Expert Consensus on Intraoperative awareness prevention and EEG Monitoring (2021) Date issued: August 6, 2021

Date issued:2021-08-06

In recent years, the anesthesia goals proposed by the American Society of Anesthesiologists are to avoid intraoperative awareness, maintain ideal hemodynamics, the best quality of anesthesia recovery, avoid postoperative cognitive dysfunction and avoid perioperative death, perioperative anesthesia depth is too shallow or too deep can increase intraoperative awareness and postoperative complications. Anesthesia depth monitoring based on EEG signal analysis has been widely used in clinical anesthesia and scientific research. This expert consensus summarizes the prevention of intraoperative knowledge and the application of EEG monitoring to provide reference for clinical anesthesia.

1 Definition and basic concepts of intraoperative knowledge

(1)(intraoperative awareness)

Specifically, it should be referred to as operative awareness under general anesthesia. In this consensus, operative awareness is defined as the patient under general anesthesia becomes conscious during the operation and can recall the events related to the operation after the operation.


Consciousness is defined as a state in which a patient can process external information in the context of his environment. Anesthesiologists judge whether a patient has consciousness by observing whether the patient has a purposeful response to various stimuli, such as open eyes in response to commands and body movements in response to painful stimuli. But this purposeful response is difficult to observe with muscle relaxants.


It can be divided into explicit memory and implicit memory. In the case of patients under general anesthesia, explicit memory refers to the ability of the patient to recall events that occurred during general anesthesia. Implicit memory refers to the fact that the patient cannot recall the events that occurred during general anesthesia, but certain intraoperative events can lead to postoperative changes in the patient's performance or behavior. Strictly speaking, intraoperative knowledge should include explicit memory and implicit memory. However, Unless the patient shows obvious mental and psychological disorders after surgery, the only way to judge whether the patient has the occurrence of implicit memory during surgery is to use special psychological testing methods such as word stem repair. Therefore, intraoperative knowledge is only defined as explicit memory, excluding implicit memory. Events that occurred before and after induction of sleep under general anesthesia were excluded, and intraoperative dreams were not considered to be intraoperative knowledge.

Consciousness and memory are two interrelated but independent functions of the brainThe fact that the patient is conscious or able to perform certain commands during surgery does not necessarily mean that the patient will be able to recall the relevant events after surgery. Studies have shown that the awareness rate of intraoperative awakening test in patients undergoing scoliosis orthopaedic surgery is only about 16.7%. Similarly, the patients who can complete the movements according to instructions using forearm isolation technology during general anesthesia rarely recall the related intraoperative events, which are known intraoperatively as one of the complications of clinical anesthesia. The two conditions of existence awareness and postoperative recall of intraoperative events should be met.

To determine whether a patient has intraoperative knowledge, in addition to listening to the patient's statement, it is necessary to check with the physician involved in the patient's anesthesia and surgery, and a panel of specialists is required to identify knowledge or suspected knowledge.

(4)The modified Brice questionnaire was used for the postoperative investigation of intraoperative awareness

①What is the last thing you remembered before you went to sleep?


②What is the first thing you remembered when you woke up?


③Can you remember anything between these two periods?


④Did you dream during your operation?


⑤What was the worst thing about your operation?


(5)Intraoperative knowledge of the investigation timing

Look up different terms, will lead to different results. In addition, if intraoperative knowledge has no impact on the patient, the patient may not actively report it. Intraoperative known memory may be delayed, and only 1/3 of intraoperative known cases are determined before PACU. In addition, about 1/3 of the intraoperative known cases were reported 1-2 weeks after surgery, and most of the patients reported within 24 hours after surgery.

Recommendation 1:Use of the modified Brice questionnaire for intraoperative awareness of postoperative investigation. The timing of the investigation should include the first day and about one week after surgery (grade A).


