We read with interest the comments by Lequeux et al.  about our article,1and we agree with them. As mentioned in the Discussion, our positive memory results, even with adequate anesthesia, may be related to learning during a period of lighter anesthesia that was “missed” by our Bispectral Index (BIS) recording and also by our BIS analysis. More precisely, to classify each word in a BIS category, we used the mean of the BIS values associated with each word played during anesthesia. Therefore, it is possible that some of the words have been played at a higher BIS value than reported in the study. Moreover, as suggested by Lequeux et al. , because of the time requirement for BIS processing, the first BIS values associated with a word should have been associated with the word played earlier. We have thus reanalyzed our data regarding memory performance for the different levels of anesthesia, eliminating the BIS values associated with the 30 first seconds of each word presentation. Moreover, we have considered only the highest value of BIS associated with each word (and not the mean of BIS values). These “Maximal BIS” values were categorized as BIS 21–40, 41–60, and 61–80, and memory scores (C and A) were recalculated. We globally replicated our results despite these changes. That is, we found no evidence of memory during deep anesthesia (BIS 21–40, C = 0.05 ± 0.1 and A = 0.09 ± 0.14). However, memory for words was significant during adequate anesthesia (BIS 41–60), with a significant contribution of implicit memory, because the automatic influence score was significantly greater than the base rate (P < 0.05; A = 0.18 ± 0.19). During light anesthesia (BIS 61–80), the automatic influence was greater than the base rate, but not significantly (P = 0.09; A = 0.17 ± 0.17). However, this nearly significant result for light anesthesia can be explained by the insufficient number of words that could be included in this analysis of memory performance. Finally, we found no evidence of explicit memory contribution regardless of the level of anesthesia (C = 0.04 ± 0.09 at BIS 41–60 and C = 0.04 ± 0.09 at BIS 61–80). This last analysis emphasizes the necessity of further investigations on persistence of implicit memory during light and adequate anesthesia.

*University Hospital of Geneva, Geneva, Switzerland. irene.iselin-chaves@hcuge.ch

Iselin-Chaves IA, Willems SJ, Jermann FC, Forster A, Adam SR, Van der Linden M: Investigation of implicit memory during isoflurane anesthesia for elective surgery using the process dissociation procedure Anesthesiology 2005; 103:925–33