Unconscious memory formation during anaesthesia

Neuro-functional modeling of near-death experiences in contexts of altered states of consciousness

Near-death experiences (NDEs) including out-of-body experiences (OBEs) have been fascinating phenomena of perception both for affected persons and for communities in science and medicine. Modern progress in the recording of changing brain functions during the time between clinical death and brain death opened the perspective to address and understand the generation of NDEs in brain states of altered consciousness. Changes of consciousness can experimentally be induced in well-controlled clinical or laboratory settings. Reports of the persons having experienced the changes can inform about the similarity of the experiences with those from original NDEs. Thus, we collected neuro-functional models of NDEs including OBEs with experimental backgrounds of drug consumption, epilepsy, brain stimulation, and ischemic stress, and included so far largely unappreciated data from fighter pilot tests under gravitational stress generating cephalic nervous system ischemia. Since we found a large overlap of NDE themes or topics from original NDE reports with those from neuro-functional NDE models, we can state that, collectively, the models offer scientifically appropriate causal explanations for the occurrence of NDEs. The generation of OBEs, one of the NDE themes, can be localized in the temporo-parietal junction (TPJ) of the brain, a multimodal association area. The evaluated literature suggests that NDEs may emerge as hallucination-like phenomena from a brain in altered states of consciousness (ASCs).

Anesthesia and the neurobiology of fear and posttraumatic stress disorder

PURPOSE OF REVIEW

Dysfunction of fear memory systems underlie a cluster of clinically important and highly prevalent psychological morbidities seen in perioperative and critical care patients, most archetypally posttraumatic stress disorder (PTSD). Several sedative-hypnotics and analgesics are known to modulate fear systems, and it is theoretically plausible that clinical decisions of the anesthesiologist could impact psychological outcomes. This review aims to provide a focused synthesis of relevant literature from multiple fields of research.

RECENT FINDINGS

There is evidence in some contexts that unconscious fear memory systems are less sensitive to anesthetics than are conscious memory systems. Opiates may suppress the activation of fear systems and have benefit in the prevention of PTSD following trauma. There is inconsistent evidence that the use of propofol and benzodiazepines for sedation following trauma may potentiate the development of PTSD relative to other drugs. The benefits of ketamine seen in the treatment of major depression are not clearly replicated in PTSD-cluster psychopathologies, and its effects on fear processes are complex.

SUMMARY

There are multiple theoretical mechanisms by which anesthetic drugs can modulate fear systems and clinically important fear-based psychopathologies. The current state of research provides some evidence to support further hypothesis investigation. However, the absence of effectiveness studies and the inconsistent signals from smaller studies provide insufficient evidence to currently offer firm clinical guidance.

Propofol and sevoflurane affect intra-operative memory formation of words differently: A prospective cohort study

BACKGROUND

Memory priming seems possible even during apparently adequate anaesthesia. However, the effects of different anaesthetics and type of stimuli, by virtue of their specific neural underpinnings, have not been considered.

OBJECTIVE

To determine if intra-operative implicit memory is affected by the type of anaesthesia (propofol or sevoflurane) or by the type of stimuli (abstract or concrete words).

DESIGN

Two consecutive, randomised controlled experiments.

SETTING

Neurological institute in Milan, Italy.

PATIENTS

Forty-three patients undergoing anaesthesia with propofol (experiment 1) and 32 patients undergoing anaesthesia with sevoflurane (experiment 2). Patients were ASA I or II, age 18 to 65 years, native Italian speakers, right-handed and without any condition affecting memory or hearing.

INTERVENTION

During anaesthesia, the patients heard a list of either concrete or abstract words or no words at all (controls). Explicit memory was tested with an explicit recall task and the Brice Interview; implicit memory was assessed through a word stem completion test.

OUTCOME MEASURES

The number of explicitly recalled words, positivity to the Brice Interview, the proportion of target and nontarget hits, and a derived implicit memory score.

