Comparison of auditory evoked potential parameters for predicting clinically anaesthetized state

BACKGROUND: Most of the research efforts to monitor the depth of anaesthesia using the mid-latency auditory evoked potential (MLAEP) signal in humans are based on the detection of the amplitudes and latencies of the signal peaks. Attempts have also been made to combine different time-domain and frequency-domain parameters. A comparison of different parameters is required to identify those which best discriminate the awake state from the anaesthetized state. METHODS: Although the sensitivity of MLAEP signal peaks is appreciable in awake and light anaesthesia states, it is reduced considerably at the moderate anaesthesia level, rendering this method unsuitable for predicting the surgical stage of anaesthesia. To overcome this problem, a numerically derived quantity--the morphology index--was used which does not require location of the peaks of the signal, but, at the same time, reflects the changes in both the latency and amplitude of the peaks. AEPs were recorded in the hospital for 18 patients during various states, i.e. awake, induction, unconscious and after regaining consciousness from halothane anaesthesia. The peak latencies, amplitudes, morphology index and peak power frequency (PPF) were calculated. RESULTS: The sensitivity and specificity of PPF (89% and 95%, respectively) were found to be better than those for Pa and Nb peak amplitudes, their latencies and the morphology index. In addition, PPF showed minimum inter-patient variation. The mean value (standard deviation) of this parameter was 26.9 (0.67) during the awake state, decreased to 17.1 (1.2) during the anaesthetized state, and increased again to 26.1 (0.93) when the patients regained full consciousness. CONCLUSION: PPF is the best of the four studied MLAEP parameters for the clinical characterization of the anaesthetized state during surgery.     

Midlatency auditory evoked potentials during anaesthesia with increasing endexpiratory concentrations of desflurane

Background. Under general anaesthesia with the volatile anaesthetics halothane, enflurane and isoflurane, midlatency auditory evoked potentials (MLAEP) are suppressed dose-dependently. Therefore, MLAEP have been used to measure depth of anaesthesia and to indicate intraoperative awareness. Desflurane is a new volatile anaesthetic and its effects on MLAEP have not been studied previously.
Methods. We have studied MLAEP during general anaesthesia with increasing endexpiratory concentrations of desflurane in 12 patients scheduled for elective gynaecological surgery. Auditory evoked potentials were recorded in the awake state and during anaesthesia with endexpiratory steady state concentrations of 1.5, 3.0, 4.5 and 6.0 vol %, of desflurane on vertex (positive) and mastoids on both sides (negative). Latencies of the peaks V, Na, Pa, Nb, Pl (ms) and amplitudes Na/Pa, Pa/Nb and Nb/Pl (μV) were measured.
Results. In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. During general anaesthesia with increasing endexpiratory concentration of desflurane, the latency of the brainstem response V increased only slightly. In contrast, MLAEP showed a marked dose-dependent and statistically significant increase in latencies of Na, Pa, Nb and Pl and decrease in amplitudes of Na/Pa, Pa/Nb and Nb/Pl. Under 6.0 vol % of desflurane MLAEP were severely attenuated or even abolished.
Conclusion. Based on these observations, endexpiratory concentrations of ≥4.5 vol % desflurane should suppress awareness phenomena such as auditory perceptions during anaesthesia.     

Comparison of bispectral EEG analysis and auditory evoked potentials for monitoring depth of anaesthesia during propofol anaesthesia

We have compared the auditory evoked potential index (AEPIndex) and bispectral index (BIS) for monitoring depth of anaesthesia in spontaneously breathing surgical patients. Twenty patients (aged 17-49 yr) undergoing day surgery were anaesthetized with computer-controlled infusions of propofol. The mean (SD and range) of each measurement was determined during consciousness and unconsciousness and at specific times during the perioperative period. Mean values for AEPIndex during consciousness and unconsciousness were 74.5 (SD 14.7) 36.7 (7.1), respectively. BIS had mean values of 89.5 (SD 4.6) during consciousness and 48.8 (16.4) during unconsciousness. AEPIndex and BIS were greater during consciousness compared with during unconsciousness. The average awake values of AEPIndex were significantly higher than all average values during unconsciousness but this was not the case for BIS. BIS increased gradually during emergence from anaesthesia and may therefore be able to predict recovery of consciousness at the end of anaesthesia. AEPIndex was more able to detect the transition from unconsciousness to consciousness.      