1)Recommendation 1: (1)Incidence of intraoperative awareness

In recent years, the incidence of intraoperative awareness reported abroad is 0.1% ~ 0.4%, and the high risk population (patients receiving cardiac surgery, cesarean section, emergency neurosurgical trauma surgery, shock patients, short-effect otorhinolaryngology and other surgery patients, etc.) can be more than 1%, especially young women. Domestic single-center and small-sample studies reported that the incidence of intraoperative awareness was as high as 1.5% ~ 2.0%. The investigation of intraoperative awareness in children is special, and the incidence of intraoperative awareness is 0.2% ~ 1.2%, higher than that in adults. The incidence of intraoperative awareness of cesarean section under general anesthesia was about 0.26%.

Recommendation 2:Although the incidence of intraoperative awareness is only 0.1% ~ 0.4%, the actual number of intraoperative awareness should be paid great attention by anesthesiologists (Grade A), especially for high-risk groups, given the huge annual volume of general anesthesia operations.

2)Potential hazards known intraoperatively

Intraoperative awareness can cause serious emotional and mental health problems. It has been reported that up to 30% to 71% of patients with intraoperative awareness developed post-traumatic Stress Disorder (PTSD), with symptoms lasting an average of 4.7 years. In addition, patients often hear, pain, paralysis, anxiety, and even near death, suffocation and other memories. About 70% of patients who experience intraoperative awareness experience sleep disturbances, nightmares, flashbacks, anxiety, fear of surgery and even refusal of medical services.

The serious psychiatric/medico-legal problems caused by intraoperative knowledge have developed into a social problem in recent years. Of ASA closed claims in 1999 and 2006, 1.9% were known intraoperatively (79/4183 in 1999 and 129/6811 in 2006).

Recommendation 3: Attention should be paid to severe emotional and mental health problems associated with intraoperative awareness (grade B).


Intraoperative known mechanisms and risk factors
1)Medical history and history of anesthesia

Patients with known intraoperative history of occurrence, heavy drug use or abuse (opioids, benzodiazepines, and cocaine), history of high doses of opioids in patients with chronic pain, expected or known airway difficulties, ASA grade 4 to 5, and limited hemodynamic reserve.

2) The operation type

General anesthesia surgery may occur, including heart surgery, cesarean section, craniocerebral trauma surgery, ear, nose and throat surgery, emergency surgery and other higher incidence.

3) Anesthesia management

All-intravenous anesthesia, n2O-opioid anesthesia, muscle relaxant use, underdosage of hypnotics, no benzodiazepine preadministration.

At present, there is a lack of large sample studies of children's intraoperative knowledge of risk factors. Based on 5 independent investigations of children's intraoperative awareness, foreign scholars proposed that the use of N2O anesthesia and endotracheal intubation are risk factors for children's awareness.

Recommendation 4:Although the mechanism and risk factors leading to intraoperative awareness have not been finalized, the incidence of intraoperative awareness in high-risk patients is 5-10 times higher than that in general patients, that is, from 0.1% to 0.4% to 1%. The anesthesiologist must identify known intraoperatie risk factors based on history, history of anesthesia, type of surgery, and anesthesia management (level A).


Strategies for reducing intraoperative awareness
1)Preoperative assessment

During the preoperative visit, according to the above possible risk factors known during the operation, the high-risk groups were analyzed from the aspects of medical history, anesthesia history, operation type and anesthesia management. If the patient has known risk factors during surgery:

①Inform the patient of the possibility of intraoperative awareness;

②Prophylactic use of benzodiazepines such as midazolam before surgery. 

Recommendation 5:Before general anesthesia, the anesthesiologist should evaluate each patient for intraoperative awareness of risk factors and inform high-risk groups of the risk of intraoperative awareness (grade C).

Recommendation 6:Prophylactic preoperative benzodiazepine use can reduce the incidence of intraoperative awareness, but benzodiazepine use may cause delayed awakening and postoperative delirium in some patients (class B).

