Implicit memory formation in sedated ICU patients after cardiac surgery

Background. Recent research into memory formation under sedation has generated conflicting results. We investigated explicit and implicit memory in ICU patients during moderate to deep propofol sedation following cardiac surgery. Methods. Two different methods of memory testing were used; (1) free-association (F-A) word-pair testing ( n =33) to test conceptual implicit memory and (2) process dissociation procedure (PDP) ( n =26) to detect perceptual implicit and explicit memory. One hour after surgery, whilst sedated, the F-A group received one of two lists of 10 category-exemplar word-pairs through headphones, while the PDP group was presented with one of two lists of 16 five-letter words. When awake and co-operative, the F-A group was tested using F-A testing, and the PDP group was tested using the PDP. Results. The F-A group had a mean (sd) correct response rate of 7 (9)% for the target list, and 9 (8)% for the distractor list. The PDP group had a mean (sd) correct response rate of 11 (14) and 10 (13)% for the inclusion and exclusion lists, respectively, with mean correct response rates of 13 (14)% for both the corresponding distractor lists. Neither group showed any significant differences between their responses and a list of distractor words (Wilcoxon tests). Conclusion. We found no evidence for memory formation in post-cardiac surgery patients under moderate to deep propofol sedation.