Absence of implicit and explicit memory during propofol/remifentanil anaesthesia

BACKGROUND AND OBJECTIVE: High doses of opioid associated with low doses of hypnotic is a popular anaesthetic technique since the use of remifentanil has become widespread. This type of anaesthesia could result in a higher incidence of implicit memory. METHODS: Ten patients were anaesthetised with a target-controlled infusion of remifentanil (target concentration of 8 ng mL(-1)) combined with a target-controlled infusion of propofol with progressive stepwise increases until loss of consciousness was reached. A tape containing 20 words was then played to the patients. Bispectral index (BIS, Aspect Medical Systems, Newton, MA, USA) was continuously monitored during the whole study period. Implicit and explicit memories were tested between 2 and 4 h after recovery. RESULTS: Loss of consciousness was obtained with a mean calculated propofol plasma concentration of 1.3 +/- 0.4 microg mL(-1). At this low hypnotic concentration no implicit or explicit memory was found in the three postoperative memory tests. Median (range) BIS value during word presentation was 93 (80-98). CONCLUSIONS: In our group of young American Society of Anesthesiologists (ASA) I/II patients, no explicit or implicit memory was found when the calculated concentration of propofol combined with a high concentration of remifentanil was maintained at the level associated with loss of consciousness with high BIS values.