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

Background
Several studies that investigate the implicit memory under general anesthesia revealed conflicting results. Limitations may be due to failure to control depth of anesthesia. This prospective randomized study was designed to compare the implicit memory during total intravenous versus inhalational anesthesia.
Method
Fourty patients ASA I and II undergoing orthopedic procedures under BIS-guided (40–60) general anesthesia were tested for implicit memory of previously introduced auditory material. Patients were divided into two groups (each group 20 patient). Group I, anesthesia was induced with propofol and maintained with propofol, fentanyl and cis-atracurium. Group II, anesthesia was induced with propofol and maintained with sevoflurane, fentanyl and cis-atracurium. Explicit memory was evaluated by asking four standard questions regarding intra-operative awareness. Free recall and recognition tests for implicit memory testing were carried out 30 min and 120 min after recovery.
Results
No participant manifested explicit recall according to the 4-question interview. None of the patients gave a free recall of the presented items during anesthesia in the immediate and delayed tests of memory. In the recognition tests, immediate test (5%) and (0%) of the patients indicated correctly the presented words in TIVA and sevoflurane group respectively, in delayed test (10%) in TIVA group and (5%) in sevoflurane group. These results are not significant for implicit memory.
Conclusion
There was no significant evidence of implicit memory in the two groups. BIS – controlled anesthesia appears to abolish implicit memory.