Relationship between awareness and middle latency auditory evoked responses during surgical anaesthesia

Implicit Memory and Anesthesia: A Systematic Review and Meta-Analysis

General anesthesia should induce unconsciousness and provide amnesia. Amnesia refers to the absence of explicit and implicit memories. Unlike explicit memory, implicit memory is not consciously recalled, and it can affect behavior/performance at a later time. The impact of general anesthesia in preventing implicit memory formation is not well-established. We performed a systematic review with meta-analysis of studies reporting implicit memory occurrence in adult patients after deep sedation (Observer's Assessment of Alertness/Sedation of 0-1 with spontaneous breathing) or general anesthesia. We also evaluated the impact of different anesthetic/analgesic regimens and the time point of auditory task delivery on implicit memory formation. The meta-analysis included the estimation of odds ratios (ORs) and 95% confidence intervals (CIs). We included a total of 61 studies with 3906 patients and 119 different cohorts. For 43 cohorts (36.1%), implicit memory events were reported. The American Society of Anesthesiologists (ASA) physical status III-IV was associated with a higher likelihood of implicit memory formation (OR:3.48; 95%CI:1.18-10.25, p < 0.05) than ASA physical status I-II. Further, there was a lower likelihood of implicit memory formation for deep sedation cases, compared to general anesthesia (OR:0.10; 95%CI:0.01-0.76, p < 0.05) and for patients receiving premedication with benzodiazepines compared to not premedicated patients before general anesthesia (OR:0.35; 95%CI:0.13-0.93, p = 0.05).

Perception of auditory stimuli during general anesthesia and its effects on patient outcomes: a systematic review and meta-analysis

Purpose

Interest in implicit memory formation and unconscious auditory stimulus perception during general anesthesia has resurfaced as perioperative music has been reported to produce beneficial effects. We conducted a systematic review and meta-analysis of randomized controlled trials (RCTs) evaluating explicit and implicit memory formation during general anesthesia and its effects on postoperative patient outcomes and recovery.

Source

We performed a systematic literature search of Embase, Ovid Medline, and Cochrane Central from inception date until 15 October 2020. Eligible for inclusion were RCTs investigating intraoperative auditory stimulation in adult surgical patients under general anesthesia in which patients, healthcare staff, and outcome assessors were all blinded. We used random effects models for meta-analyses. This study adhered to the PRISMA guidelines and was registered in PROSPERO (CRD42020178087).

Principal findings

Fifty-three (4,200 patients) of 5,859 identified articles were included. There was evidence of implicit memory formation in seven out of 17 studies (41%) when assessed using perceptual priming tasks. Mixed results were observed on postoperative behavioural and motor response after intraoperative suggestions. Intraoperative music significantly reduced postoperative pain (standardized mean difference [SMD], -0.84; 95% confidence interval [CI], -1.1 to -0.57; P < 0.001; I² = 0; n = 226) and opioid requirements (SMD, -0.29; 95% CI, -0.57 to -0.015; P = 0.039; I² = 36; n = 336), while positive therapeutic suggestions did not.

Conclusion

The results of this systematic review and meta-analysis show that intraoperative auditory stimuli can be perceived and processed during clinically adequate, general anesthesia irrespective of surgical procedure severity, leading to implicit memory formation without explicit awareness. Intraoperative music can exert significant beneficial effects on postoperative pain and opioid requirements. Whether the employed intraoperative anesthesia regimen is of influence is not yet clear.

Electronic supplementary material

The online version of this article (10.1007/s12630-021-02015-0) contains supplementary material, which is available to authorized users.

Anaesthetic interventions for prevention of awareness during surgery

BACKGROUND

General anaesthesia is usually associated with unconsciousness. 'Awareness' is when patients have postoperative recall of events or experiences during surgery. 'Wakefulness' is when patients become conscious during surgery, but have no postoperative recollection of the period of consciousness.

