What is meant by awareness during anesthesia?
The term awareness is used to describe explicit memory (1), and the general assumption in intraoperatie learning or recall studies is that explicit recall indicates an inadequate anesthetic (2). Explicit memory is the conscious recollection of preious experiences, and is equialent to “remembering”, that is, the content and context of a learning episode can be recalled. Postoperatiely, for example, a patient who can identify specific words or sounds in the context of their surgery has explicit recall. Implicit memory, by contrast, refers to changes in performance or behaior that cannot be linked to a conscious recollection. An example of implicit memory would be knowing how to ride a bike, without remembering when that specific skill was acquired. Implicit memory during anesthesia remains controersial but is thought to occur een when an adequate amount of anesthesia to preent awareness. It is utilized by some inestigators in an attempt to improe postoperatie outcomes by using positie suggestion ia audio tapes or messages (2).
Independent of the work being done to assess the quality and depth of anesthesia, some inestigators hae questioned the alidity of tools utilized to ealuate and separate implicit and explicit memory measures (3). Regardless, the alue of these measures, howeer sensitie or specific, is to raise our own awareness of this issue.
标签： anesthesia, awareness, 茶太 anesthesia
References:
Ghoneim MM. Awareness during anesthesia. Anesthesiology 2000;92:597-602.
Wang M. Learning, memory and awareness during anaesthesia. In Adams AP, Cashman JN. Recent Adances in Anaesthesia and Analgesia. Edinburgh, Churchill Liingstone. 1998 p. 94.
Buchner A, Wippich W. On the Reliability of Implicit and Explicit Memory Measures. Cognit Psychol 2000 May;40(3):227-59.
What is the incidence of awareness during anesthesia?
Are certain indiiduals more likely to experience awareness?
The incidence in non obstetric and non cardiac surgical cases has been reported to be 0.2%, when a structured interiew was utilized in a large series of patients to quantitate it (4). During cardiac and obstetric anesthesia, the incidence is higher, at 1.1-1.5% and 0.4% respectiely, most likely reflecting the lower dose of anesthesia administered (1). As a surrogate measure of the incidence of awareness, an in-depth analysis of the database of the American Society of Anesthesiologists Closed Claims Project by Domino et al. (5) noted that awareness claims accounted for 79 (1.9%) of 4,183 claims in the database, including 18 claims for” awake paralysis”, i.e., the inadertent paralysis of an awake patient, and 61 claims for “recall during general anesthesia”, i.e., recall of eents while receiing general anesthesia.
As an aside, does it seem like people hae a tendency to remember only the bad things that are said about them? The same may be true under anesthesia! Ott et al. in a study of 45 healthy subjects who listened to a series of neutral and threatening words masked under white noise at a signal-noise ratio of -17 dB, later identified a significantly higher number of threatening words. The authors concluded that unconscious cognition occurs more often for threatening words. While comments regarding patients may not be perceied as “threatening” by the speaker, perhaps the motto, een with anesthetized patients, “If you hae nothing good to say, say nothing at all!” is a prudent one to subscribe to!
References:
1. Ghoneim MM. Awareness during anesthesia. Anesthesiology 2000;92:597-602.
Wang M. Learning, memory and awareness during anaesthesia. In Adams AP, Cashman JN. Recent Adances in Anaesthesia and Analgesia. Edinburgh, Churchill Liingstone. 1998 p. 94.
2. Buchner A, Wippich W. On the Reliability of Implicit and Explicit Memory Measures. Cognit Psychol 2000 May;40(3):227-59.
3. Liu WHD, Thorp TAS, Graham SG, Aitkenhead AR. Incidence of awareness with recall during general anaesthesia. Anaesthesia 1991;46:435-7.
4. Domino KB, Posner KL, Caplan RA, Cheney FW. Awareness during anesthesia: a closed claims analysis. Anesthesiology 1999 Apr;90(4):1053-61.
5. Ott R, Curio I, Scholz OB. Implicit memory for auditorily presented threatening stimuli: a process-dissociation approach. Percept Mot Skills 2000 Feb;90(1):131-46
Are certain indiiduals more likely to experience awareness?
