The effect of analgesic state on implicit learning during propofol anesthesia in volunteers

Noxious stimulation may enhance implicit learning during general anesthesia. It is unknown, however, whether analgesic state can influence this memory processing. Twenty healthy adult volunteers were enrolled our prospective, double-blinded, controlled experiments. Anesthesia was induced with a propofol target controlled infusion (TCI), titrated in step-wise increments to loss of consciousness. In phase A, a 10-word list was played to the subjects while a noxious stimulus was applied (hand immersion in cold water at 2-4 degrees C). In phase B, a remifentanil TCI infusion was added to the steady-state propofol TCI anesthesia, and titrated to loss of hand movement on cold water immersion. A second 10-word list was then played while maintaining the hand in cold water. Memory testing, 2 hours post-recovery revealed no evidence of explicit memory in any subject during either phase of the study. During phase A, the word stem completion test revealed implicit learning for played words. In contrast, no implicit memory was detected during phase B. This study indicates that analgesia with remifentanil TCI (titrated to loss of movement on noxious stimulation), prevented implicit memory processing during stable propofol anesthesia in healthy adult volunteers.