Auditory evoked potentials during isoflurane anaesthesia

Brainstem auditory evoked potentials (BAEP) were determined in 12 volunteers. The effect of isoflurane anaesthesia on BAEP was determined in six patients. Body temperature and end-tidal CO2% were controlled. Increasing end-tidal isoflurane concentration from 0.6-2.4% increased BAEP wave I, III and V latencies. The amplitude of wave V decreased with increasing isoflurane concentration. Thus a dose-related change was demonstrated between end-tidal concentration of isoflurane and BAEP latencies.     

Effects of the auditory stimuli of an auditory evoked potential system on levels of consciousness, and on the bispectral index

Investigators in the field of depth of anaesthesia monitoringsometimes measure the auditory evoked potential (AEP) and theBispectral Index (BIS) concurrently. However, the auditory stimulirequired to generate an AEP may increase the level of consciousness,and cause an increase in the BIS. They may also alter the BISby producing phase-locked harmonics in the surface electroencephalogram.The aim of this study was to determine if AEP stimuli have clinicallysignificant effects on levels of consciousness and BIS valuesduring sedation and general anaesthesia. Ten healthy adult patientswere studied by measuring and recording the BIS for 6 epochsof 5 min each. The first 3 epochs took place during steady-statesedation, during which time the Observer’s Assessmentof Awareness/Sedation (OAA/S) score was also measured. The second3 epochs took place during steady-state anaesthesia. Duringalternate epochs, patients were subjected to the auditory stimuligenerated by an AEP system. The auditory stimuli were not associatedwith a change in BIS values (during sedation and anaesthesia)or OAA/S scores (sedation). Br J Anaesth 2001; 87: 778–80     

Awareness detected by auditory evoked potential monitoring

We report a case of awareness detected by the Alaris AEP Monitor,a device that measures anaesthesia by tracking changes of thewaveform of the mid-latency auditory evoked potential. Br J Anaesth 2003; 91: 290–2     

Auditory evoked potentials

Auditory evoked potentials (AEPs) are an electrical manifestation of the brain response to an auditory stimulus. The waveform represents the passage of electrical activity provoked by auditory stimuli from the cochlea to cortex. The waves represented by I-VII are generated mainly in the brainstem. These waves are called the brain stem auditory evoked potentials (BAEPs) or the auditory brain stem response (ABR). The middle latency AEPs (MLAEP) are generated from the medial geniculate and primary auditory cortex. The long latency AEPs (LLAEP) are generated from the frontal cortex and association areas. The BAEPs appear to be an exquisitely sensitive monitor for pathological events during surgery. Anesthetics and mild hypothermia have minimum effect, if any, on the BAEPs. The BAEPs are useful during the microvascular decompression of the fifth or seventh cranial nerve, resection of acoustic neuroma and posterior fossa operations. Because the auditory pathway occupies a small area in the brainstem, combined use of other evoked potentials such as short latency sensory evoked potentials is recommended. The MLAEPs are most promising evoked responses for monitoring awareness or depth of anesthesia. When the concentration of anesthetics is increased, the amplitudes of the MLAEP's peaks are decreased and their latencies are elongated. Commercially developed A-line AEP monitor or aepEX can extract the AEPs waveform in a short period and automatically analyze the changes in the MLAEPs. These AEP based monitors may be superior to bispectral index (BIS) in detecting the transition from unconsciousness to consciousness.     

Auditory evoked potentials

Auditory evoked potentials (AEPs) are an electrical manifestation of the brain response to an auditory stimulus. Mid-latency auditory evoked potentials (MLAEPs) and the coherent frequency of the AEP are the most promising for monitoring depth of anaesthesia. MLAEPs show graded changes with increasing anaesthetic concentration over the clinical concentration range. The latencies of Pa and Nb lengthen and their amplitudes reduce. These changes in features of waveform are similar with both inhaled and intravenous anaesthetics. Changes in latency of Pa and Nb waves are highly correlated to a transition from awake to loss of consciousness. MLAEPs recording may also provide information about cerebral processing of the auditory input, probably because it reflects activity in the temporal lobe/primary cortex, sites involved in sounds elaboration and in a complex mechanism of implicit (non declarative) memory processing. The coherent frequency has found to be disrupted by the anaesthetics as well as to be implicated in attentional mechanism. These results support the concept that the AEPs reflects the balance between the arousal effects of surgical stimulation and the depressant effects of anaesthetics. However, AEPs aren't a perfect measure of anaesthesia depth. They can't predict patients movements during surgery and the signal may be affected by muscle artefacts, diathermy and other electrical operating theatre interferences. In conclusion, once reliability of the AEPs recording became proved and the signal acquisition improved it is likely to became a routine feature of clinical anaesthetic practice.     