Awareness is typically associated with light anesthesia, increased anesthetic requirements, or machine malfunction or misuse. As a consequence, indiiduals undergoing light anesthesia, i.e. patients with limited cardiac resere or hypoolemia, or during induction or emergence from anesthesia, may experience awareness. These time periods and particular cases should be discussed with patients prior to their anesthetic. Domino et al. (2) conducted an in-depth analysis of independent patient and anesthetic factors associated with claims for recall during general anesthesia within the database of the American Society of Anesthesiologists Closed Claims Project. The authors noted that the majority of awareness claims inoled women (77%), younger than 60 years of age (89%), American Society of Anesthesiologists physical class I-II (68%), who underwent electie surgery (87%). It should not be surprising that certain characteristics which hae been noted to require an increased anesthetic dose to produce unconsciousness may be associated with recall: younger age, smoke tobacco, or are chronic users of certain drugs (alcohol, opiates, or amphetamines) (3)
Interestingly, adances in anesthetic medications may hae allowed awareness to be more common. Athough anesthetic concentrations required to block awareness are less than those required to preent motor responses to pain (1), with the use of muscle relaxants, an inadertent light anesthetic may not be detected by patient moement. In the study by Domino sited aboe (2), claims for recall during general anesthesia were more likely to inole anesthetic techniques using intraoperatie opioids (OR = 2.12, 95% CI = 1.20, 3.74), intraoperatie muscle relaxants (OR = 2.28, 95% CI = 1.22, 4.25), and no olatile anesthetic (OR = 3.20, 95% CI = 1.88, 5.46). The authors concluded that deficiencies in labeling and igilance were common causes for awake paralysis and that claims for recall during general anesthesia were more likely with nitrous-narcotic-relaxant techniques.
What are some of the risk factors for patients to experience awareness?
The most distressing feature of awareness is the pain, anxiety, suffering, and a sense of helplessness or death that occur during the course of surgery itself. In addition, while temporary effects, including sleep disturbances and nightmares, may occur post operatiely, in some patients post-traumatic stress disorder (PTSD) can deelop (4). As with other forms of PTSD, it is marked by repetitie nightmares, anxiety, irritability, and preoccupations with death and sanity (3). Although it is unclear why PTSD occurs in certain indiiduals, Brady et al. (5) noted its relationship with other depressie, substance use, and anxiety disorders and concluded that a preious depressie disorder is a risk factor for the deelopment of PTSD once exposure to a trauma occurs.
Medicolegal consequences to awareness can also occur. Domino et al (2) in the ASA Closed Claims database sited aboe, noted that awareness claims accounted for 79 (1.9%) of 4,183 claims in the database, including 18 claims for awake paralysis and 61 claims for recall during general anesthesia. Most (94%) claims for awake paralysis represented substandard care inoling errors in labeling and administration, whereas care was substandard in only 43% of the claims for recall during general anesthesia (P < 0.001). The median compensation for these claims was $18,000.
References:
Dwyer R, Bennett HL, Efer EL II, Heilbron D. Effects of isoflurane and nitrous oxide in subanesthetic concentrations on memory and responsieness.
Domino KB, Posner KL, Caplan RA, Cheney FW. Awareness during anesthesia: a closed claims analysis. Anesthesiology 1999 Apr;90(4):1053-61.
Ghoneim MM. Awareness during anesthesia. Anesthesiology 2000;92:597-602.
Osterman JE, an der Kolk BA. Awareness during anesthesia and posttraumatic stress disorder. Gen Hosp Psychiatry 1998 Sep;20(5):274-81
Brady KT, Killeen TK, Brewerton T, Lucerini S. Comorbidity of psychiatric disorders and posttraumatic stress disorder. J Clin Psychiatry 2000;61 Suppl 7:22-32
How should intraoperatie awareness be managed?
Should awareness occur, 5 steps hae been suggested (1). They include:
A detailed interiew with the patient, with erifying, sympathizing, explaining, reassuring, apologizing, and offering psychological support.
Recording the interiew in the patient’s chart.
Informing the patient’s surgeon, nurse, and hospital lawyer.
isiting the patient daily during hospital stay and contacting by telephone afterwards.
