Minimum alveolar concentration threshold of sevoflurane for postoperative dream recall

Background. Many factors affect postoperative dream recall, including patient characteristics, type of anesthesia, timing of postoperative interview and stress hormone secretion. Aims of the study were to determine whether Bispectral Index (BIS)-guided anesthesia might decrease sevoflurane minimum alveolar concentration (MAC) when compared with hemodynamically-guided anesthesia, and to search for a MAC threshold useful for preventing arousal, dream recall and implicit memory. Methods. One hundred thirty patients undergoing elective thyroidectomy were enrolled. Anesthesia was induced with propofol 2 mg kg-1, fentanyl 3 mcg kg-1and cis-atracurium 0.15 mg kg-1. For anesthesia maintenance, patients were randomly assigned to one of two groups: a BIS-guided group in which sevoflurane MAC was adjusted on the basis of BIS values, and a hemodynamic parameters (HP)-guided group in which MAC was adjusted based on HP. An auditory recording was presented to patients during anesthesia maintenance. Dream recall and explicit/implicit memory were investigated upon awakening and approximately after 24 h. Results. Mean sevoflurane MAC during auditory presentation was similar in the two groups (0.85±0.16 and 0.87±0.17 [P=0.53] in BIS-guided and HP-guided groups, respectively). Frequency of dream recall was similar in the two groups: 27% (N.=17) in BIS-guided group, 18% (N.=12) in HP-guided group, P=0.37. In both groups, dream recall was less probable in patients anesthetized with MAC values ≥0.9 (area under ROC curve=0.83, sensitivity=90%, and specificity=49%). Conclusion. BIS-guided anesthesia was not able to generate different MAC values compared to HP-guided anesthesia. Independent of the guide used for anesthesia, a sevoflurane MAC over 0.9 was required to prevent postoperative dream recall.