Introduction: Unconscious processing of words during general anaesthesia has been suggested. We used the process dissociation procedure (PDP) to test memory performance during sevoflurane and propofol anaesthesia in relation to hypnotic depth.
Material and methods: One hundred participants anaesthetised for elective surgery (50 with propofol and 50 with sevoflurane) and 50 non-anaesthetized listened to a list of words. The bispectral index (BIS) of the anaesthetised patients was recorded. Within 36 h after word presentation, memory was assessed using a word stem completion task, based on Buchner’s model applied on the PDP.
Results: There was evidence of memory for words presented during light (BIS 61-80) (p = 0.001) and adequate (BIS 41-60) (p = 0.008) but not deep anaesthesia (BIS 21-40) (p = 0.09). The PDP showed a significant implicit but not explicit memory contribution (mean total explicit memory scores: 0.04 ±0.07 in all BIS categories; mean implicit memory scores: 0.01 ±0.04, 0.1 ±0.08, and 0.05 ±0.09 at BIS = 21-40, 41-60, and 61-80, respectively). There was a statistically significant difference between the mean implicit memory score (I) of the propofol and sevoflurane group in the BIS category 41-60 in general (p = 0.016), and after incision (I_A.I.) (p = 0.005) in particular, with propofol depressing I more than sevoflurane in both cases. Memory performance of nonanaesthetized participants was better, with a higher contribution of explicit and a comparable contribution of implicit memory.
Conclusions: During general anaesthesia, implicit memory persists even in adequate hypnotic states. Sevoflurane affects the implicit memory of adequately anaesthetised subjects less than propofol.