## 4 Discussion  
In the present study, NIRS was used for monitoring patients undergoing spinal surgery in the prone position. Three out of 44 (6%) patients showed intraoperative cerebral desaturation, indicating that rScO\({}_{2}\) monitoring might not be useful in all spinal surgery patients and careful patient selection could be necessary. On the other hand, no correlation between cerebral oxygen saturation and other intraoperatively monitored parameters, like, MAP, SpO\({}_{2}\) and EtCO\({}_{2}\) were seen. Therefore, without rScO\({}_{2}\) monitoring, cerebral desaturation episodes would have remained unnoticed.  
In patients experiencing intraoperative cerebral desaturation, longer operation time (not statistically significant) and greater blood loss (statistically significant) were recognized as possible predictors of cerebral oxygen desaturation during spinal surgery. [PERSON] et al. [11] showed that NIRS might be a valuable tool for guiding the decision regarding the necessity of blood transfusion during cardiac surgery. In his study significantly fewer patients monitored with INVOS received blood transfusion and significantly fewer red blood cell units were transfused in comparison with the control group with no NIRS monitoring [11].  
The patients who experienced intraoperative cerebral desaturation episodes were 24, 54 and 57 years of age. Only one patient had comorbidities. That was adjoosities and arterial hypertension, which was treated with one antihypertensive medicine. Therefore, it is not possible to assess whether one or another comorbidity is related to intraoperative cerebral oxygen desaturation. Nevertheless, our study demonstrates that also young people without any known comorbidities can be at risk for cerebral desaturation.  
Regarding NIRS interventional protocol, in one patient rScO\({}_{2}\) values raised above the threshold after verifying correct head position. The importance of correct head position during prone position  
Figure 4: Receiver operating characteristic (ROC) curve for blood loss as a predictor of cerebral desaturation.  
also has been emphasized by [PERSON] et al., where they strongly recommend a neutral head position, as head rotation, flexion or extension can significantly alter cerebral oxygenation [12].  
The limitation of our study is the lack of analysis of the postoperative period as one or another monitoring device implementation in clinical practice should also improve postoperative period or organ-related outcome [13]. The majority of studies show clear benefit of r5 CO2 intraoperative monitoring. Thus, less renal and respiratory failures and a shorter length of stays in intensive care units have been recognized in patients in whom NIRS guided interventions were used during surgery [14]. A lower probability of death and major organ dysfunction also have been reported [15; 16]. A number of studies have been performed focusing on cerebral oxygen saturation intraoperative monitoring and postoperative cognitive outcome as postoperative cognitive decline or dysfunction (POCD) impair patient postoperative recovery and can lead to long-term consequences [17]. A randomized study has shown that patients suffering from intraoperative cerebral desaturation also have an increased risk of POCD [2]. What is more important, lower incidence of cognitive deficiencies in patients monitored with NIRS was reported in a study that included spinal surgery patients [18].  
Every monitoring device, that can be used intraoperatively, should work in favor of additional patient safety. NIRS devices can help physicians to guide making decisions during the surgery. However, NIRS is believed to be best used as a trend monitor, without following strict thresholds [19]. It seems to be better for recognizing patients at risk than guiding interventions [20].