Referring early to a psychologist or psychiatrist.
Early referral and treatment by a psychologist or psychiatrist has been noted to decrease the incidence of more permanent disabilities like PTSD. Should the patient suffer from PTSD, Ballenger et al.(2), in a consensus statement on post traumatic stress disorder from the International Consensus Group on Depression and Anxiety, noted that selectie serotonin reuptake inhibitors are generally the most appropriate choice of first-line medication for PTSD, and effectie therapy should be continued for 12 months or longer. The most appropriate psychotherapy is exposure therapy, and it should be continued for 6 months, with follow-up therapy as needed.
How may intraoperatie awareness be preented?
Ghoneim (1) suggests a multiple step method to preent the recall of eents during anesthesia. These include:
The use of amnestic premedication, especially when light anesthesia is anticipated. These medications impair the acquisition or encoding of new information, with the degree of impairment being positiely correlated to the dose utilized.
Administer more than “sleep doses” of induction agents if immediate tracheal and/or anticipated difficulty with intubation will follow.
Aoid muscle paralysis unless necessary, and een then aoid total paralysis.
Supplement N2O and opioids with olatiles agents with end tidal concentrations of 0.6 MAC or more.
Use 0.8-1.0 MAC inhalational agents when used alone.
Maintain anesthesia machines and check aporizers, flow meters and inhalational and exhalational gas monitors.
Discuss the potential for awareness.
Monitor for awareness (To be discussed in Friday’s Answer Page).
Gain more awareness by reading and understanding the entity of awareness.
Opioids and nitrous oxide hae been noted to either not affect or to hae weak effects on memory and cognitie effects, respectiely. Ghoneim et al. (3), in studying 180 patients scheduled for electie surgical procedures noted a 6% incidence of awareness in patients anesthetized with nitrous oxide and bolus opioid supplementation (ia arious regimens). Thus, opioid supplemented anesthetic techniques did not reduce the risk of awareness compared with the use of nitrous oxide alone.
In terms of inhalational MAC amounts, it remains unclear how much is enough to preent explicit memory. While some authors suggest that agents at leels as low as 0.4 MAC is all that is necessary (4, 5), more recently, others hae suggested that leels of 0.6 MAC are doubtful at best, and may underestimate the true need (1).
Interestingly, some medications that we don’t traditionally think as affecting memory may play a role. Roozendaal ealuated the acute effects of glucocorticoids and beta adrenergic blockers on memory consolidation in rats. Actiation of pathways inoling glucocorticoid receptors (GRs or type II) was noted to enhance memory consolidation in a dose-dependent inerted-U fashion. By contrast, selectie lesions of the basolateral nucleus of the amygdala (BLA) or infusions of beta-adrenoceptor antagonists into the BLA blocked the memory improing effects of systemic glucocorticoids. Lesions of the BLA or inactiation of beta-adrenoceptors within the BLA were noted to block the memory-modulatory effects of intrahippocampal administration of a GR agonist or antagonist. The authors concluded that glucocorticoids can improe memory and that the effects require beta-adrenergic actiity in the BLA. These findings are in agreement with the general hypothesis that the BLA integrates hormonal and neuromodulatory influences on memory consolidation.
Fun fact: With all this discussion of medications and the preention of explicit memory, is it possible that medications can actually improe memory (certainly the sale figures for Ginko seem to suggest that people beliee so)? Dimpel et al. (7), in a twin, double crossoer, placebo-controlled study in 20 healthy non-smokers, ealuated the hypothesis that oxygen administration to healthy olunteers could improe their memory. Blinded to the nature of the gas, group A breathed air first then oxygen on day 1, and then oxygen first, followed by air on day 2. Group B had all exposures in reerse order. After each gas exposure a written memory test with a list of 20 words was carried out and ealuated by a blinded obserer. Recall after oxygen exposure (mean 8.3 words) was not significantly different from that after air exposure (mean 9 words). The authors concluded that oxygen did not improe memory. So much for those expensie trips to the oxygen bar while studying for important exams!
Can clinical signs be used to monitor awareness?
