Cerebral Monitoring (also see Chapter 31 )

The impact of monitoring the level of consciousness during general anesthesia on recovery remains controversial.^{[450] [451] [452] [453] [454]} Monitoring patient vital signs remains the most common method for determining the "depth of anesthesia" during surgery. Although the value of cerebral monitoring as an adjuvant to monitoring autonomic responses to noxious surgical stimuli has been questioned,^{[450] [452] [454]} recent studies have suggested that the use of cerebral monitoring improves early recovery after general anesthesia in the ambulatory setting because of its ability to minimize both "overdosing" and "underdosing" with both intravenous (e.g., propofol) and inhaled anesthetic (e.g., sevoflurane and desflurane) drugs during the maintenance period. ^{[455] [456] [457]}

Several different electroencephalographic (EEG)-based algorithms have been evaluated in an attempt to correlate EEG-derived indices and anesthetic drug concentrations with clinical signs of the depth of anesthesia.^{[458] [459] [460]} The bispectral index (BIS), physical state index (PSI), spectral and response entropy, and auditory evoked potential (AEP) index (AAI) are processed variables derived from the EEG that have been used to quantify the sedative and hypnotic effects of anesthetic drugs on the central nervous system (CNS).^[461] The BIS and PSI values are both dimensionless numbers that vary from 0 to 100, with values less than 60 associated with "adequate" hypnosis under general anesthesia and values greater than 75 typically observed during emergence from anesthesia.^[462] Chen and coworkers^[460] found a very similar pattern of changes in the BIS and PSI indices during general anesthesia involving propofol and desflurane. Although BIS and PSI values have been correlated with the degree of sedation-hypnosis produced by intravenous sedatives and volatile anesthetics, the EEG-based indices are less useful in assessing the effects of opioid analgesics, ketamine, and nitrous oxide on the CNS.^{[458] [460] [463]} The AAI values vary over a narrower range, with values of 15 to 25 associated with an adequate hypnotic effect during general anesthesia. Struys and coauthors^[462] reported that the BIS and AAI indices were also similar with respect to their ability to track levels of sedation and loss of consciousness with propofol. However, these investigators reported that both indices had poor predictive power with respect to purposeful movement in response to noxious stimuli.

In contrast to the BIS and PSI monitoring devices, the AEP monitor uses the middle latency auditory evoked potential (MLAEP) response to assess the depressant effects of anesthetic drugs on the CNS. Preliminary studies suggest that the AAI changes in a consistent and dose-dependent manner with the administration of sedative-hypnotic and volatile anesthetics, but not with opioid analgesics.^{[464] [465] [466] [467] [468] [469] [470]} Another EEG-based cerebral monitoring technology known as spectral entropy has recently been introduced as a novel measure of anesthetic effect on the CNS.^[471] However, clinical utility studies comparing the entropy device with the existing technology are needed to determine whether it offers any advantages with respect to facilitating titration or expediting the recovery process, or both.

Despite the introduction of these cerebral monitoring devices, there is still no universally accepted "gold standard" for measuring the depth of anesthesia. For a monitor to be of the most clinical utility, it should display a strong correlation between the observed variable (index) and the patient's state of consciousness and physiologic responses (e.g., blood pressure, heart rate, catecholamine levels) to noxious stimuli during surgery independent of the anesthetic technique and with minimal interpatient variability. In an editorial on cerebral monitoring, Kalkman and Drummond^[454] suggested that these conditions have not yet been achieved with any of the currently available monitoring devices.

Because the cerebral indices are generally maintained at lower values in patients with standard clinical monitoring than in cerebral-monitored patients, ^{[464] [472] [473]} these data suggest that patients are maintained at a "lighter" level of anesthesia (or hypnosis) when the anesthesiologist has access to the information provided by a cerebral monitor. The availability of the information provided by cerebral monitors influenced the anesthesiologist's use of not only

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One of the concerns in "minimizing" the use of anesthetics relates to the possibility that patients may experience more purposeful movement during surgery (i.e., less optimal operating conditions). In studies involving nonparalyzed patients, EMG activity interferes with the ability of the monitor to obtain reliable BIS, PSI, or AAI values. It has further been suggested that the use of cerebral monitors to minimize the administration of anesthetic and analgesic drugs may result in increased autonomic (stress) responses and adverse clinical outcomes (e.g., myocardial ischemia, intraoperative awareness). Previous studies have demonstrated that patient outcome is similar whether general anesthetic, opioid analgesic, or sympatholytic drugs were used to control acute "stress responses" during surgery.^{[474] [475]} Interestingly, a recent study by Weldon and colleagues^[476] suggested that there was a correlation between the length of time during anesthesia that the BIS value was less than 45 and the incidence of adverse clinical outcomes in an elderly surgical population. In addition, despite the emphasis on rapid recovery after ambulatory anesthesia, outpatients do not appear to be at increased risk for awareness when compared with inpatients receiving general anesthesia.^[477] From the perspective of the patient, the quality of recovery appears to be improved with cerebral monitoring versus traditional monitoring practices.^{[472] [473]} However, large-scale cost-benefit analyses would be required to determine whether the routine use of these cerebral monitoring devices can be justified in the current cost-conscious practice environment.