OBJECTIVES

To evaluate the efficacy of two types of anaesthetic interventions in reducing clinically significant awareness:- anaesthetic drug regimens; and- intraoperative anaesthetic depth monitors.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL, ISSUE 4 2016); PubMed from 1950 to April 2016; MEDLINE from 1950 to April 2016; and Embase from 1980 to April 2016. We contacted experts to identify additional studies. We performed a handsearch of the citations in the review. We did not search trial registries.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) of either anaesthetic regimens or anaesthetic depth monitors. We excluded volunteer studies, studies of patients prior to skin incision, intensive care unit studies, and studies that only randomized different word presentations for memory tests (not anaesthetic interventions).Anaesthetic drug regimens included studies of induction or maintenance, or both. Anaesthetic depth monitors included the Bispectral Index monitor, M-Entropy, Narcotrend monitor, cerebral function monitor, cerebral state monitor, patient state index, and lower oesophageal contractility monitor. The use of anaesthetic depth monitors allows the titration of anaesthetic drugs to maintain unconsciousness.

DATA COLLECTION AND ANALYSIS

At least two authors independently scanned abstracts, extracted data from the studies, and evaluated studies for risk of bias. We made attempts to contact all authors for additional clarification. We performed meta-analysis statistics in packages of the R language.

MAIN RESULTS

We included 160 studies with 54,109 enrolled participants; 53,713 participants started the studies and 50,034 completed the studies or data analysis (or both). We could not use 115 RCTs in meta-analytic comparisons because they had zero awareness events. We did not merge 27 of the remaining 45 studies because they had excessive clinical and methodological heterogeneity. We pooled the remaining 18 eligible RCTs in meta-analysis. There are 10 studies awaiting classification which we will process when we update the review.The meta-analyses included 18 trials with 36,034 participants. In the analysis of anaesthetic depth monitoring (either Bispectral Index or M-entropy) versus standard clinical and electronic monitoring, there were nine trials with 34,744 participants. The overall event rate was 0.5%. The effect favoured neither anaesthetic depth monitoring nor standard clinical and electronic monitoring, with little precision in the odds ratio (OR) estimate (OR 0.98, 95% confidence interval (CI) 0.59 to 1.62).In a five-study subset of Bispectral Index monitoring versus standard clinical and electronic monitoring, with 34,181 participants, 503 participants gave awareness reports to a blinded, expert panel who adjudicated or judged the outcome for each patient after reviewing the questionnaires: no awareness, possible awareness, or definite awareness. Experts judged 351 patient awareness reports to have no awareness, 87 to have possible awareness, and 65 to have definite awareness. The effect size favoured neither Bispectral Index monitoring nor standard clinical and electronic monitoring, with little precision in the OR estimate for the combination of definite and possible awareness (OR 0.96, 95% CI 0.35 to 2.65). The effect size favoured Bispectral Index monitoring for definite awareness, but with little precision in the OR estimate (OR 0.60, 95% CI 0.13 to 2.75).We performed three smaller meta-analyses of anaesthetic drugs. There were nine studies with 1290 participants. Wakefulness was reduced by ketamine and etomidate compared to thiopental. Wakefulness was more frequent than awareness. Benzodiazepines reduces awareness compared to thiopental, ketamine, and placebo., Also, higher doses of inhaled anaesthetics versus lower doses reduced the risk of awareness.We graded the quality of the evidence as low or very low in the 'Summary of findings' tables for the five comparisons.Most of the secondary outcomes in this review were not reported in the included RCTs.

AUTHORS' CONCLUSIONS

Anaesthetic depth monitors may have similar effects to standard clinical and electrical monitoring on the risk of awareness during surgery. In older studies comparing anaesthetics in a smaller portion of the patient sample, wakefulness occurred more frequently than awareness. Use of etomidate and ketamine lowered the risk of wakefulness compared to thiopental. Benzodiazepines compared to thiopental and ketamine, or higher doses of inhaled anaesthetics versus lower doses, reduced the risk of awareness.