Analysis of the EEG bispectrum, auditory evoked potentials and the EEG power spectrum during repeated transitions from consciousness to unconsciousness

We have compared the auditory evoked potential (AEP) index (a numerical index derived from the AEP), 95% spectral edge frequency (SEF), median frequency (MF) and the bispectral index (BIS) during alternating periods of consciousness and unconsciousness produced by target- controlled infusions of propofol. We studied 12 patients undergoing hip or knee replacement under spinal anaesthesia. During periods of consciousness and unconsciousness, respective mean values for the four measurements were: AEP index, 60.8 (SD 13.7) and 37.6 (6.5); BIS, 85.1 (8.2) and 66.8 (10.5); SEF, 24.2 (2.2) and 18.7 (2.1); and MF, 10.9 (3.3) and 8.8 (2.0). Threshold values with a specificity of 100% for a state of unconsciousness were: AEP index, 37 (sensitivity 52%); BIS, 55 (sensitivity 15%); and SEF, 16.0 (sensitivity 9%). There was no recorded value for MF that was 100% specific for unconsciousness. Of the four measurements, only AEP index demonstrated a significant difference (P < 0.05) between all mean values 1 min before recovery of consciousness and all mean values 1 min after recovery of consciousness. Our findings suggest that of the four electrophysiological variables, AEP index was best at distinguishing the transition from unconsciousness to consciousness.      

Midlatency auditory evoked potentials do not allow the prediction of recovery from general anesthesia with isoflurane

PURPOSE: To investigate midlatency auditory evoked potentials (MLAEP) waveforms during recovery from anesthesia. The hypothesis was that MLAEP are sensitive variables to discriminate between states of consciousness and unconsciousness during emergence from anesthesia. METHODS: MLAEP were recorded in the awake state and during the wake-up phase from isoflurane anesthesia in 22 female patients undergoing ophthalmologic surgery. During emergence from anesthesia the changes in latency and amplitude of MLAEP components Na, Pa and Nb were compared with the awake level. The next day the patients were asked for explicit memory for the recovery period. RESULTS: In 72% of the patients the MLAEP waveforms were completely suppressed during isoflurane anesthesia. When the patients responded and opened their eyes spontaneously 38 +/- 12 min after anesthesia, the latencies of Na (18.3 +/- 1.2 vs 17.6 +/- 1.3; P = 0.013) and Nb (47.4 vs 7.1 vs 44.7 +/- 7.8; P = 0.048) remained prolonged compared with awake values. In contrast, the amplitudes NaPa and PaNb had regained baseline level. Nine patients had explicit memory for the immediate recovery period. However, there was no difference for any MLAEP component between patients with and without memory at any time. CONCLUSIONS: The persistent changes of MLAEP latency components Na and Nb indicated impaired auditory signal processing 38 min after isoflurane anesthesia. There was a marked intra- and inter-individual variability during reversal of the anesthetic induced MLAEP changes. This limits the prediction of recovery of consciousness in the individual patient during emergence from anesthesia.     

The performance of electroencephalogram bispectral index and auditory evoked potential index to predict loss of consciousness during propofol infusion.

The bispectral index (BIS) of the electroencephalogram and middle latency auditory evoked potentials are likely candidates to measure the level of unconsciousness and, thus, may improve the early recovery profile. We prospectively investigated the predictive performance of both measures to distinguish between the conscious and unconscious state. Twelve patients undergoing lower limb orthopedic surgery during regional anesthesia additionally received propofol by target-controlled infusion for sedation. The electroencephalogram BIS and the auditory evoked potential index (AEPi), a mathematical derivative of the morphology of the auditory evoked potential waveform, were recorded simultaneously in all patients during repeated transitions from consciousness to unconsciousness. Logistic regression procedures, receiver operating characteristic analysis, and sensitivity and specificity were used to compare predictive ability of both indices. In the logistic regression models, both the BIS and AEPi were significant predictors of unconsciousness (P < 0.0001). The area under the receiver operating characteristic curve for discrete descending index threshold values was apparently, but not significantly (P > 0.05), larger for the AEPi (0.968) than for the BIS (0.922), indicating a trend of better discriminatory performance. We conclude that both the BIS and AEPi are reliable means for monitoring the level of unconsciousness during propofol infusion. However, AEPi proved to offer more discriminatory power in the individual patient. IMPLICATIONS: Both the bispectral index of the electroencephalogram and the auditory evoked potentials index are good predictors of the level of sedation and unconsciousness during propofol infusion. However, the auditory evoked potentials index offers better discriminatory power in describing the transition from the conscious to the unconscious state in the individual patient.     