2)Intraoperative anesthesia management

①Check anaesthesia equipment to reduce errors, especially inhalation anaesthesia leakage, etc.

②The prophylactic use of benzodiazepine medications, including pre-surgery and during light anesthesia; Prophylactic use of cholinergic receptor antagonists, such as pentadiol, may have a role;

③Sedative drugs should be added when intraoperative risk is known, such as difficulty in endotracheal intubation.

④Simple hemodynamic data is not the index to judge the depth of anesthesia.

⑤Muscle relaxants can mask the depth of anesthesia by anesthesiologists.

⑥Monitor end-expiratory anaesthetic concentration and maintain age-adjusted end-expiratory concentration > 0.7MAC.

⑦It is recommended to use depth monitoring means based on eeg signal analysis to avoid too shallow or too deep anesthesia.

⑧Reduce unnecessary stimulation (sound and light) to patients during operation. The use of earplugs may have a preventive effect on intraoperative awareness;

⑨Anesthesiologists are vigilant against the use of beta blockers, calcium channel blockers, and drugs that mask the physiological effects of anesthesia.

⑩All operating room personnel should avoid inappropriate jokes, discussion of other patients or irrelevant remarks

3)Postoperative treatment

术后处理包括分析患者的知晓报告,向质控部门汇报,为患者提供适当的术后随访和相应治疗。Postoperative management includes analyzing the patient's awareness report, reporting to quality control, and providing appropriate follow-up and treatment for the patient.

Recommendation 7: Adopt the above multimodal measures to effectively reduce the incidence of intraoperative awareness.


Definition and types of eeg monitoring

1)Electrical activity changes synchronously during sleep or anesthesia

At present, the mechanism of intraoperative awareness is not clear. Most of the claims resolved by the ASA did not show signs of shallow anesthesia in cases known intraoperatively. Therefore, it is one-sided to think that knowledge during prophylaxis can be solved by simply deepening anesthesia. In addition, recent studies have shown that deep anesthesia may be associated with long-term disability rate and mortality.

The electrical activity of cortical nerve cells reflected by electroencephalography (EEG) has been proved to be directly related to the depth of sleep or anesthesia, that is, the electrical activity of the brain changes synchronously during sleep or anesthesia. As the depth of general anesthesia increased, the eeg showed slow wave sign (frequency gradually slowed down, at the same time the amplitude increased), then burst suppression, and finally the isopotential line. Therefore, EEG signal - based analysis techniques have been widely used in perioperative anesthesia depth monitoring.

2)EEG analyzer for clinical use to monitor depth of anesthesia

①bispectral index, BISmonitor

②Entropy Entropy model

③Narcotrend monitor

④NeuroSENSE monitor

⑤Conview monitor

⑥SEDline monitor

⑦SNAPII monitor

⑧qCON 2000 monitor

⑨BOSpro和NOX monitor

Based on electroencephalogram (pEEG) processing, the monitoring devices use different time and frequency domain analysis and/or burst suppression data to derive correlation indices to quantify the depth of anesthesia in patients.

3)Common monitoring indexes

①EEG power spectrum

②Burst suppression rate(BS)

③EEG bispectral index(BIS)

④Reaction entropy(Response entropy, RE)

⑤The state of entropy(State entropy, SE)

⑥Narcotrend index

⑦WAVcns index

⑧Patient status index(Patient state index, PSI)

⑨SNAP index

⑩AiDepth of anesthesia index

Auditory evoked potential (AEP) can be used to reflect anesthesia and awakening state.


Clinical application of EEG monitoring
Since 1990, eeg monitors have been developed and put into clinical use to reduce the risk of intraoperative awareness. With the popularization of eeg monitoring during general anesthesia, researchers have found that deep anesthesia may be associated with long-term morbidity and mortality. At present, the anesthesia depth monitor based on EEG analysis is mainly applied as follows.

