Brain function monitors for assessing the depth of anesthesia or hypnosis have been commercially available since the 1990s. The processed electroencephalogram (pEEG) has been used to monitor the depth of anesthesia by numerical representation. Many available anesthetic depth monitors including the BIS monitor, M-Entropy, Narcotrend monitor, cerebral function monitor, CSI, patient state index, and lower esophageal contractility monitor have been introduced, and studied extensively for clinical utility and performance. Initial studies suggested that the use of pEEG technology could significantly decrease the incidence of unintended intraoperative awareness events during general anesthesia. However, recently published trials examining the use of the pEEG, such as BIS during general anesthesia using inhaled anesthetic agent, have not been able to show superiority over the technique of maintaining an end-tidal anesthetic-agent concentration of > 0.7 age-adjusted minimum alveolar concentration (MAC) [2]. Recently, neither careful monitoring of a patient to reduce the frequency of intraoperative awareness, nor depth monitoring of anesthesia, are routinely indicated in patients undergoing procedures under general anesthesia. The ASA’s Practice Advisory for intraoperative awareness recommended that the decision on whether to use a brain function monitor should be made on the basis of a case-by-case by tailoring each patient’s anesthetic protocol based on one’s comorbidities and the procedure performed [3]. Selective use of pEEG may be useful for the prevention of intraoperative awareness during the administration of total intravenous anesthesia (TIVA) combined with neuromuscular blockade, and for avoiding drug overdose in the elderly. The incidence of intraoperative awareness with explicit recall is two-fold to four-fold higher when using the TIVA technique with propofol, when compared to the use of a volatile anesthetic. In addition, the use of a neuromuscular blockade drug can dramatically increase the risk of awareness while pEEG monitoring significantly reduces that risk [4]. The intraoperative use of pEEG can be recommended for the elderly patient to minimize postoperative delirium by avoiding deep levels of anesthesia that may result in unnecessary burst suppression [5]. Several studies show that exposure to deep anesthesia is associated with the occurrence of delirium, especially in vulnerable patients, such as the elderly [6,7]; but all results have not been consistent [8].

Studies have found that BIS values correlated with outcomes in other ways. The ‘triple low’ (the combination of intraoperative mean arterial pressure < 75 mmHg, BIS < 45, and MAC < 0.8) is associated with an increased risk of perioperative mortality, particularly if the ‘triple low’ lasts for more than 15 min [9,10]. Another use is that, the pEEG can predict the outcome of cardiac arrest and traumatic brain injury. Mean BIS values in the first 24 h after resuscitation are higher in patients with good outcomes, when compared with those with poor outcomes [11]; and patients with high BIS values (> 60) after traumatic brain injury have a high survival rate and Glasgow Coma Score [12]. BIS monitors can also be used to guide burst suppression therapy (burst suppression ratio of 15–30) applied during deep coma for the treatment of increased intracranial pressure. Intensive care unit (ICU), pEEG monitoring in patients receiving continuous infusions of sedatives and neuromuscular blockade can give both economic and personal benefits, even though BIS variability is relatively high in ICU patients. Using pEEG for titrating the dosage of sedating agents can also decrease the risk of medical complications associated with over-sedation, such as hypotension, ventilator-associated pneumonia, and longer ICU stays, with a secondary benefit of financial savings from reduced drug doses and decreased time to tracheal extubation [13,14]. Some studies have shown that a decrease in BIS value may correlate with cerebral ischemia, suggesting that pEEG is a useful tool in detecting cerebral ischemia or injury [15,16]. Others, however, have failed to demonstrate an association [17,18], which could be partly explained by the inability of pEEG to assess subcortical regions of the brain. Therefore, current literature cannot support pEEG monitoring as a sensitive modality for detecting cerebral ischemia, whether the monitor uses unilateral or bilateral EEG channels.

For more accurate interpretation of the pEEG, clinicians should recognize its several limitations. Patients’ pathophysiologic states (e.g., Hypothermia, hypoglycemia, acid-base abnormalities and pre-existing neurologic disorder), degree of muscle activity, and muscle relaxants can affect the pEEG indices. Other limitations include undetected artifacts (e.g., electrocautery, vibration from forced air warmers) and time lag for the index to display after the occurrence of an event that may affect the pEEG [19]. When there are changes in the pEEG, the clinicians might be able to determine from the raw EEG trace, whether the change is caused by an artifact or by true changes in the hypnotic state. It should also be noted that changes occur in the raw EEG trace about one minute before they are reflected in the pEEG indices.