Nitrous oxide-based versus nitrous oxide-free general anaesthesia and accidental awareness during general anaesthesia in surgical patients

BACKGROUND

Accidental awareness during general anaesthesia (AAGA) is when a patient unintentionally becomes conscious during a procedure performed with general anaesthesia and subsequently has explicit recall of this event. Incidence estimates for AAGA vary, with the most common estimate being one to two cases per 1000 general anaesthetics. Evidence linking nitrous oxide use and an increased risk of AAGA has come from observational studies data but the literature is contradictory, with some studies finding a protective effect of nitrous oxide.

OBJECTIVES

To assess the effect of general anaesthesia including nitrous oxide on the risk of AAGA in patients aged five years and over.

SEARCH METHODS

We searched the following databases: Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE and trial registers ((www.clinicaltrials.gov), the WHO International Clinical Trials Registry Platform (www.who.int/ictrp/network/en/) and Current Controlled Trials (www.isrctn.com/)) for eligible studies on December 9 2015. In addition, we conducted forward and backward citation searching using key identified papers.

SELECTION CRITERIA

We considered all randomized controlled trials (RCTs), including quasi-randomized studies and cluster-randomized studies, of participants aged five years or older receiving general anaesthesia for any type of surgery.We included trials in which participants receiving general anaesthesia that included nitrous oxide for maintenance at a concentration of at least 30% were compared with participants receiving no nitrous oxide during general anaesthesia. The intervention group must have received nitrous oxide in conjunction with an additional anaesthetic. We excluded studies where the depth of anaesthesia differed between the study arms. For inclusion in the review, studies needed to state in their methods that they planned to assess AAGA. We defined this as when a patient becomes conscious during a procedure performed with general anaesthesia and subsequently has explicit recall of this event.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures expected by Cochrane to identify studies. We extracted data and conducted 'Risk of bias' assessment using the Covidence database.

MAIN RESULTS

We included 15 studies. The total number of participants included in the analyses was 3520. Most studies were small with fewer than 120 participants, although two larger studies with 2012 and 671 participants were included. There was considerable variation in many of the study characteristics, including the anaesthetics used. The concentrations of nitrous oxide varied between 50% and 70%, and half of the studies used clinical signs and haemodynamic changes to monitor depth of anaesthesia.As it was not possible to blind the anaesthetist to the anaesthetic used, we rated all studies at high risk of performance bias and we therefore downgraded the quality of evidence by one level for risk of bias using the GRADE approach. Other types of bias were generally low, or were rated unclear due to missing information.No studies were designed to measure AAGA as the primary outcome, and were therefore statistically underpowered to answer this review question. Despite the inclusion of 3520 participants, only three awareness events were reported by two studies. In one study the event was due to technical failure. Due to the rarity of the events, we did not consider it appropriate to pool the data, and we therefore downgraded the quality of evidence by a further level for imprecision using GRADE.

AUTHORS' CONCLUSIONS

It is not possible to draw any conclusions from this review. The included studies were mainly small (fewer than 120 participants) and there were limited estimates of effect, with only two studies reporting any events. We cannot therefore determine whether the use of nitrous oxide in general anaesthesia increases, decreases or has no effect on the risk of accidental awareness.

Use of auditory evoked potentials for intra-operative awareness in anesthesia: a consciousness-based conceptual model

Auditory evoked potentials (AEPs) have been used as a measure of the depth of anesthesia during the intra-operative process. AEPs are classically divided, on the basis of their latency, into first, fast, middle, slow, and late components. The use of auditory evoked potential has been advocated for the assessment of Intra-operative awareness (IOA), but has not been considered seriously enough to universalize it. It is because we have not explored enough the impact of auditory perception and auditory processing on the IOA phenomena as well as on the subsequent psychological impact of IOA on the patient. More importantly, we have seldom tried to look at the phenomena of IOP from the perspective of consciousness itself. This perspective is especially important because many of IOA phenomena exist in the subconscious domain than they do in the conscious domain of explicit recall. Two important forms of these subconscious manifestations of IOA are the implicit recall phenomena and post-operative dreams related to the operation. Here, we present an integrated auditory consciousness-based model of IOA. We start with a brief description of auditory awareness and the factors affecting it. Further, we proceed to the evaluation of conscious and subconscious information processing by auditory modality and how they interact during and after intra-operative period. Further, we show that both conscious and subconscious auditory processing affect the IOA experience and both have serious psychological implications on the patient subsequently. These effects could be prevented by using auditory evoked potential during monitoring of anesthesia, especially the mid-latency auditory evoked potentials (MLAERs). To conclude our model with present hypothesis, we propose that the use of auditory evoked potential should be universal with general anesthesia use in order to prevent the occurrences of distressing outcomes resulting from both conscious and subconscious auditory processing during anesthesia.

