Does a BIS-guided Maintenance of Anesthetic Depth Prevent Implicit Memory?

Abstract

Recent studies investigating the relationship between depth of anesthesia and implicit memory have conflicting results. Limitations of these studies include lack of standardization in surgical procedures and failure to control depth of anesthesia prospectively. We assessed implicit memory function at two different (BIS-guided and prospectively controlled) anesthetic depths during surgical stimulus. A list of words was played via headphones to 37 patients during orthopedic surgery under general anesthesia. The Bispectral (BIS) Index was monitored and patients were randomized to remain in a deeper (BIS 40-45) or lighter (BIS 55-60) plane of surgical anesthesia during word presentation. Postoperatively, implicit memory performance was tested using a simple auditory word-stem completion test for presented as well as non-presented words. Absence of explicit memory was evaluated by asking four standard questions regarding intraoperative awareness. All patients received sevoflurane and fentanyl for general anesthesia. There was no evidence of implicit memory in either study group. Hit rates for presented and non-presented words were 0.23 ± 0.14 and 0.25 ± 0.09, respectively. No explicit memory was reported by any patient. Depth of anesthesia did not affect implicit memory formation in anesthetized patients undergoing surgery. General anesthesia, even at a higher BIS range, appears to abolish implicit memory as assessed by a simple word-stem completion test.

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

“…From the 1167 potentially relevant studies that we initially identified in the literature, we excluded 1106 studies because they did not meet the inclusion criteria, were duplicates, or were incomplete in their methods or in their outcome data. Therefore, 61 studies, [ 9 , 10 , 11 , 12 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , ...…”

“…A total of 61 studies [ 9 , 10 , 11 , 12 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , …”

Comparative study of implicit memory during bispectral index guided total intravenous anesthesia versus sevoflurane inhalation anesthesia

“…Furthermore, maintaining the BIS below 60 may be an adequate strategy to prevent the formation of implicit as well as explicit memories [2]. Nevertheless, it may be reasonable to caution anesthesiologist, nurses and surgeons to restrain their conversations as some of them may be retained by the unconscious patients [14].…”

“…Did you dream?'' The questions were similar to those used in previous studies [2,6]. Testing was done through postoperative interviews 30 min (immediate test) and 120 min (delayed test) carried out to assess cognitive functions as follows:…”

“…The term memory usually includes the acquisition of new information, its storage and subsequent retrieval. There are two distinct forms of memory: explicit memory which is the deliberate recollection of an experience and implicit (also called none declarative memory), which is the influencing of a response by memory of a previous experience without the person knowing that he or she is being influenced [1,2]. Formation of explicit memories during general anesthesia is associated with intra-operative awareness.…”