Postoperative reading speed does not indicate implicit memory in elderly cardiac patients after propofol and remifentanyl anaesthesia

Background: A recent study in young patients undergoing propofol-alfentanil -nitrous oxide anaesthesia demonstrated implicit memory for stories present ed during operation using a post-operative reading speed task. In this stud y we investigated whether patients who tolerate only small amounts of anaes thetics are prone to develop implicit and explicit memories about intraoper ative events. Methods: Thirty patients with poor physical status (ASA III-IV) undergoing cardioverter defibrillator implantation were included in the study. Patient s were premedicated with intravenous midazolam and anaesthesia was maintain ed using propofol and remifentanil infusion. During surgery one of two audi o-tapes containing two short stories was played to the patients. Reading sp eed for the stories played surgery and two similar stories from the other t ape was tested 4 h later. Explicit memory was tesyted at 4 h and 24 h after audiotape preentation using a structured interview and a forced-choice rec ognition test pertaining to the story content. Thirty additional awake subj ects served as controls. Results: Although half of the patients seemed to be awake one or more times during the operation, no explicit memories of intraoperative events were r eported. The forced-choice recognition of the stories was at chance level. No effects on reading speed was found in either or the control subjects. Conclusion: The possible reasons for reduced explicit and implicit memory p erformance in elderly patients and the modility change between study and te st phases. Non-anaesthetised control group of the same age and physical sta tus ahould therefore be included in all studies of implicit memory.