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TECHNOLOGY, COMPUTING, AND SIMULATION

The Effect of Cerebral Monitoring on Recovery After General Anesthesia: A Comparison of the Auditory Evoked Potential and Bispectral Index Devices with Standard Clinical Practice

From the Department of Anesthesiology and Pain Management University of Texas Southwestern Medical Center at Dallas

Address correspondence to Dr. Paul F. White, Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390–9068. Address email to paul.white{at}utsouthwestern.edu

	Abstract

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The use of cerebral monitoring may improve the ability of anesthesiologists to titrate anesthetic drugs. However, there is controversy regarding the impact of the alleged anesthetic-sparing effects of cerebral monitoring on the recovery process and patient outcome. We designed this prospective double-blinded, sham-controlled study to evaluate the impact of intraoperative monitoring with the electroencephalogram bispectral index (BIS^TM) or auditory evoked potential (AEP) device on the usage of desflurane and the time to discharge from the recovery room, as well as on patient satisfaction with their anesthetic experience and recovery. Ninety healthy patients undergoing laparoscopic general surgery procedures using a standardized anesthetic technique were randomly assigned to one of three monitoring groups: standard clinical practice (control), BIS-guided, or AEP-guided. Both the BIS and AEP monitors were connected to all patients before induction of general anesthesia. In the control group, the anesthesiologists were not permitted to observe the BIS or AEP index values during the intraoperative period. In the BIS-guided group, the volatile anesthetic was titrated to maintain a BIS value in the range of 45–55. In the AEP-guided group, the targeted AEP index range was 15–20. The BIS and AEP indices, as well as end-tidal desflurane concentration, were recorded at 3–5 min intervals. Recovery times to awakening, tracheal extubation, fast-track score 12, and postanesthesia care unit (PACU) discharge criteria were recorded at 1–10 min intervals. In addition, patient satisfaction with anesthesia and quality of recovery were evaluated on 100- and 18-point scales, respectively, at 24 h after surgery. The AEP- and BIS-guided groups were administered significantly smaller average end-tidal desflurane concentrations than the control group (3.8 ± 0.9 and 3.9 ± 0.6 versus 4.7 ± 1.7, respectively) (P < 0.01). Although the emergence times to eye opening, tracheal extubation, and obeying commands were consistently shorter in the AEP and BIS groups (6 ± 4 and 6 ± 5 versus 8 ± 8 min; 6 ± 5 and 6 ± 4 versus 11 ± 10 min; and 8 ± 4 and 7 ± 4 versus 12 ± 9 min, respectively), only the extubation times were significantly different from the control group (P < 0.05). More importantly, the length of the PACU stay was significantly shorter in both the AEP- and BIS-guided groups (79 ± 43 and 80 ± 47 versus 108 ± 58 min, respectively) (P < 0.05). The patients’ quality of recovery was also significantly higher in the two monitored groups (15 ± 2 versus 13 ± 3 in the control group, P < 0.05). We concluded that cerebral monitoring with either the BIS or AEP devices reduced the maintenance anesthetic (desflurane) requirement, resulting in a shorter length of stay in the PACU and improved quality of recovery after laparoscopic surgery. However, there were no significant outcome differences between the two cerebral monitored groups.

IMPLICATIONS: Compared with standard monitoring practices, use of an auditory evoked potential or bispectral index monitor to titrate the volatile anesthetic led to a significant reduction in the anesthetic requirement. The anesthetic-sparing effect of cerebral monitoring resulted in a shorter postanesthesia care unit stay and improved quality of recovery from the patient’s perspective.

	Introduction

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The impact of monitoring the level of consciousness during general anesthesia on recovery remains controversial (1–5). The monitoring of patients’ vital signs remains the most common method for determining "depth of anesthesia" during surgery. The value of cerebral monitoring as an adjuvant to clinical monitoring of autonomic responses during surgery has been questioned (1,3,5). However, recent studies have suggested that use of cerebral monitoring could improve early recovery after general anesthesia because of the ability to minimize both over- and underdosage with anesthetic drugs during the maintenance period (6–8).