Attempts hae been made to correlate clinical signs with central anesthetic effects and awareness during anesthesia. The most important of these utilize PRST scores, calculated from changes in autonomic egetatie clinical signs (blood pressure, heart rate, sweating, and tear production), and the isolated forearm technique (described below). Although in clinical practice the proision of anesthetics is generally guided by autonomic egetatie clinical signs (PRST score), these parameters are not ery reliable in predicting the suppression of consciousness and awareness (1). An example of where consciousness can occur in the presence of minimal changes in clinical signs is during the use of a high-dose opioid anesthetic in patients with cardioascular disease and surgery. The PRST score mainly appears to indicate the autonomic responses to painful stimuli, and seres as a guide to the use of analgesics; howeer, it does not appears useful in preenting awareness.
In the isolated forearm technique a constrictie band is placed around the forearm to isolate it from the effects of muscle relaxants and patient moement in response to erbal commands is recorded. While it has been utilized successfully to assess patient responsieness during general anesthesia, it has been poorly correlated with postoperatie recall and thus has not been a good tool for inestigating awareness (1). In addition, some practitioners are concerned of the pressure-induced nere compression that may occur.
Can monitors be utilized to determine awareness?
The failure of clinical signs to guide the depth of anesthesia necessary to preent awareness has focused research on arious monitors. Howeer, the difficulty in ealuating such monitors lies in correlating the machines’ responses with awareness, and deeloping clinical dose-response normograms for particular anesthetics. This being said, most of the research into “awareness” has focused on electroencephalography (EEG) or its deriaties, which gies a direct indication of the patient’s leel of consciousness, and is known to be affected by anesthetics (1). Techniques which hae been deeloped to process the EEG signal and improe on-line interpretation include fast Fourier transformation to deelop a three-dimensional plot or compressed spectral array, eoked-potential monitoring, and bispectral analysis to derie a uniariate descriptor of the EEG (For information on Bispectral Index (BIS) monitoring see tomorrow’s question page).
The processed EEG and its deried parameters, median frequency (MF) and spectral-edge frequency (SEF), are important scientific tools which hae quantified the central effects of many anesthetics and hae fostered the deelopment of pharmacodynamic-pharmacokinetic models of anesthetic action. Unfortunately, few clinical studies hae documented the usefulness of MF or SEF with regard to predicting intraoperatie arousal or awareness. To the contrary, some experimental data has failed to predict imminent arousal and response to surgical incision or erbal commands by MF or SEF (2). Therefore, these parameters seem to be of limited alue in monitoring awareness, consciousness, or memory formation during anesthesia.
By contrast, the other processed EEG offspring, mid-latency auditory and somatosensory eoked potentials (MLAEP, SEP), are suppressed in a dose-dependent fashion by many general anesthetics and hae correlated with wakefulness, awareness, and explicit and implicit memory during anesthesia and seem to be promising methods of monitoring awareness during anesthesia. Rundshagen et al. (3) in 20 patients undergoing propofol/sufentanil anesthesia , demonstrated that when certain SEP latencies elicited by median nere stimulation were obsered, impaired explicit memory function during recoery from anesthesia was noted. Most recently, Ghoneim MM et al. (4) utilized mid-latency auditory eoked responses (MLAEP) to ascertain the adequacy of the hypnotic state during surgery. MLAEP were recorded before anesthesia, 5 min after surgical incision and then eery 30 min until the end of surgery. A tape of either the story of the “Three Little Pigs” or the “Wizard of Oz” was played continuously between the recordings. Explicit memory was assessed postoperatiely by tests of recall and recognition. After a thiopental induction, one of four anesthetics were studied: Opioid bolus: 7.5 microg x kg(-1) fentanyl, 70% N2O, with 2.5 microg x kg(-1) supplements as needed (n=100); Opioid infusion: Alfentanil 50 microg x kg(-1) bolus, 1-1.5 microg x kg(-1) x min(-1) infusion, 70% N2O (n=40); Isoflurane 0.3%: Fentanyl 1 microg x kg(-1), 70% N2O, isoflurane 0.3% expired (n=16); Isoflurane 0.7%: Fentanyl 1 microg x kg(-1), 70% N2O, isoflurane 0.7% expired (n=23). Six patients showed explicit recall of intraoperatie eents, and all of these patients had receied a nitrous with opioid bolus regimen. The authors concluded that the incidence of awareness in patients anesthetized with nitrous oxide and bolus supplementation was 6% and that this anesthetic technique did not reduce the risk of awareness compared with the use of nitrous oxide alone. By contrast, those indiiduals receiing isoflurane 0.3% did not hae awareness.