RESULTS

With propofol, the proportion of target hits was significantly greater than the proportion of nontarget hits for the concrete word experimental group (P = 0.018). The implicit memory score of the concrete word experimental group was significantly higher than the score of both the abstract word experimental group (P  = 0.000) and the concrete word control group (P = 0.023). With sevoflurane, the proportion of target hits was significantly higher than the proportion of nontarget hits for the abstract word experimental group only (P = 0.027). No patients had a BIS above 60 and no one could recall intra-operative events or words.

CONCLUSION

Intra-operative memory for words can form during apparently adequate BIS-guided anaesthesia but is modified by propofol or sevoflurane acting on different brain targets. Further studies on larger samples and using neuroimaging techniques are needed.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NCT03727464.

Clinical Pharmacokinetics and Pharmacodynamics of Propofol

Propofol is an intravenous hypnotic drug that is used for induction and maintenance of sedation and general anaesthesia. It exerts its effects through potentiation of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) at the GABAA receptor, and has gained widespread use due to its favourable drug effect profile. The main adverse effects are disturbances in cardiopulmonary physiology. Due to its narrow therapeutic margin, propofol should only be administered by practitioners trained and experienced in providing general anaesthesia. Many pharmacokinetic (PK) and pharmacodynamic (PD) models for propofol exist. Some are used to inform drug dosing guidelines, and some are also implemented in so-called target-controlled infusion devices, to calculate the infusion rates required for user-defined target plasma or effect-site concentrations. Most of the models were designed for use in a specific and well-defined patient category. However, models applicable in a more general population have recently been developed and published. The most recent example is the general purpose propofol model developed by Eleveld and colleagues. Retrospective predictive performance evaluations show that this model performs as well as, or even better than, PK models developed for specific populations, such as adults, children or the obese; however, prospective evaluation of the model is still required. Propofol undergoes extensive PK and PD interactions with both other hypnotic drugs and opioids. PD interactions are the most clinically significant, and, with other hypnotics, tend to be additive, whereas interactions with opioids tend to be highly synergistic. Response surface modelling provides a tool to gain understanding and explore these complex interactions. Visual displays illustrating the effect of these interactions in real time can aid clinicians in optimal drug dosing while minimizing adverse effects. In this review, we provide an overview of the PK and PD of propofol in order to refresh readers' knowledge of its clinical applications, while discussing the main avenues of research where significant recent advances have been made.

5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: patient experiences, human factors, sedation, consent, and medicolegal issues

The 5th National Audit Project (NAP5) of the Royal College of Anaesthetists and the Association of Anaesthetists of Great Britain and Ireland into accidental awareness during general anaesthesia (AAGA) yielded data related to psychological aspects from the patient, and the anaesthetist, perspectives; patients' experiences ranged from isolated auditory or tactile sensations to complete awareness. A striking finding was that 75% of experiences were for <5 min, yet 51% of patients [95% confidence interval (CI) 43-60%] experienced distress and 41% (95% CI 33-50%) suffered longer term adverse effect. Distress and longer term harm occurred across the full range of experiences but were particularly likely when the patient experienced paralysis (with or without pain). The patient's interpretation of what is happening at the time of the awareness seemed central to later impact; explanation and reassurance during suspected AAGA or at the time of report seemed beneficial. Quality of care before the event was judged good in 26%, poor in 39%, and mixed in 31%. Three-quarters of cases of AAGA (75%) were judged preventable. In 12%, AAGA care was judged good and the episode not preventable. The contributory and human factors in the genesis of the majority of cases of AAGA included medication, patient, and education/training. The findings have implications for national guidance, institutional organization, and individual practice. The incidence of 'accidental awareness' during sedation (~1:15,000) was similar to that during general anaesthesia (~1:19,000). The project raises significant issues about information giving and consent for both sedation and anaesthesia. We propose a novel approach to describing sedation from the patient's perspective which could be used in communication and consent. Eight (6%) of the patients had resorted to legal action (12, 11%, to formal complaint) at the time of reporting. NAP5 methodology provides a standardized template that might usefully inform the investigation of claims or serious incidents related to AAGA.