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 depth of anesthesia (9–11). The bispectral index (BIS) and physical state index (PSI) are processed variables derived from spontaneously-evoked EEG signals that have been used to quantify the sedative and hypnotic effects of anesthetic drugs on the central nervous system (CNS) (12). The BIS and PSI values are both dimensionless numbers that vary from 0 to 100, with values <60 associated with "adequate" hypnosis under general anesthesia. Values larger than 75 are typically observed during emergence from anesthesia (11). Although the BIS and PSI values have been correlated with the degree of sedation produced by IV hypnotics and volatile anesthetics, these EEG-based indices are less useful in assessing the effects of opioid analgesics, ketamine, and nitrous oxide (N₂O) on the CNS (10,12).

In contrast to the BIS and PSI monitoring devices, the auditory evoked potential (AEP) monitor uses the AEP index (AAI), an averaging of the EEG signal recovered from a mastoid-vertex electrode montage in response to auditory stimulation (i.e., clicking in the ear), to assess the depressant effects of anesthetic drugs on the CNS (13,14). This auditory stimulus produces a highly reproducible sequence of EEG waveforms in the brain. The early cortical response, also known as the mid-latency of the AEP (MLAEP), varies in a consistent manner in response to the administration of both IV and inhaled anesthetic drugs (15). The AAI value is a numerical variable between 0 and 100 that is calculated from the sum of the square root of the absolute difference between two successive segments of the MLAEP waveform using an autoregressive model (with an exogenous input adaptive method) to perform a fast extraction of the AEP-evoked EEG signal (16). Therefore, in contrast to the BIS and PSI values (which reflect spontaneous EEG activity), the AAI value additionally represents an evoked EEG response.

Preliminary studies suggest that the AAI changes in a consistent and dose-dependent manner in response to the administration of sedative-hypnotic and volatile anesthetics but not opioid analgesics (13–15,17,18). Although Gajraj et al. (19) compared AEP and BIS monitoring in spontaneously breathing surgical patients, there was no control group in their study. Furthermore, these investigators did not evaluate the impact of either AEP or BIS monitoring on recovery from general anesthesia.

The aim of the present study was to evaluate the impact of using the BIS and AEP monitors as adjuvants to standard clinical practices on the volatile anesthetic requirement and the quality of recovery after general anesthesia in patients undergoing major laparoscopic surgery with a standardized anesthetic technique. Therefore, the hypothesis being tested was that the adjunctive use of the BIS or AAI values to guide the administration of desflurane would reduce the anesthetic requirement and facilitate a faster recovery after general anesthesia compared with standard clinical monitoring practices alone.

	Methods

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After obtaining approval from the IRB at the University of Texas Southwestern Medical Center at Dallas, written informed consent was obtained from 90 patients undergoing laparoscopic general surgery procedures (e.g., cholecystectomy, gastric bypass/banding, hernia repair). These patients were randomly assigned to one of three study groups: standard clinical practice (control), AEP-guided, or BIS-guided. Exclusionary criteria included a history of CNS disease (e.g., hearing loss, seizure disorders), chronic use of psychoactive medication, and any clinically significant cardiovascular, renal, hepatic, or endocrinologic disorders.

In the preoperative holding area, two cutaneous electrodes were placed on the forehead and one behind the left ear over the mastoid bone, and acceptable contact impedance was confirmed when the electrodes were connected to the AEP monitor (A-line^TM; Danmeter, Denmark). The AAI was automatically calculated based on the amplitude and latency of the MLAEP responses. The BIS monitor used a standard disposable sensor (BIS^TM sensor XP, Aspect Medical Systems, Newton, MA) that was applied to the patient’s forehead as recommended by the manufacturer. There was no positional interference between the electrodes for the two cerebral monitoring devices (Fig. 1). On arrival to the operating room, the standard clinical monitoring devices were applied (including a noninvasive blood pressure cuff, electrocardiogram, pulse oximeter, and capnograph), and the AEP and BIS monitors were connected to the electrodes. The earbuds were placed bilaterally in the external meatus. Although the AAI or BIS values were recorded at 3–5 min intervals by one of the investigators in all patients, the real-time AAI or BIS values were only made available during the procedure to those anesthesiologists caring for patients in the AEP or BIS-guided groups, respectively.

