The speechless brain : behavioral studies of memory and emotion during anesthesia

This thesis describes investigations into memory function in surgical patients under anesthesia. In a worst case scenario, sleep (hypnosis) is insufficiently induced and/or maintained and the patient regains consciousness, is aware of surgery and possibly in pain. Such an experience is often traumatic and may have devastating, long-term psychological consequences. Ideally, anesthesia induces unconsciousness and recollection of intraoperative events is lost. The latter scenario provided the research setting for this thesis, which specifically addresses two questions: 1) Does memory function depend on hypnotic adequacy? 2) Does memory function depend on the type of information? Outline Studies were conducted in healthy, ambulatory patients scheduled for relatively short elective surgical procedures under general anesthesia. In all cases, anesthesia was induced and maintained with propofol, an intravenous anesthetic with favorable clinical and pharmacodynamic properties for this type of patient and surgery. Novel psychological and neurophysiological techniques were combined to assess memory function during two levels of anesthesia: adequate hypnosis and deep sedation. Each level was examined in separate studies, as was the effect of non-emotional and emotional information. Both the influence of hypnotic state and type of information on memory function (2 x 2 factorial design) were addressed, and four experiments were conducted. The first chapter introduces behavioral studies of memory and emotion in general, and during anesthesia in particular. Part one provides a background to memory assessment and reviews recent technological developments in monitoring hypnotic state during anesthes·la. Part two briefly reviews human and animal responses to emotional stimuli, and describes the neurobiological basis of emotion. The chapters that follow report on the experiments that were conducted to address the aims of this thesis. With the use of a memory priming paradigm with relatively familiar Dutch words, chapters 2 and 3 describe investigations into memory function during adequate hypnosis and deep sedation, respectively. In chapter 3, individual differences in memory function are addressed as well. Chapter 4 highlights patient awareness that was observed during deep sedation from a phenomenological and neurophysiological point of view, and describes its relation to postoperative conscious recall. Chapter 5 investigates processing of emotional information and its effect on memory function during adequate hypnosis. A similar paradigm is adhered to during deep sedation, which is reported on in chapter 6. In chapter 7, finally, the main findings and conclusions of the studies presented in this thesis are discussed

Auditory information processing during adequate propofol anesthesia monitored by electroencephalogram bispectral index

Memory for intraoperative events may arise from inadequate anesthesia when the hypnotic state is not continuously monitored. Electroencephalogram bispectral index (BIS) enables monitoring of the hypnotic state and titration of anesthesia to an adequate level (BIS 40 to 60). At this level, preserved memory function has been observed in trauma patients. We investigated memory formation in elective surgical outpatients during target-controlled propofol anesthesia supplemented with alfentanil. While BIS remained between 40 and 60, patients listened to a tape with either familiar instances (exemplars) from two categories (Experimental [E] group, n = 41) or bird sounds (Control [C] group, n = 41). After recovery, memory was tested directly and indirectly. BIS during audio presentation was on average (+/- SD) 44 +/- 5 and 46 +/- 5 for Groups E and C, respectively. No patient consciously recalled the intraoperative period, nor were presented words recognized reliably (Group E, 0.9 +/- 0.8 hits; Group C, 0.8 +/- 0.8 hits) (P = 0.7). When asked to generate category exemplars, Group E named 2.10 +/- 1.0 hits versus 1.98 +/- 1.0 in Group C (P = 0.9). We found no explicit or implicit memory effect of familiar words presented during adequate propofol anesthesia at BIS levels between 40 and 60 in elective surgical patients. IMPLICATIONS: This study suggests that stable levels of adequate hypnosis may prevent information processing and memory formation during general anesthesia and supports the feasibility of electroencephalogram bispectral index as a monitor of adequate anesthesia