However, the literature contains contrasting results regarding what has a significant impact on implicit memory formation during surgery and anesthesia. They can be related to (i) anesthesia (like the type and dosage of hypnotic/analgesic drugs delivered for induction and/or maintenance, the usage of NMBA, anesthesia duration, or the use of ABM), (ii) the timing of the auditory task adopted for implicit memory formation (before skin incision or during surgical stimulation, repeated presentation of the words during the whole anesthesia period or otherwise), and (iii) the time point of implicit memory testing after the return of consciousness. Several variables during anesthesia seem to interfere with implicit memory formation. Implicit memory refers to any change in experience, thought, or action that is attributable to a past event and can be detected with psychological tests, such as free association test, category member generation, word stem completion test, or the process dissociation procedure. While explicit memory is recalled spontaneously, or may be provoked by postoperative events or questioning, implicit memory is not consciously recalled, and may affect behavior or performance at a later time. Therefore, the gap between the incidence of anesthesia awareness and explicit recall, and the incidence of connected consciousness can be explained by the fact that patients may be aware of surgical events at the time they occur but may be unable to remember them later. The explanation for the low incidence of surgical recall may be due to the amnestic properties of the benzodiazepines and other hypnotics administered during general anesthesia. The experience of such conscious episodes can lead to an increased risk of post-traumatic stress disorders. While anesthesia awareness has an incidence of only 0.1–0.2%, connected consciousness, as detected by the isolated forearm technique, has a higher incidence, up to 34.8%. The use of neuromuscular blocking agents (NMBAs), decreasing muscle tone and preventing sudden muscle movements in response to a noxious stimulus, allows surgery under light general anesthesia, which can increase the risk of awareness and connected consciousness, even when an anesthesia brain monitor (ABM) is used. The OAA/S score, which evaluates the patient’s behavioral response, speech, facial expressions, and ocular activity, ranges from a score of 5 (awake state and responsiveness) to 0 (unconscious state and unresponsiveness even to noxious stimuli). Ī popular assessment tool to evaluate responsiveness during anesthesia is the Observer’s Assessment of Alertness/Sedation Scale (OAA/S). One of the most important goals of general anesthesia is to ensure the patient’s unconsciousness and unresponsiveness during induction and maintenance as well as to achieve post-operative amnesia, usually detected by the absence of an explicit recall. 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). 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. For 43 cohorts (36.1%), implicit memory events were reported. We included a total of 61 studies with 3906 patients and 119 different cohorts. The meta-analysis included the estimation of odds ratios (ORs) and 95% confidence intervals (CIs). We also evaluated the impact of different anesthetic/analgesic regimens and the time point of auditory task delivery on implicit memory formation. 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. The impact of general anesthesia in preventing implicit memory formation is not well-established. Unlike explicit memory, implicit memory is not consciously recalled, and it can affect behavior/performance at a later time. Amnesia refers to the absence of explicit and implicit memories. General anesthesia should induce unconsciousness and provide amnesia.
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