View larger version (125K): [in this window] [in a new window]   Figure 1. The electrodes for both the bispectral index (BIS) and the auditory evoked potential (AEP) monitors were applied simultaneously on all the patients as illustrated.

Intermittent bolus doses of fentanyl, 0.5 µg/kg IV, were also given as needed to maintain stable hemodynamic variables (mean arterial blood pressure [MAP] and heart rate [HR] within 15% of the baseline values) during surgery. In addition, labetalol, 5 mg IV boluses, was available in all three groups to control acute sympathetic responses to noxious surgical stimuli in the presence of an "adequate" hypnotic state as assessed using clinical signs (control group) and/or one of the cerebral monitors (AEP- or BIS-guided groups). At the end of the surgical procedure, residual neuromuscular block was antagonized with neostigmine, 50 µg/kg IV, and glycopyrrolete, 5 µg/kg IV, desflurane was discontinued, and the oxygen flow rate was increased to 5 L/min.

The HR, systolic blood pressure (SBP), diastolic blood pressure (DBP), and MAP values, as well as BIS and AAI values, were recorded at 3–5 min intervals during the induction, maintenance, and emergence periods, as well as immediately before and after any changes in the inspired concentration of desflurane. The end-tidal concentrations of desflurane and oxygen saturation values were also recorded at 3–5 min intervals during the maintenance period. Emergence times (e.g., opening eyes, obeying simple verbal commands, and orientation to person and place) were determined at 1-min intervals after discontinuation of desflurane by a blinded observer. Tracheal extubation time was calculated as the time from cessation of desflurane until the tracheal tube was removed.

The time to achieve a White fast-track score 12 (20) and an Aldrete discharge score of 10 (21), as well as the length of stay in the postanesthesia care unit (PACU), were assessed at 5–10 min intervals by a PACU nurse. Finally, the occurrence of postoperative side effects (e.g., pain, nausea, vomiting, dizziness) and the requirements for "rescue" analgesic and antiemetic drugs were recorded at the time of discharge from the PACU. Patients were also asked to evaluate their pain and nausea using 11-point verbal rating scales (0 = none and 10 = highest), and their level of satisfaction with anesthesia using a 100-point verbal rating scales (1 = poor to 100 = excellent). A standardized 18-point questionnaire was used to evaluate the patient’s quality of recovery (22).

An a priori power analysis based on a preliminary clinical utility study involving the AEP monitor (23) estimated that 28 patients would be required in each group to detect a 25% difference in the volatile anesthetic requirement with a power of 0.8 ( = 0.05). Continuous data were analyzed and compared using repeated-measures analysis of variance followed by multiple comparisons with Bonferroni’s correction. Categorical data were analyzed using the ² test with Fisher’s exact test where appropriate. The relationship between AAI values, BIS values, and desflurane concentrations and the relationship between BIS and AAI values during the maintenance period were analyzed using Spearman’s correlation and linear regression to determine the correlation coefficients (r-value).

Power and probability level were analyzed using NCSS software (Number Cruncher Statistical Systems for Windows, Kaysville, UT). Data are presented as mean values (± SD), median values (with interquartile ranges), numbers, and percentages, with P values < 0.05 considered statistically significant.

	Results

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The three study groups were comparable with respect to their demographic characteristics, types of laparoscopic operations, and the durations of surgery and anesthesia (Table 1). The AEP- and BIS-guided groups received significantly smaller average end-tidal concentrations of desflurane than the control group (3.8 ± 0.9% and 3.9 ± 0.6% versus 4.7 ± 0.7%, respectively, P < 0.01). The usage of fentanyl and labetalol to control the acute hemodynamic responses did not differ significantly among the three groups. Although the preinduction (baseline) AAI and BIS values were comparable among the three study groups, the averaged AAI and BIS values during the maintenance period were significantly larger in the AEP- and BIS-guided groups than in the control group (15 ± 5 and 16 ± 3 versus 13 ± 7 for the AAI, and 48 ± 11 and 49 ± 13 versus 40 ± 11 for the BIS, respectively, P < 0.01) (Table 2).

