## 3 Results  
In total, 44 patients scheduled for spinal surgery were included. Demographic characteristics and preoperative laboratory findings, duration of operation, intraoperative blood loss, intraoperative MAP, SpO\({}_{2}\) and EtCO\({}_{2}\) are shown in Table 1. We determined rScO\({}_{2}\) at the following time points: (1) preoperative baseline; (2) after induction of anesthesia; (3) during prone position and (4) at the end of the surgery in supine position, as shown in Table 2.  
In 3 out of 44 patients, we observed intraoperative cerebral desaturation. In two patients rScO\({}_{2}\) decreased under 20% from individual baseline. One patient was 57 years old undergoing microdiscectomy, the other one was 24 years old undergoing transpedicular fixation due to trauma. In one patient, 54 years old undergoing exitrpation of spinal meningioma, rScO\({}_{2}\) decreased to below 50%. In all 3 patients the NIRS interventional protocol was initiated. The first step, the verification of correct  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Patients (n), Sex (Male/Female)** & **44 (20/24)** \\ \hline Age (years) & \(54\pm 14\) \\ \hline Hb level (g/dL) & \(13\pm 1\) \\ \hline Ht (\%) & \(41\pm 4\) \\ \hline Duration of operation (min) & \(121\pm 47\) \\ \hline Blood loss (mL) & \(259\pm 99\) \\ \hline Intraoperative MAP (mmHg) & \(83\pm 10\) \\ \hline Intraoperative SpO\({}_{2}\) (\%) & \(99\pm 0.3\) \\ \hline Intraoperative EtCO\({}_{2}\) (mmHg) & \(34\pm 2\) \\ \hline \end{tabular} Values are shown as mean \(\pm\) standard deviation. Hb—hemoglobin, Ht—hematocrit, MAP—mean arterial pressure, SpO\({}_{2}\)—peripheral oxygen saturation, EtCO\({}_{2}\)—end-tidal carbon dioxide.  
\end{table}
Table 1: Demographic characteristics and preoperative laboratory findings, duration of operation, intraoperative blood loss, intraoperative MAP, SpO\({}_{2}\) and EtCO\({}_{2}\) in patients undergoing spinal surgery under general anesthesia.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Position** & **Cerebral Hemisphere** & **Mean rScO\({}_{2}\) (\%)** \\ \hline Before induction of anesthesia & Right & \(72\pm 9\) \\  & Left & \(72\pm 10\) \\ \hline During induction of anesthesia & Right & \(74\pm 9\) \\  & Left & \(73\pm 9\) \\ \hline Prone position during anesthesia & Right & \(73\pm 6\) \\  & Left & \(73\pm 6\) \\ \hline Supine at the end of the anesthesia and surgery & Right & \(73\pm 8\) \\  & Left & \(73\pm 8\) \\ \hline \end{tabular} Values are shown as mean \(\pm\) standard deviation. rScO\({}_{2}\)—regional cerebral oxygen saturation.  
\end{table}
Table 2: Determination of rScO\({}_{2}\) before induction of anesthesia, during induction of anesthesia, during anesthesia with prone positioning of the patient and at the end of anesthesia and surgery with the patient in the supine position.  
head position resulted in no changes in rScO\({}_{2}\) in two of the patients while rScO\({}_{2}\) increased above the threshold value in one patient. In the remaining two patients, MAP was raised using Ephedrin boluses (5-20 mg) and rScO\({}_{2}\) raised above threshold and no further interventions were necessary.  
Comparing to other spinal surgery patients who experienced only mild cerebral oxygenation fluctuations intraoperatively, which did not reach desaturation threshold, the patients with desaturation episodes had significantly longer operation medium time--\(114\pm 35\) min in patients without cerebral desaturation compared to medium \(200\pm 98\) min in patients with intraoperative cerebral desaturation (\(p=0.01\)). [PERSON]'s correlation showed a trend towards a negative correlation between rScO\({}_{2}\) and duration of operation, although not statistically significant (r = \(-0.9\), \(p=0.2\)).  
Receiver operating characteristic (ROC) curve analysis showed blood loss to be a strong predictor of possible cerebral desaturation (AUC: 0.947, 95% CI: 0.836-1.000, \(p=0.04\); Figure 4).