Does anesthetic regimen influence implicit memory during general anesthesia?

BACKGROUND

Implicit learning of intraoperative auditory stimuli during general anesthesia is very difficult to quantify but may require the presence of noxious stimulation. We hypothesized that an anesthetic regimen with a low dose of opioid would enhance implicit memory, while a regimen with a high dose of opioid would not.

METHODS

One hundred-twenty patients were randomized into 3 groups. All patients were anesthetized with a target-controlled infusion of propofol and remifentanil, targeting a Bispectral Index (BIS) value of 50. The remifentanil effect-site concentration (in ng/mL) was always double that of propofol (in μg/mL) in the first group and half of that in the second group. Patients in these 2 groups were played a list of 20 words via headphones during surgery. The third group served as control for memory tests and was not played any word during anesthesia. BIS was recorded during word presentation.

RESULTS

No statistical difference was found among the 3 groups regarding 3 different memory tests although 67.5% [50.7%; 80.9%] of the patients of the high-opioid group and 72.5% [55.9%; 84.9%] of the low-opioid group had at least 1 episode of BIS >60.

CONCLUSIONS

We could not demonstrate the presence of implicit or explicit memorization under propofol-remifentanil anesthesia either with a low- or a high-dose opioid anesthetic regimen.

Neurophysiological and Neuropsychological Correlates of Subconscious Auditory Processing During Anesthesia and Their Implications in Anesthesia Awareness

Like any other unusual state of consciousness, the condition of anesthesia remains a mystery, especially regarding the information processing events of the brain. Evoked potentials are the only known way to understand the neurophysiological events of brain in this condition. Auditory evoked potentials (AEPs) have been used as a measure of the depth of anesthesia during the intra-operative process. AEPs have been classically divided, on the basis of their latency, into first, fast, middle, slow, and late components. Auditory evoked potential has been advocated for the assessment of intra-operative awareness (IOA) but has not been considered seriously enough to be universalized. It is because we have not explored enough the impact of auditory perception and auditory information processing on the IOA phenomena as well as on the subsequent psychological impact of IOA on the patient. This limitation is because we have poor understanding of the subconscious auditory processing itself. This perspective is especially important because more of the IOA phenomena exist in the subconscious domain than they do in the conscious domain of explicit recall. Two important forms of these subconscious manifestations of IOA are the implicit recall phenomena and post-operative dreams related to the operation. Here we present a review of the neurophysiological and neuropsychological correlates of auditory processing during anesthesia. We start with a brief description of auditory awareness and the factors affecting it. Further, we proceed to the evaluation of conscious and subconscious information processing by auditory modality and how they interact during and after intra-operative period. Further, we show that both conscious and subconscious auditory processing affect the IOA experience and both have serious psychological implications on the patient subsequently. These effects could be prevented using auditory evoked potential during monitoring of anesthesia, especially the midlatency auditory evoked potentials. To conclude, we propose that the use of Auditory evoked potential should be universal with general anesthesia use in order to prevent the occurrences of distressing outcomes resulting from both conscious and subconscious auditory processing during anesthesia.

Anaesthetic regimens for day-procedure laparoscopic cholecystectomy

BACKGROUND

Day surgery involves admission of selected patients to hospital for a planned surgical procedure with the patients returning home on the same day. An anaesthetic regimen usually involves a combination of an anxiolytic, an induction agent, a maintenance agent, a method of maintaining the airway (laryngeal mask versus endotracheal intubation), and a muscle relaxant. The effect of anaesthesia may continue after the completion of surgery and can delay discharge. Various regimens of anaesthesia have been suggested for day-procedure laparoscopic cholecystectomy.