Comparison of auditory evoked potentials and the A-line ARX Index for monitoring the hypnotic level during sevoflurane and propofol induction

BACKGROUND: Extraction of the middle latency auditory evoked potentials (AEP) by an auto regressive model with exogenous input (ARX) enables extraction of the AEP within 1.7 s. In this way, the depth of hypnosis can be monitored at almost real-time. However, the identification and the interpretation of the appropriate signals of the AEP could be difficult to perform during the anesthesia procedure. This problem was addressed by defining an index which reflected the peak amplitudes and latencies of the AEP, developed to improve the clinical interpretation of the AEP. This index was defined as the A-line Arx Index (AAI). METHODS: The AEP and AAI were compared with the Modified Observers Assessment of Alertness and Sedation Scale (MOAAS) in 24 patients scheduled for cardiac surgery, anesthetized with propofol or sevoflurane. RESULTS: When comparing the AEP peak latencies and amplitudes and the AAI, measured at MOAAS level 5 and level 1, significant differences were achieved. (mean(SD) Nb latency: MOAAS 5 51.1 (7.3) ms vs. MOAAS 1: 68.6 (8.1) ms; AAI: MOAAS 5 74.9 (13.3) vs. MOAAS 1 20.7 (4.7)). Among the recorded parameters, the AAI was the best predictor of the awake/anesthetized states. CONCLUSION: We conclude that both the AAI values and the AEP peak latencies and amplitudes correlated well with the MOAAS levels 5 (awake) and 1 (anesthetized).     

Midlatency auditory evoked potentials in children: effect of age and general anaesthesia

BACKGROUND: Midlatency auditory evoked potentials (MLAEP) are a promising tool for monitoring suppression of sensory processing during anaesthesia and might help to avoid awareness. MLAEP in children are different to those in adults and the exact changes during general anaesthesia are unknown. METHODS: In 49 children of age between 2 and 12 yr, MLAEP were recorded before anaesthesia, during tracheal intubation, at steady-state balanced anaesthesia, and after extubation. RESULTS: MLAEP were recordable in all children in the awake (premedicated) state with latencies but not amplitudes dependent on children's age. MLAEP latencies significantly increased during tracheal intubation and steady-state anaesthesia. Changes in amplitudes were inconsistent. All MLAEP variables returned to near baseline values after extubation. CONCLUSIONS: The results of this study imply that MLAEP can successfully be recorded during anaesthesia in children above the age of 2 yr. Further studies are necessary before MLAEP might be applicable for monitoring purposes in paediatric anaesthesia.     

颈胸椎手术中体感诱发电位监护的影响因素分析及准确性观察

目的探讨体感诱发电位监护在颈、胸椎手术中的应用价值,对其准确性及影响因素进行分析。方法颈、胸椎疾病患者采用皮层体感诱发电位（CSEP）及皮层下体感诱发电位（Sub-CSEP）术中监测,根据麻醉前、后及不同手术阶段体感诱发电位的变化与术后功能相结合,判断体感诱发电位（SEP）的准确性。结果麻醉因素引起的SEP波幅降低主要影响CSEP,双侧刺激均改变,但基本未达到手术预警标准值,而Sub-SEP变化不明显。手术高危操作侵及脊髓,SEP波幅降低主要影响高危操作的同侧,表现为同侧CSEP及Sub—CSEP波幅同时降低,并且降低幅度较大,达到预警标准值,而对侧CSEP及Sub-CSEP波幅变化不甚明显。其他非手术原因如低体温、局部冷盐水冲洗脊髓,引起SEP潜伏期延长,波幅变化不明显。失血过多致平均动脉压降低可引起SEP波幅降低,潜伏期变化不明显。局部低温及低血压引起的SEP改变,均未达到预警标准值。结论颈、胸椎手术中采用体感诱发电位监测排除各种干扰,可较准确地反映脊髓的生理或病理状况。     

