# What Do We Know about the Use of EEG Monitoring during Equine Anesthesia: A Review
## 3 EEG Monitoring during General Anesthesia: Does It Improve Anesthetic Management?  
According to the classic definition of general anesthesia, it has to fulfill four basic conditions: lack (or weakening) of reflexes, analgesia, muscle relaxation, and loss of consciousness [22]. The loss of consciousness during anesthesia is obtained by a mechanism of inhibition of the central nervous system, in particular, reticular formation, thalamus, and cerebral cortex [21]. Especially the cortex is sensitive to anesthetic modulation [23]. In animals, the level of consciousness loss is assessed on the vital signs' changes and through the evaluation of observed reflexes, but it is not completely reliable since some reflexes have a neurological source outside the brain [24; 25]. The lack of active reflexes also does not mean that the depth of anesthesia is appropriate, because agents such as isoflurane cause the immobilization first at the level of the spinal cord, and only then inhibit the transmission of nociceptive stimuli (this results from different effects on sensory and motor pathways), which is the analgesic action--one of the conditions of general anesthesia [21; 26].  
The use of EEG has been extensively studied in terms of its usefulness in assessing the level of general anesthesia in humans [6; 14; 23; 26; 27; 28; 29; 30; 31; 32; 33], whereas much less of those focused on clinical application in veterinary medicine have been conducted in animals so far. The research can be divided as to the groups that have different objectives:  
* techniques of good EEG practice [9]
* normal EEG measurement of healthy animal [34; 35; 36]
* EEG measurement during different procedures and anesthesia schemes [25; 37; 38; 39; 40; 41; 42; 43]
* reactions on specific agents during general anesthesia [21; 44; 45; 46; 47; 48; 49; 50].  
Over 75% of neurons in the nervous system are located in the cerebral cortex. Such a large number of neurons create a highly complex multilayer structure with countless connections between themselves with constant transmission of the signals. In the autonomic nervous system, neurotransmission occurs mainly among motor plates. This results in a difference in the time of the reaction of changes in vital signs to the depression of the central nervous system by anesthetics. Life support parameters such as heart stroke volume and frequency (heart rate), constriction and dilatation of blood vessels (blood pressure, body temperature), and lung ventilation (respiratory gases level in blood, blood pH) require a complex pathway of signals collected from the whole organism and are controlled by various centers of the nervous system, often cooperating with each other (for example in the blood pressure control). In case of the inhibition of the eye reflexes, there is a direct route, which significantly shortens the reaction time, because it works through the reflex arc and muscle tension.  
Currently, the most common method to assess the level of anesthesia is the analysis of hemodynamic and respiratory parameters (PaO\({}_{2}\), heart rate, breaths per minute, EtCO\({}_{2}\), pulse oximetry, arterial blood pressure). The dynamics of their changes depends on the stimulation of the autonomic nervous system and is partially related to the intensity of nociceptive signals. Therefore, it seems reasonable to introduce an EEG measurement due to its high specificity and sensitivity (limited number of wave patterns and a large number of variables affecting them) to show the changes of the level of consciousness, which in combination with cardiopulmonary parameters will enable accurate assessment of the patient's clinical condition during general anesthesia [40].