View larger version (21K): [in this window] [in a new window]   Figure 2. Correlation between the individual bispectral index (BIS) and the auditory evoked potential index (AAI) values during the maintenance period (r = 0.36).

	Discussion

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Despite the introduction of several new cerebral monitoring technologies in anesthesia (e.g., BIS, physical state analyzer [PSA], AEP), there is still no universally accepted gold standard for measuring depth of anesthesia or level of consciousness during anesthesia. The so-called minimum alveolar concentration (MAC) is an ED₅₀ value that reflects the potency of inhaled anesthetics (24). However, the MAC value of a volatile anesthetic does not predict the level of hypnosis during general anesthesia. For a cerebral monitor to be clinically reliable in assessing the depth of anesthesia, it should display a strong correlation between the observed variable (e.g., BIS, PSI, AAI) and the patient’s state of consciousness, as well as their physiologic responses to noxious stimuli during surgery, independent of the anesthetic drugs and with minimal interpatient variability. In a recent editorial, Kalkman and Drummond (5) suggested that these conditions have not yet been achieved with any of the currently available cerebral monitoring devices.

Initial studies in volunteers have demonstrated a dose-response relationship between the MLAEP and increasing concentrations of both inhaled and IV anesthetics (15). Analogous to the BIS (and PSI), the AAI appears to be a good predictor of the level of sedation/hypnosis and unconsciousness (25). In a study involving patients undergoing cardiac surgery, Schwender et al. (26) also found an association between the auditory response and implicit memory. More recently, Maattanen et al. (27) reported decreased desflurane consumption during spine surgery when AEP monitoring was used to guide the administration of the anesthetic. However, none of the published studies involving the AEP monitor have found consistent benefits with respect to anesthetic-sparing (8) and meaningful outcome variables such as the length of the PACU recovery stay and quality of recovery (6,7).

Clinical utility studies involving cerebral monitoring with the BIS and PSA devices have demonstrated improved titration of both IV (6,9) and inhaled (7) anesthetics and suggest that these devices may be useful in expediting the early recovery process. In a recent study, Chen et al. (11) found a very similar pattern of changes in the BIS and PSI values during general anesthesia involving propofol and desflurane. Struys et al. (14) 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, both of these indices had poor predictive power with respect to movement in response to noxious stimuli. Schraag et al. (25) found that the AEP index actually possessed better discriminatory power than the BIS in describing the transition from the conscious to the unconscious state.

Analogous to the early comparative studies (14,19,25), this prospective study demonstrated that both the BIS and AEP monitors were able to distinguish between the awake and anesthetized states. Even though the AAI was consistently lower than the BIS during the maintenance period, a positive correlation was found between these indices, suggesting that both respond in a similar fashion to changes in the level of hypnosis. A weak negative correlation was also found between AEP and BIS values and desflurane concentrations during surgery, suggesting that these indices reflect the effects of the volatile anesthetic on the CNS. Of interest, the AAI displayed a larger degree of variability during the intraoperative period and was slower to return to baseline values after surgery compared with the BIS. This observation would suggest that the AAI might possess an increased sensitivity to the residual CNS effects of the anesthetic drugs. Alternatively, the AAI may simply be less stable over time, and the failure to return to the baseline may reflect intraoperative "drift."

As the average BIS and AAI values were significantly smaller in the control group compared with the two monitored groups, these data indicated that patients were maintained at a lighter level of anesthesia (hypnosis) when the anesthesiologists had access to the information provided by these cerebral monitors. One of the concerns in minimizing the use of anesthetics relates to the possibility that patients may experience more purposeful movements during surgery. In a pilot study involving nonparalyzed patients, the electromyelogram activity recorded by the AEP monitor interfered with our ability to obtain reliable AAI values. Therefore, in the current study all the patients were paralyzed during their laparoscopic procedures, and it was not possible to assess whether the use of the AEP device would be associated with increased movements in response to noxious surgical stimuli. Further studies are clearly needed to assess the effect of AEP monitoring on the adequacy of surgical conditions.