Acceptably aware during general anaesthesia: 'dysanaesthesia'--the uncoupling of perception from sensory inputs

This review makes the case for 'dysanaesthesia', a term encompassing states of mind that can arise in the course of anaesthesia during surgery, characterised by an uncoupling of sensation and perceptual experience. This is reflected in a macroscopic, functional model of anaesthetically-relevant consciousness. Patients in this state can be aware of events but in a neutral way, not in pain, sometimes personally dissociated from the experiences. This makes events associated with surgery peripheral to their whole experience, such that recall is less likely and if it exists, makes any spontaneous report of awareness unlikely. This state of perception-sensation uncoupling is therefore broadly acceptable (a minimum requirement for acceptable anaesthesia) but since it is likely a dose-related phenomenon, may also represent a precursor for awareness with adverse recall. This hypothesis uniquely explains the often inconsistent responses seen during the experimental paradigm of the 'isolated forearm technique', wherein apparently anaesthetised patients exhibit a positive motor response to verbal command, but no spontaneous movement to surgery. The hypothesis can also explain the relatively high incidence of positive response to relatively direct questions for recall (e.g., using the Brice questionnaire; ∼1:500; the vast majority of these being neutral reports) versus the very low incidence of spontaneous reports of awareness (∼1:15,000; a higher proportion of these being adverse recollections). The hypothesis is consistent with relevant notions from philosophical discussions of consciousness, and neuroscientific evidence. Dysanaesthesia has important implications for research and also for the development of appropriate monitoring.

Implicit memory formation using the word stem completion task during anesthesia in children

AIMS

To determine whether there is evidence of implicit memory formation during pediatric anesthesia using the word stem completion task.

BACKGROUND

In adults, there is mixed evidence for implicit memory formation during anesthesia; however, there is no evidence in children. Implicit memory in adults has been detected using the word stem completion task. This test has not been used in a pediatric anesthetic setting.

METHODS

A total of 200 patients aged 7-12 were randomized to hear one of the two lists of 10 words played continuously while anesthetized. Group 1 heard List A and Group 2 heard List B. Postoperatively, 194 completed a word stem completion task where they were required to complete the stems (the first part of words) corresponding to the words on the two word lists combined, with the first word that came to mind.

RESULTS

Group 1 completed a mean of 2.78 words correctly from List A and a mean of 2.70 words correctly from List B. Group 2 completed a mean of 3.29 words correctly from List A and a mean of 3.66 words correctly from List B. For List A, there was no evidence (P = 0.70) for an association between intraoperative exposure to this list and the odds of successfully completing a stem from this list with the corresponding target word. There was little evidence (P = 0.09) for an association with List B.

CONCLUSION

This study found no strong evidence that children form implicit memories for auditory words during anesthesia. Given the difference between lists, future research is warranted using carefully chosen word stems.

Unconscious learning processes: mental integration of verbal and pictorial instructional materials

This review aims to provide an insight into human learning processes by examining the role of cognitive and emotional unconscious processing in mentally integrating visual and verbal instructional materials. Reviewed literature shows that conscious mental integration does not happen all the time, nor does it necessarily result in optimal learning. Students of all ages and levels of experience cannot always have conscious awareness, control, and the intention to learn or promptly and continually organize perceptual, cognitive, and emotional processes of learning. This review suggests considering the role of unconscious learning processes to enhance the understanding of how students form or activate mental associations between verbal and pictorial information. The understanding would assist in presenting students with spatially-integrated verbal and pictorial instructional materials as a way of facilitating mental integration and improving teaching and learning performance.