OBJECTIVES

To compare the benefits and harms of different anaesthetic regimens (risks of mortality and morbidity, measures of recovery after surgery) in patients undergoing day-procedure laparoscopic cholecystectomy.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library (Issue 10, 2013), MEDLINE (PubMed) (1987 to November 2013), EMBASE (OvidSP) (1987 to November 2013), Science Citation Index Expanded (ISI Web of Knowledge) (1987 to November 2013), LILACS (Virtual Health Library) (1987 to November 2013), metaRegister of Controlled Trials (http://www.controlled-trials.com/mrct/) (November 2013), World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) portal (November 2013), and ClinicalTrials.gov (November 2013).

SELECTION CRITERIA

We included randomized clinical trials comparing different anaesthetic regimens during elective day-procedure laparoscopic cholecystectomy (irrespective of language or publication status).

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trials for inclusion and independently extracted the data. We calculated the risk ratio, rate ratio or mean difference with 95% confidence intervals based on intention-to-treat or available data analysis.

MAIN RESULTS

We included 11 trials involving 1069 participants at low anaesthetic risk. The sample size varied from 40 to 300 participants. We included 23 comparisons. All trials were at a high risk of bias. We were unable to perform a meta-analysis because there were no two trials involving the same comparison. Primary outcomes included perioperative mortality, serious morbidity and proportion of patients who were discharged on the same day. There were no perioperative deaths or serious adverse events in either group in the only trial that reported this information (0/60). There was no clear evidence of a difference in the proportion of patients who were discharged on the same day between any of the comparisons. Overall, 472/554 patients (85%) included in this review were discharged as day-procedure laparoscopic cholecystectomy patients. Secondary outcomes included hospital readmissions, health-related quality of life, pain, return to activity and return to work. There was no clear evidence of a difference in hospital readmissions within 30 days in the only comparison in which this outcome was reported. One readmission was reported in the 60 patients (2%) in whom this outcome was assessed. Quality of life was not reported in any of the trials. There was no clear evidence of a difference in the pain intensity, measured by a visual analogue scale, between comparators in the only trial which reported the pain intensity at between four and eight hours after surgery. Times to return to activity and return to work were not reported in any of the trials.

AUTHORS' CONCLUSIONS

There is currently insufficient evidence to conclude that one anaesthetic regimen for day-procedure laparoscopic cholecystectomy is to be preferred over another. However, the data are sparse (that is, there were few trials under each comparison and the trials had few participants) and further well designed randomized trials at low risk of bias and which are powered to measure differences in clinically important outcomes are necessary to determine the optimal anaesthetic regimen for day-procedure laparoscopic cholecystectomy, one of the commonest procedures performed in the western world.

Does anaesthesia with nitrous oxide affect mortality or cardiovascular morbidity? A systematic review with meta-analysis and trial sequential analysis

BACKGROUND

The role of nitrous oxide in modern anaesthetic practice is contentious. One concern is that exposure to nitrous oxide may increase the risk of cardiovascular complications. ENIGMA II is a large randomized clinical trial currently underway which is investigating nitrous oxide and cardiovascular complications. Before the completion of this trial, we performed a systematic review and meta-analysis, using Cochrane methodology, on the outcomes that make up the composite primary outcome.

METHODS

We used conventional meta-analysis and trial sequential analysis (TSA). We reviewed 8282 abstracts and selected 138 that fulfilled our criteria for study type, population, and intervention. We attempted to contact the authors of all the selected publications to check for unpublished outcome data.

RESULTS

Thirteen trials had outcome data eligible for our outcomes. We assessed three of these trials as having a low risk of bias. Using conventional meta-analysis, the relative risk of short-term mortality in the nitrous oxide group was 1.38 [95% confidence interval (CI) 0.22-8.71] and the relative risk of long-term mortality in the nitrous oxide group was 0.94 (95% CI 0.80-1.10). In both cases, TSA demonstrated that the data were far too sparse to make any conclusions. There were insufficient data to perform meta-analysis for stroke, myocardial infarct, pulmonary embolus, or cardiac arrest.