1)Prevention of intraoperative awareness

BIS monitoring has been shown to reduce the incidence of intraoperative awareness in patients undergoing all intravenous anesthesia. BIS monitoring can also reduce the risk of intraoperative awareness in patients with hemodynamic instability. However, there was no difference in reducing intraoperative awareness between maintaining an age-adjusted end-expirative anesthetic concentration > 0.7MAC and BIS < 60 in patients receiving inhalation anesthesia. At present, there is still a lack of eeg monitors to know the high sensitivity and specificity during prophylaxis. It is necessary to pay close attention to the operation process and be familiar with the operation schedule. When the operation enters the period of obvious injury stimulation, the depth of anesthesia must be guaranteed. 

Recommendation 8:Eeg monitors with high sensitivity and specificity are still lacking. Eeg monitoring is not recommended routinely for all patients under general anesthesia to prevent known intraoperative occurrences, and should be determined on a case-by-case basis for each patient (Grade B).

Recommendation 9:Recommendation 9: Electroencephalograph-based depth of anesthesia can reduce the incidence of intraoperative awareness in patients undergoing all-intravenous anesthesia or in patients with hemodynamic instability (level A).

Recommendation 10:Recommendation 10: There was no difference in reducing intraoperative awareness between age-adjusted end-expirative anesthetic concentration > 0.7MAC and BIS < 60 in patients receiving inhalation anesthesia (level A)

2)Monitoring depth of anesthesia

Studies have shown that using eeg monitors during general anesthesia may prevent adverse consequences such as delayed awakening, postoperative delirium and cognitive problems, and even death. However, BIS value as the only objective index of general anesthesia depth has its own limitations. Patients at high risk are more sensitive to anesthetics than healthy patients, and the use of eeg monitors to monitor the depth of anesthesia (sedation) can avoid excessive inhibition. The European Society of Anesthesiology (ESA) recommends that intraoperative EEG monitoring be used in elderly patients to avoid suppression of eeg bursts caused by excessive anesthesia to reduce postoperative delirium. However, BIS value as the only objective index of general anesthesia depth has its own limitations.

Recommendation 11:The relationship between the depth of general anesthesia and the occurrence of postoperative delirium remains to be further studied, and BIS depth monitoring of general anesthesia does not reliably predict the occurrence of postoperative delirium (grade B).

Recommendation 12:BIS is not associated with 1-year mortality under inhalation general anesthesia. High-risk patients are more sensitive to anesthesia than healthy patients, and it is necessary to use eeg monitor to monitor the depth of anesthesia in high-risk patients (grade A).


Limitations of EEG monitoring
Currently, all EEG monitors rely on complex mathematical algorithms of raw EEG signals to derive indices for clinical use. These EEG signals are all from the frontal or temporal brain regions of the patients, leading to limitations in spatial resolution and regional correlation assessment of brain function, which makes it impossible to accurately determine anesthesia status/depth of all patients using uniform STANDARD EEG features. In addition, a number of factors can interfere with the reliability of EEG signals as indicators of depth of anesthesia by influencing them:

1)Patient factors

Existing EEG monitors are based on data from healthy volunteers. Therefore, abnormal EEG in patients with Alzheimer's disease, vascular dementia, and cerebral ischemia may lead to abnormal EEG monitoring indicators. In addition, advanced age, hypothermia, hypoglycemia, and abnormal acid-base balance can also interfere with EEG and corresponding indices.

2)The anesthetic

Different anesthetics have different effects on EEG. For example, a small dose of ketamine can enhance high-frequency EEG activity, resulting in an increase in BIS index.

3)Muscle relaxant drugs

In the absence of muscle relaxation, electrical activity generated by the scalp interferes with the EEG, making the BIS monitor unable to distinguish between electromyographic signals (EMG) and high-frequency EEG activity.


The reliability of eeg monitoring can be affected by electrocoagulation and cardiac pacemaker.

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