脊髓纵裂手术前后胫后神经体感诱发电位分析

２０例脊髓纵裂患者手术前后进行了两下肢胫后神经皮层体感诱发电位（ＣＳＥＰ）检查，并选择了２０例正常人作为对照组，结果发现手术组与对照组ＣＳＥＰ有显著性差异，手术治疗后患者ＣＳＥＰ的Ｐ４０峰潜伏期及波幅明显改善，患者两下肢间的ＣＳＥＰ亦有明显差异。表明ＣＳＥＰ是一敏感、客观、可靠的诊断指标，可用来判断神经损害的程度，评价手术疗效。文中并讨论了神经缺陷的机理。     

Effect of sevoflurane on the mid-latency auditory evoked potentials measured by a new fast extracting monitor

BACKGROUND: Mid-latency auditory evoked potentials (MLAEP) are widely suppressed during general anesthesia and may therefore be useful for assessment of the depth of anesthesia. However, interpretation of amplitudes and latencies in the AEP signal is time consuming. A new monitor (A-line) that quantifies the MLAEP into an index has therefore been developed. The present study aimed to assess the precision of a prototype of the new monitor and to test the hypothesis that the depth of anesthesia index shows a graded response with changing steady-state end-expiratory concentrations of sevoflurane. METHODS: We studied 10 ASA physical status I or II patients undergoing elective hysterectomy under combined epidural and general anesthesia by sevoflurane. Baseline auditory evoked potentials were recorded in the conscious patient immediately before induction of general anesthesia. Depth of anesthesia indices were recorded before anesthesia and at decreasing end-expiratory steady-state sevoflurane concentrations of 2.0%, 1.5%, 1.0% and 0.5%. All indices were recorded in duplicate 6 s apart. By use of an autoregressive model with exogenous input (ARX-model), the monitor extracted the AEP within 6 s. The depth of anesthesia AEP index calculated in this way was defined as the A-line ARX index (AAI). RESULTS: Approximately 95% of the differences between repeated recordings were 5 AAI-units or less. A wide interindividual variation was observed at each observation point. AAI at 1%, 1.5% and 2% end-expiratory concentration was significantly less than the baseline AAI obtained before induction of anesthesia (P < 0.001). AAI did not change significantly in the 1-2% concentration range. CONCLUSION: The new monitor was precise. Attenuation of the A-line ARX-index (AAI) for mid-latency auditory evoked potentials (MLAEP) during general anesthesia was profound. However, the monitor did not show a graded response with changing end-expiratory steady-state concentrations of sevoflurane.     

The auditory evoked response as an awareness monitor during anaesthesia

We investigated the relationship between the latency of theNb wave of the auditory evoked response (AER) and periods ofawareness during propofol anaesthesia. In the anaesthetic roombefore cardiac surgery the AER was recorded continuously in14 patients. Awareness was measured by the ability of the patientto respond to command using the isolated forearm technique (IFT).The Nb latencies were shorter when the patients were able torespond than at loss of response (P<0.001). In six patientswho repeated this transition from response to loss of response,there was a high and significant correlation between Nb latencies.None of the patients had any recollection of events after theinitial induction of anaesthesia as measured by explicit andimplicit memory tests. These results suggest that the Nb latencyof the AER may represent an indication of awareness in individualpatients, but wide inter-patient variability limits its practicalusefulness. In addition, because no evidence of memory was demonstrated,even when patients were known to be awake, the relationshipbetween AER and memory processing remains unclear. Br J Anaesth 2001; 86: 513–8     

Midlatency median nerve evoked responses during recovery from propofol/sufentanil total intravenous anaesthesia

BACKGROUND: Median nerve somatosensory evoked responses (MnSSER) are frequently used to monitor the integrity of the somatosensory pathway during surgery. We investigated MnSSER components during the wakeup phase from anaesthesia with propofol/sufentanil, because detailed information is lacking about the reversibility of anaesthetic induced changes of MnSSER. The aim of the study was to document precisely the MnSSER waves in relation to the clinical awakening. The hypothesis was that anaesthetic induced MnSSER changes are reversed when the patient becomes responsive after anaesthesia. METHODS: In 20 gynaecological patients anaesthesia was maintained with propofol 8 mg kg(-1) h(-1) supplemented by bolus injections of sufentanil. MnSSER were recorded at C4' (N20, P25, N35, P45, N50) following electrical median nerve stimulation on the day before surgery, after the end of surgery during anaesthesia and every 5 min during recovery, till the patients were responsive again and able to identify a shown object. RESULTS: While the primary cortical MnSSER complex N20P25 regained baseline values, the cortical latencies > or =35 ms remained prolonged (P<0.001) and the amplitudes P45N50 were suppressed (P< or =0.013), when the patients were responsive after 26+/-7 min following anaesthesia. However, the amplitudes P25N35 exceeded their corresponding baseline value (P<0.01) CONCLUSION: Persistent changes of MnSSER waves > or =35 ms reflect impaired signal processing along the somatosensory pathway following propofol/sufentanil anaesthesia when the patients are responsive again. Further studies combining MnSSER recording with distinct neuro-psychological tests are needed to define the clinical relevance of these findings.     

