## 1 Introduction  
Intraoperative patient management is a challenging undertaking. Among a multitude of laboratory tests and technologies available to monitor patients during surgery, clear selection criteria for one or another monitoring technique is necessary. The human brain is one of the most metabolically active organs consuming in average 20% of all the body oxygen [1]. The dangers of prolonged brain hypoxia, also intraoperatively, are already well documented in numerous studies. A large randomized study including 265 patients undergoing coronary artery bypass surgery showed significant correlation between prolonged intraoperative hypoxia and early cognitive decline and prolonged hospital stay [2]. Intraoperative cerebral desaturation has been proven to predict postoperative cognitive dysfunction and other postoperative complications also in patients undergoing non-cardiac surgery like thoracic, major orthopedic and major abdominal surgery [3]. Still, brain remains one of the least monitored organ during anesthesia and surgery, as the majority of available methods are invasive, like, jugular venous oximetry, requiring cannulation of the jugular bulbus, brain tissue oxygen monitoring using the Clark electrode in the brain parenchyma or cerebral microdialysis [4]. Near infrared spectroscopy (NIRS) has been increasingly implemented in clinical practice. NIRS devices are non-invasive, easy to use and monitor cerebral oxygen saturation (rScO2) continuously. A NIRS device cerebral oximeter consists of a monitor and two adhesive sensors, which are attached to the patients' forehead: one above the left and one above the right cerebral hemisphere overlying the frontal lobe (Figure 1). In each sensor there is an incorporated light diode-emitting near infrared light and two light detectors proximal and distal collecting the scattered light returning to the surface, permitting to separate data of shallow and deep optical signals [5] (Figure 2). Near infrared light, used in the cerebral oximetry, is at wave length 730 and 810 nm, which is capable of penetrating the skull [6]. In the brain tissue, light is either absorbed by hemoglobin (Hb) molecules or scattered back to the brain surface depending on the degree of oxygenation [5]. Cerebral oxygenation values are calculated using the Beer Lambert law. [PERSON]'s law claims that the light intensity decreases as the concentration of the substance the light passes through increases [5]. [PERSON]'s law incorporates that the intensity of the light decreases as the distance, which the light is traveling, increases [5]. Based on both laws, oxygen can be measured by how much of the emitted light the substance absorbs and by the amount of light, which is travelling through it. Light absorption will be proportional to the oxygenation status of the tissue [5,7]. Values, at which cerebral oxygen saturation is considered to be normal, differ amongst studies published. Normal baseline cerebral  
Figure 1: INVOS (in vivo optical spectroscopy) sensor placement on the patients’ forehead (picture taken from the INVOS Cerebral/Somatic Oximeter Operations Manual, Covidien 2010). oximetry values range between 60% and 80% [5]. Abnormal rSCo\({}_{2}\) values are defined as rSCo\({}_{2}\) drop from ones' individual baseline values for 20% or an absolute drop under a rSCo\({}_{2}\) of 50% [7], marking those as a trigger values for an intervention [6] (Figure 3). The NIRS interventional protocol is used to guide patient management in order to mitigate cerebral desaturation and avoid hypoxic brain injury. The most commonly applied protocol has been published by [PERSON] and colleagues [8]. The protocol includes steps that should be considered when cerebral desaturation occurs. The protocol also proposes a plan of actions. Interventions aimed at preserving normal cerebral oxygenation within baseline values, can help to avoid postoperative cognitive disturbances, reduce the days spent in the intensive care unit [9] and avoid complications, like, kidney failure or wound infections [3].