patients scheduled for spinal surgery were recruited to a prospective study including patients over 18 years of age scheduled for elective spinal surgery. Exclusion criteria were patients undergoing urgent spinal surgery, patients with a history of cerebral disease, like previous stroke, cerebral hematoma or transient ischemic stroke.  
In our study INVOS (in vivo optical spectroscopy) 4100 (Covidien, Minneapolis, USA) two channel cerebral oximeter was used. In the operating room two INVOS sensors were attached to the patients' forehead, one above the left (L) and one above the right (R) eyebrow, before induction of anesthesia, while breathing room air. Individual baseline values were obtained. Meanwhile other presurgical measures were assessed, such as percutaneous oxygen saturation (SpO2) and mean arterial pressure (MAP) by measuring non-invasive blood pressure. Anesthesia was induced using fentanyl 0.1-0.2 mg and propofol l-2 mg/kg. Tracheal intubation was facilitated with cisatracurium 0.2 mg/kg. Anesthesia was maintained with continuous infusion of fentanyl 0.03-0.06 mg/kg/min, cisatracurium 0.06-0.1 mg/kg/h and sevoflurane at MAC 0.6-0.8. Mechanical lung ventilation was set to 8 mL/kg and fraction of inspired oxygen (FiO2) to 0.4. ventilation was set to keep end-tidal carbon dioxide (EtCO2) in the range of 35-45 mmHg and SpO2 96-100%.  
During the surgery, SpO2, MAP, EtCO2 and rSco2 were documented in the study protocol every 5 min. If the rSco2 values dropped more than 20% from the patients' individual baseline value, or if there was an absolute drop of rSco2 to below 50%, near infrared spectroscopy based interventional protocol was activated to restore rSco2 values above the threshold. In the current study, we followed the NIRS protocol proposed by [PERSON] et al. [8]. As the protocol was developed for cardiac surgery patients, we adjusted it to spinal surgery patients. Firstly, the head position is verified, in order to exclude head flexion, extension or rotations to the left or right side. Secondly, MAP is raised by injecting boluses of ephedrine. Thirdly, systemic oxygen saturation is tested and arterial blood oxygen partial pressure (PaO2) is measured, and if low, FiO2 is raised above 0.4. Fourthly, if EtCO2 or PaCO2 are low, hyperventilation is corrected. Fifthly, hemoglobin concentration is assessed and red blood cell transfusion is considered. Sixthly, the cardiac function is optimized by using vasopressor infusion. Seventhly, causes of excessive cerebral O2 consumption, such as convulsions and hyperthermia, are evaluated and treated [8].  
In the study protocol, we also noted patients' demographic data (age and sex), comorbidities, daily medication, the type of spinal surgery, preoperative hemoglobin and hematocrit levels, intraoperative blood loss and duration of operation.  
Statistical analysis was performed using SPSS V.23. Values were presented as mean \(\pm\) standard deviation (SD). Statistical significance was assumed if \(p<0.05\). The receiver operating characteristic (ROC) curves, their respective 95% confidence intervals, and significance were compared between patients to identify predicting factors for intraoperative cerebral desaturation.  
Study protocol and informed consent form was approved by the Medical Research Ethics Committee of Riga Stradins University (Approval No. 85/29.12.2016).