CONCLUSION

This systematic review demonstrated that we currently do not have robust evidence for how nitrous oxide used as part of general anaesthesia affects mortality and cardiovascular complications.

Stress-related biomarkers of dream recall and implicit memory under anaesthesia

The aim of this study was to investigate whether auditory presentation of a story during general anaesthesia might influence stress hormone changes and thus affecting dream recall and/or implicit memory. One hundred and ten patients were randomly assigned either to hear a recording of a story through headphones or to have routine care with no auditory recording while undergoing laparoscopic cholecystectomy. Anaesthesia was standardised. Blood samples for cortisol and prolactin assays were collected 20 min before anaesthesia and 5 min after pneumoperitoneum. Dream recall and explicit/implicit memory were investigated upon awakening from anaesthesia and approximately 24 h after the end of the operation. Auditory presentation was associated with lower intra-operative serum prolactin concentration compared with control (p = 0.0006). Twenty-seven patients with recall of dreaming showed higher intra-operative prolactin (p = 0.004) and lower cortisol (p = 0.03) concentrations compared with those without dream recall. The knowledge of this interaction might be useful in the quest to ensure postoperative amnesia.

Comparative effects of sevoflurane and propofol based general anaesthesia for elective surgery on memory

INTRODUCTION

Unconscious processing of words during general anaesthesia has been suggested. We used the process dissociation procedure (PDP) to test memory performance during sevoflurane and propofol anaesthesia in relation to hypnotic depth.

MATERIAL AND METHODS

One hundred participants anaesthetised for elective surgery (50 with propofol and 50 with sevoflurane) and 50 non-anaesthetized listened to a list of words. The bispectral index (BIS) of the anaesthetised patients was recorded. Within 36 h after word presentation, memory was assessed using a word stem completion task, based on Buchner's model applied on the PDP.

RESULTS

There was evidence of memory for words presented during light (BIS 61-80) (p = 0.001) and adequate (BIS 41-60) (p = 0.008) but not deep anaesthesia (BIS 21-40) (p = 0.09). The PDP showed a significant implicit but not explicit memory contribution (mean total explicit memory scores: 0.04 ±0.07 in all BIS categories; mean implicit memory scores: 0.01 ±0.04, 0.1 ±0.08, and 0.05 ±0.09 at BIS = 21-40, 41-60, and 61-80, respectively). There was a statistically significant difference between the mean implicit memory score (I) of the propofol and sevoflurane group in the BIS category 41-60 in general (p = 0.016), and after incision (IA.I.) (p = 0.005) in particular, with propofol depressing I more than sevoflurane in both cases. Memory performance of nonanaesthetized participants was better, with a higher contribution of explicit and a comparable contribution of implicit memory.

CONCLUSIONS

During general anaesthesia, implicit memory persists even in adequate hypnotic states. Sevoflurane affects the implicit memory of adequately anaesthetised subjects less than propofol.

Unconscious memory formation during anaesthesia

Do patients form memories of intra-operative events when they are adequately anaesthetized? Studies of memory priming during anaesthesia with depth or awareness monitoring provide some evidence that they do, although only the most basic form of memory function, perceptual priming, persists when patients are unconscious. The probability of memory encoding increases as depth of anaesthesia decreases. There is a theoretical possibility that patients can be adversely affected, through memory priming, by comments made in the operating theatre, and some evidence that positive intra-operative suggestions can benefit patients.

Dreaming during anaesthesia in adult patients

Dreaming during anaesthesia is defined as any recalled experience (excluding awareness) that occurred between induction of anaesthesia and the first moment of consciousness upon emergence. Dreaming is a commonly-reported side-effect of anaesthesia. The incidence is higher in patients who are interviewed immediately after anaesthesia (approximately 22%) than in those who are interviewed later (approximately 6%). A minority of dreams, which include sensory perceptions obtained during anaesthesia, provide evidence of near-miss awareness. These patients may have risk factors for awareness and this type of dreaming may be prevented by depth of anaesthesia monitoring. Most dreaming however, occurs in younger, fitter patients, who have high home dream recall, who receive propofol-based anaesthesia and who emerge rapidly from anaesthesia. Their dreams are usually short and pleasant, are related to work, family and recreation, are not related to inadequate anaesthesia and probably occur during recovery. Dreaming is a common, fascinating, usually pleasant and harmless phenomenon.