It has been suggested that the use of cerebral monitors to minimize the administration of anesthetic drugs may result in increased autonomic stress responses and adverse clinical outcomes (e.g., myocardial ischemia, intraoperative awareness). Interestingly, a recent study by Weldon et al. (28) found that there was an apparent positive correlation between the length of time during anesthesia that the BIS value was <45 and the incidence of adverse clinical outcomes in an elderly surgical population. In the present study, the hemodynamic variables during the maintenance period were similar in all three groups despite the fact that the AEP- and BIS-guided groups received 20% and 18% less desflurane, respectively. Although more patients in the monitored groups received labetalol, this difference was not statistically significant. Importantly, there were no serious adverse cardiovascular outcomes after surgery and none of the patients reported recall of any intraoperative events.

Analogous to the earlier studies with the BIS (6,7) and PSA (9) monitors, these intraoperative data suggest that cerebral monitoring with the AEP device could be useful in improving the titration of anesthetic drugs, thereby facilitating a more rapid recovery from anesthesia. Of interest, these previously mentioned studies (6,7,9,23) failed to find significant differences in the length of the recovery stay or the time to discharge home. In the present study, patients in both cerebral-monitored groups more rapidly achieved surrogate recovery end-points (e.g., fast-track eligibility, Aldrete discharge criteria) and experienced a shorter PACU stay even though none of these patients were allowed to bypass the PACU. If the patients who had satisfied the fast-track criteria (20) had been allowed to bypass the PACU, the length of the recovery stay would have been further reduced in the cerebral-monitored groups. From the perspective of the patients, the quality of recovery was also significantly improved in the cerebral-monitored groups compared with the control group. However, there were no significant differences between the AEP and BIS monitors with respect to any of the clinical end-points we studied.

The simultaneous application of both the AEP- and BIS-monitoring electrodes may lead to concerns regarding the possibility of interference with signal recognition. However, Absalom et al. (29) reported that the AEP clicks did not alter the clinical signs of depth of anesthesia, and were not associated with changes in the BIS values. Although some anesthesia practitioners may have concerns about using labetalol to control acute sympathetic responses during anesthesia, it is well known that patients can manifest hemodynamic responses to noxious surgical stimuli (i.e., a brainstem function) in the presence of an adequate anesthetic (hypnotic) state.

This study could also be criticized because it was not adequately powered to examine adverse outcomes such as myocardial ischemia (or infarction) and intraoperative awareness. Given the infrequent incidence of these untoward events, it is obviously impossible to draw meaningful conclusions from a study involving only 90 patients. Finally, a cost-benefit analysis would be required to determine if the routine use of these monitoring devices in clinical practice can be justified (30). However, the improved patient satisfaction with quality of recovery may be a more important justification for the increased cost associated with routine use of cerebral monitoring than the cost-savings related to reduced anesthetic drug usage and shortened PACU stay. In contrast to the BIS monitor, there is only a relatively limited clinical experience using the AEP monitor to guide the administration of anesthetic drugs. Therefore, additional studies with the AEP monitor in larger, more diverse surgical populations are clearly needed to determine the future role of this cerebral monitoring device in clinical practice.

In summary, the availability of the processed EEG information provided by the AEP and BIS monitors reduced the volatile anesthetic requirement and contributed to a faster recovery from general anesthesia after laparoscopic surgery. More importantly, use of these cerebral monitors lead to a significant improvement in the patient’s assessment of their quality of recovery.

	Acknowledgments

 
Supported, in part, by an educational grant from Alaris Medical Systems (San Diego, CA), and salary support for Dr. P. F. White from the Margaret Milam McDermott Distinguished Chair of Anesthesiology.

We appreciate the support of Pat Embree at Aspect Medical Systems and Louise Kenney at Alaris Medical System. In addition, the secretarial assistance of Dolly Tutton was greatly appreciated.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999