For the patients undergoing propofol sedation under regional anesthesia, continuous monitoring of the hypnotic level is required for adequate sedation and rapid recovery. We evaluated the correlation between the observer's assessment of alertness/sedation (OAA/S) and the EEG-entropy.

We studied 40 patients who were scheduled for spinal anesthesia. Premedication with intramuscular midazolam 0.04 mg/kg was carried out 30 minutes before spinal anesthesia. When the anesthesia level was adequate for surgery but lower than T6, an infusion of propofol 10 mg/kg/h was started and this was decreased to 5 mg/kg/h after 1 minute. We measured the response entropy (RE), the state entropy (SE) and the OAA/S score. When the OAA/S score fell to 1, the infusion dose was decreased to half. After this, the infusion dose was decreased or increased to half or twice, respectively, to keep the OAA/S score at 2 or 3.

The OAA/S was well correlated with the RE (Spearman's rho = 0.913) and also the SE (Spearman's rho = 0.915). With the increasing depth of sedation, there was a progressive decrease in the RE and SE (the OAA/S score/the mean of the RE/the mean of the SE = 5/98/89, 4/92/85, 3/85/78, 2/78/70, 1/66/59).

EEG-entropy provided good information for monitoring the hypnotic level for the patients undergoing propofol sedation under spinal anesthesia.