Dreams recall and auditory evoked potentials during propofol anaesthesia

It is unclear whether shorter wave latencies of middle-latency-auditory-evoked-potentials may be associated to cognitive function other than nondeclarative memory. We investigated the presence of declarative, nondeclarative and dreaming memory in propofol-anaesthetized patients and any relationship to intraoperatively registered middle-latency-auditory-evoked-potentials. An audiotape containing one of two stories was presented to patients during anaesthesia. Patients were interviewed on dream recall immediately upon emergence from anaesthesia. Declarative and nondeclarative memories for intraoperative listening were assessed 24 h after awakening without pointing out positive findings. Six patients who reported dream recall showed an intraoperative Pa latency less than that of patients who were unable to remember any dreams (P<0.001). A high responsiveness degree of primary cortex was associated to dream recall formation during anaesthesia.

Comparison of a new composite index based on midlatency auditory evoked potentials and electroencephalographic parameters with bispectral index (BIS) during moderate propofol sedation

BACKGROUND AND OBJECTIVE

Derived parameters of the electroencephalogram and auditory evoked potentials can be used to determine depth of anaesthesia and sedation. However, it is not known whether any parameter can identify the occurrence of awareness in individual patients. We have compared the performance of bispectral index and a new composite index derived from auditory evoked potentials and the electroencephalogram (AAI 1.61) in predicting consciousness, explicit and implicit memory during moderate sedation with propofol.

METHODS

Twenty-one patients with spinal anaesthesia received intraoperatively propofol at the age-corrected C(50) for loss of consciousness and were presented test words via headphones. Bispectral index and AAI 1.61 (auditory evoked potentials, AEP-Monitor2) were recorded in parallel as well as the Observer's Assessment of Alertness/Sedation-score. Postoperatively, testing for explicit and implicit memory formation was performed.

RESULTS

Bispectral index and AAI 1.61 correlated well with loss of consciousness defined by an Observer's Assessment of Alertness/Sedation-score of 2 (identical P(K) of 0.87), but did not allow a prediction of postoperative explicit or implicit recall.

CONCLUSIONS

Both bispectral index and AAI may be indices of depth of sedation rather than indicators of memory formation, which persists during propofol sedation even after loss of consciousness.

Objective detection of evoked potentials using a bootstrap technique

Evoked potentials are usually evaluated subjectively, by visual inspection, and considerable differences between interpretations can occur. Objective, automated methods are normally based on calculating one (or more) parameters from the data, but only some of these techniques can provide statistical significance (p-values) for the presence of a response. In this work, we propose a bootstrap technique to provide such p-values, which can be applied to a wide variety of parameters. The bootstrap method is based on randomly resampling (with replacement) the original data and gives an estimate of the probability that the response obtained is due to random variation in the data rather than a physiological response. The method is illustrated using auditory brainstem responses (ABRs) to detecting hearing thresholds. The flexibility of the approach is illustrated, showing how it can be used with different parameters, numbers of stimuli and with user-defined false-positive rates. The bootstrap method provides a new, simple and yet powerful means of detecting evoked potentials, which is very flexible and readily adapted to a wide variety of signal parameters.

Is priming during anesthesia unconscious?

General anesthesia provides an alternative to typical laboratory paradigms for investigating implicit learning. We assess the evidence that a simple type of learning--priming--can occur without consciousness. Although priming has been shown to be a small but persistent phenomenon in surgical patients (Merikle & Daneman, 1996) there is reason to question whether it occurs implicitly due to problems in detecting awareness using typical clinical signs. This paper reviews the published studies on priming during anesthesia that have included a measure of awareness or of anesthetic depth. We conclude that perceptual priming, but not conceptual priming, takes place in the absence of conscious awareness.