Reply 1

Sir, Intraoperative awareness seems to be a minor problem for patients undergoing an operation as indicated by the study of Ranta et al. (I), who reported only four claims for compensation for awareness over a six-year period in Finland. However, it might be of interest for the anaesthesist that the absence of explicit recall of intraoperative events does not rule out the existence of so-called implicit memory for events that were perceived during anaesthesia. These can not be detected by traditional memory tests requiring free recall or recognition, but by special tests for implicit memory (2). For example, in the Word Stem Completion Task the patients are presented with a list of uncommon words during anaesthesia. Postoperatively, the patients are given a page containing a list of word stems representing the three initial letters of words (some of which match the words presented during anaesthesia). Patients are asked to write the first word beginning with those letters that comes to mind. Implicit memory is shown if the patient completes stems with more "primed" words than control words that had not been presented previously. At the moment there are some reports demonstrating implicit memory during anaesthesia (3, 4), although other authors debate those findings (5). Recently, in an as of yet unpublished study we could confirm implicit memory retention during anaesthesia by a Reading Time Task (6). The patients were premedicated with midazolam and after induction with propofol, alfentanyl and tracrium the anaesthesia was maintained with continuous infusions of propofol(4 mg kg-' h-') and alfentanyl(30 pg kg-' h-l and tracrium boluses as needed by thirty patients undergoing lumbar disc surgery. Two stories were presented via headphones six times from the time of skin incision. Postoperatively, after a delay of approximately 7 hours, the patients were asked to read these two and two other identically constructed stories as fast as possible. The stories were presented per videomonitor and with words masked by strings of the letter X. These could be read by pushing the space bar on the keyboard. Reading times (ms/ word) were calculated by a computer program. Those stories presented under anaesthesia were read significantly quicker by the patients than those not presented before (old stories 727 ms, new 764 ms). The reading speed was likewise decreased for the "old' stories in the control subjects, which heard the two stories without anaesthesia (old 484 ms, new 521 ms), thus indicating an implicit memory effect of equal size for both groups. There were no signs of wakefulness or recall of any intraoperative events by the anaesthetised group. Thus conscious awareness is only one kind of perception that can occur intraoperatively. Unconscious perception and retention of memories may occur independently. The intraoperative awareness may have deleterious psychologic sequelae for the patients (7). Some anecdotal evidence suggests that unconscious perception can influence the recovery and outcome of the patients as well @), and further research should address the relation between unconscious perception and postoperative emotional and cognitive well-being. Sinikka Miinte Martin Leuwer Siegfried Piepenbrock