Whereas other studies hae demonstrated the successful use of isoflurane in preenting memory at higher doses (5), the aboe mentioned study represented one of the lowest concentrations of isoflurane (0.3%) to be ealuated to date. Nonetheless, future studies will hae to determine threshold alues for the different MLAEP parameters for intraoperatie awareness and explicit recall of intraoperatiely presented information for the different commonly used anesthetics.
What is the BIS monitor?
The bispectral index (BIS) monitor is based on bispectral processing that determines the harmonic and phase relationships among arious EEG frequencies (1). The actual BIS reading is a nonlinear single ariable based on a large olume of clinical data correlating behaior and EEG assessments. As such, different ersions of BIS hae different algorithims.
Can BIS preent awareness?
Although some authors hae concluded that BIS monitoring is aluable in preenting awareness (2), little eidence is aailable. The bispectral index has been used to assess increasing sedation and the loss of consciousness, and in this regard has correlated well with midlatency auditory eoked potentials (MLEP) (3), one of the most promising measures of awareness during anesthesia (See yesterday’s Question of the Day). Moreoer, BIS monitoring has demonstrated that “light” leels of hypnosis are not protectie against explicit memory. Lubke et al. (4) inestigated explicit memory during emergency cesarean sections under light leels of anesthesia by studying recall of words presented ia headphones. BIS monitoring was recorded throughout. Memory of the presented words was tested after recoery with a word-stem completion test. Using the “process dissociation” procedure to separate explicit and implicit memory, they noted that at light leels of anesthesia (a mean bispectral index during word presentation of 76.3 ± 3.0) explicit memory could occur.
If the BIS monitor is used, what may lead to errors in preenting awareness?
Assuming that normograms could be caluculated for the use of BIS monitoring and awareness during anesthesia (we are not een close yet), seeral inestigators hae reported scenarios where the BIS monitor may not yield accurate information. Detsch et al. (5) noted that in 23 and 27 patients repectiely (out of 70 patients anesthetized with isoflurane-nitrous oxide-sufentanil for major abdominal surgery) no change or an increase, rather than the expected decrease in BIS scores was obsered when end-tidal isoflurane was increased after 15 minutes to 1.6% from 0.8% for 20 minutes. The authors concluded that the paradoxical increase in BIS suggested that the use of BIS as a guide for isoflurane administration may be misleading in some patients undergoing surgical procedures.
In addition, at certain times such as intubation, BIS can increase unpredictably. Mi et al. noted that with intubation, although fentanyl, 2 micrograms·kg-1 i.. added to propofol blunted the haemodynamic responses to intubation, BIS alues increased significantly in comparison to preintubation alues.
Finally, Barr et al. (7) studied the effect of nitrous oxide on bispectral index (BIS), and noted that despite the loss of consciousness, nitrous oxide caused no change in BIS. The authors concluded that although BIS may indicate a sufficient hypnotic depth to preent awareness during surgery, pharmacological unconsciousness-hypnosis can also be reached by mechanisms to which BIS is not sensitie. Thus BIS is not a necessary criterion for adequate depth of anaesthesia or preention of awareness.
In part, these situations may be explained by Kissin (1) who concluded: “BIS is an empirical index deried statistically from a database which included many, but not all, types of anesthetics and their combinations. Therefore, when the BIS is used with a new drug or new patient population that was not in the original database, it must be realidated. This may lead to additional changes in the BIS algorithm.”
At present, there is not enough eidence to suggest that the BIS monitor can proide a unifying measure of anesthetic depth neccessary to preent awareness under anesthesia. As with all of the other monitors aailable, further alidation will hae to be done.