The auditory evoked potential (AEP) index and bispectral (BIS) index have been proposed as methods to measure the depth of sedation. A prospective study was designed to assess the performance of both these methods for measuring the depth of sedation induced by propofol patient-controlled sedation (PCS) under spinal anesthesia.

Forty ASA I and II adult patients under spinal anesthesia using 0.5% hyperbaric bupivacaine were studied. Group 1 (10 mg bolus, 30 mg loading) and Group 2 (20 mg bolus, 60 mg loading) received propofol and maintained PCS with 1min lockout interval and 100 mg/hr continuous infusion. AEP, BIS and Observer's assessment of alertness/sedation (OAA/S) scale were monitored during the operation.

AEP and BIS decreased and increased following the changes on the patient's OAA/S scores and correlated with sedation significantly. There were no significant difference in mean AEP index (group 1; 13.4 ± 8.4, group 2; 8.9 ± 6.2), BIS index (group 1; 76.2 ± 9.7, group 2; 71.2 ± 9.8), and OAA/S scale (group 1; 3.8 ± 1.3, group 2; 3.2 ± 1.5) between the groups. Incidence of perioperative respiratory depression was significantly higher in group 2 (25%) than group 1 (5%), and incidence of involuntary movement was significantly higher in group 1 (20%) than group 2 (5%) (P < 0.05).

Both AEP and BIS correlated well with the depth of sedation induced by propofol PCS under spinal anesthesia. AEP seems to be more valuable in measuring the change between consciousness and unconsciousness, and BIS seems to be more effective in measuring the depth of sedation.