Unconscious learning during surgery with propofol anaesthesia

Background. Learning during anaesthesia has been demonstrated, but little is known about the circumstances under which it may occur. This study investigated the hypothesis that learning during anaesthesia occurs during, but not before, surgical stimulation. Methods. Words were played through headphones to 64 day-surgery patients during propofol anaesthesia. Fourteen words were played repeatedly (15 times) for 1 min each either before (n=32) or during (n=32) surgical stimulation. The depth of anaesthesia was estimated using the bispectral index(TM) (BIS(TM)). Heart rate, ventilatory frequency, mean arterial pressure, end-tidal carbon dioxide concentration, and infusion rate of propofol were recorded at 1 min intervals during word presentation. On recovery, memory was assessed using an auditory word stem completion test and word recognition test. Results. The mean BIS(TM), arterial pressure, end-tidal carbon dioxide and heart rate during word presentation did not differ between the groups. The infusion rate of propofol and the ventilatory frequency were significantly greater in the during-surgical stimulation group. There was no evidence for explicit recall or recognition, nor of awareness during anaesthesia (median mean-BIS(TM)=38 in the before-surgical stimulation group and 42 in the during-surgical stimulation group). Only patients who were played words during surgical stimulation showed significant implicit memory on recovery (mean score=0.08, P<0.02) However, their scores were not significantly higher than those of the before-surgical stimulation group (mean score=0.01). Conclusions. Learning during anaesthesia seems more likely to occur during rather than before surgical stimulation at comparable anaesthetic depth. We hypothesize that surgical stimulation facilitates learning during anaesthesia, independently of its effects on anaesthetic depth.