AEP-monitor/2 derived, composite auditory evoked potential index (AAI-1.6) and bispectral index as predictors of sevoflurane concentration in children

BACKGROUND: Level of anesthesia may be predicted with the auditory evoked potential or with passive processed electroencephalogram (EEG) parameters. Some previous reports suggest the passive EEG does not reliably predict level of anesthesia in infants. The AAI-1.6 is a relatively new index derived from the AEP/2 monitor. It combines auditory evoked potentials and passive EEG parameters into a single index. This study aimed to assess the AAI-1.6 as a predictor of level of anesthesia in infants and children. METHODS: Four infants aged less than 1 year, and five older children aged between 2 and 11 years were enrolled. They all had uniform sevoflurane anesthesia for cardiac catheterization. The AAI-1.6 and bispectral index (BIS) were recorded after achieving equilibrium at 1.5%, 2% and 2.5% sevoflurane, and immediately prior to awakening. The prediction coefficient (Pk) for BIS and AAI-1.6 was calculated and compared within each age group. RESULTS: The Pk for the AAI-1.6 was low in both 0-1 and 2-11 years age groups. In the 2-12 years group, the Pk for BIS was significantly higher than the Pk for the AAI-1.6 (Pk for BIS: 0.89, Pk for AAI-1.6: 0.53, P < 0.01). In contrast in the 0-1 year age group there was no evidence for a difference between the Pk for BIS and the Pk for the AAI-1.6 (Pk for BIS: 0.74, Pk for AAI-1.6: 0.53, P = 0.25). CONCLUSIONS: This preliminary study suggests AAI-1.6 is a poor predictor of sevoflurane concentration in infants and children.     

Performance of bispectral index and auditory evoked potential monitors in detecting loss of consciousness during anaesthetic induction with propofol with and without fentanyl

BACKGROUND AND OBJECTIVE: To investigate and compare the performance of bispectral index (BIS) and auditory evoked response index (AAI) in detecting the transition from consciousness to unconsciousness during anaesthesia induction by propofol, alone and in combination with fentanyl. METHODS: Anaesthesia was induced with either an intravenous infusion of 30 mg kg(-1)h(-1) of propofol plus 2 microg kg(-1) of fentanyl (Group PF, n = 20) or an intravenous infusion of 30 mg kg(-1) h(-1) of propofol plus normal saline (Group P, n = 20). BIS, AAI and the doses of propofol administered were recorded at the end-point of unresponsiveness to verbal commands. The propofol plasma concentration was also measured. RESULTS: The propofol dose and plasma propofol concentration required to achieve loss of consciousness were significantly lower in patients pretreated with fentanyl (P < 0.001). The mean BIS value at loss of consciousness was significantly different between the two groups (74.10 in Group PF vs. 60.80 in Group P) (P < 0.001). However, no difference in the AAI was seen between the two groups at loss of consciousness (32.90 in Group PF vs. 31.80 in Group P) (P > 0.05). In both groups, the regression analysis values (r-values) between BIS and plasma propofol concentrations at the onset of unconsciousness were higher than those between AAI and propofol concentrations (0.553 vs. 0.180 in Group P; 0.432 vs. 0.308 in Group PF). CONCLUSIONS: These results show that a fentanyl bolus is effective in augmenting the hypnotic effect of propofol during anaesthesia induction. AAI appears to be able to measure the transition from consciousness to unconsciousness at similar values, regardless of whether or not fentanyl pretreatment is used whereas the BIS values were not independent of fentanyl pretreatment. This suggests that AAI may be a better indicator of conscious status during propofol/fentanyl anaesthesia, where it appears to be independent of the anaesthesia regimen.