Unconscious learning processes: mental integration of verbal and pictorial instructional materials

This review aims to provide an insight into human learning processes by examining the role of cognitive and emotional unconscious processing in mentally integrating visual and verbal instructional materials. Reviewed literature shows that conscious mental integration does not happen all the time, nor does it necessarily result in optimal learning. Students of all ages and levels of experience cannot always have conscious awareness, control, and the intention to learn or promptly and continually organize perceptual, cognitive, and emotional processes of learning. This review suggests considering the role of unconscious learning processes to enhance the understanding of how students form or activate mental associations between verbal and pictorial information. The understanding would assist in presenting students with spatially-integrated verbal and pictorial instructional materials as a way of facilitating mental integration and improving teaching and learning performance.

Light levels of anaesthesia after relaxation for tracheal intubation - comparison of succinylcholine and cis-atracurium

BACKGROUND

Use of a single bolus of a hypnotic together with non-depolarizing muscle relaxants for anaesthesia induction may cause inappropriate light levels of anaesthesia (ILLA). The purpose of this study was to compare the incidence of ILLA during anaesthesia induction using either cis-atracurium (CIS) or succinylcholine (SUC).

METHODS

Patients (n = 65) received fentanyl and propofol. Relaxants were randomly chosen and were either CIS 0.15 mg/kg, or SUC 1 mg/kg. After achieving relaxation, ILLA were assessed double-blinded by the isolated forearm technique and electroencephalogram -derived values.

RESULTS

Time from induction to complete relaxation was 335 ± 55 s with CIS and 141 ± 26 s with SUC. Nine patients in the CIS group (26%), but no patient in the SUC group responded to commands before endotracheal intubation (P < 0.01). During the entire induction up to 1 min after intubation in the CIS group, 24 of 35 patients (68%) showed 31 episodes of ILLA, as defined as responsiveness to commands and spontaneous movements. With SUC, 8 of 30 patients (27%) showed 11 episodes of ILLA (P < 0.01). In patients with ILLA, state entropy (SE) and spectral edge frequency (SEF) were 68 ± 10 (mean ± standard deviation) and 17 ± 4 Hz, respectively, and in patients without ILLA, SE and SEF were 40 ± 14 and 12 ± 3 Hz, respectively (P < 0.01).

CONCLUSION

The onset time of a muscle relaxant has substantial impact on the incidence of ILLA during induction of anaesthesia. Entropy and SEF may indicate the presence of ILLA.

Awareness under general anesthesia

BACKGROUND

Awareness while under general anesthesia, and the later recall of what happened during surgery, can be experienced by patients as horrific events that leave lasting mental trauma behind. Patients may have both auditory and tactile perception, potentially accompanied by feelings of helplessness, inability to move, pain, and panic ranging to an acute fear of death. For some patients, the experience of awareness under anesthesia has no sequelae; for others, however, it can lead to the development of post-traumatic stress disorder, consisting of complex psychopathological phenomena such as anxiety, insomnia, nightmares, irritability, and depression possibly leading to suicidality.

METHODS

The literature on the subject was selectively reviewed.

RESULTS

In the absence of risk factors awareness phenomena occur in one to two per 1000 operations under general anesthesia (0.1% to 0.2%) and are thus classed as an occasionally occurring critical event. In children, the risk of such phenomena occurring is 8 to 10 times higher. These phenomena are due to an inadequate depth of anesthesia with incomplete unconsciousness. They can be promoted by a number of risk factors that are either patient-related (ASA class III or above, medication abuse), surgery-related (Caesarean section, emergency procedures, surgery at night), or anesthesia-related (anesthesia without benzodiazepines, use of muscle relaxants).

CONCLUSION

Strategies for avoiding awareness phenomena under anesthesia include the training of staff to know about the problem and, specifically, the use of benzodiazepines, the avoidance of muscle relaxants if possible, and shielding the patient from excessive noise. EEG monitoring is effective but provides no guarantee against awareness. If awareness under anesthesia occurs despite these measures, the patient must be given expert, interdisciplinary treatment as soon after the event as possible in order to minimize its potential sequelae.