Comparative effects of pentazocine on the auditory evoked potential index (AEP) and the bispectral index (BIS) under nitrous oxide sevoflurane anesthesia

Background : The auditory evoked potential index (AEP) and the bispectral index (BIS) can be used to assess the depth of general anesthesia Pentazocine is reported to increase the BIS value in anesthetized patients and may confound use of the index as a guide to administering hypnosis. In this study, we compared the effects of pentazocine on AEP values and BIS values in the absence of surgical stimulation under nitrous oxide sevoflurane anesthesia. Methods : The study was approved by the ethics committee of our institution, and written informed consent was obtained from all patients. Patients with known psychiatric, cardiovascular, neurological or endocrine disorders were excluded from the study. Twenty adult patients (ASA I) scheduled for elective oral surgery were studied, and assigned to one of two groups : AEP group (n = 10) and BIS group (n = 10). Anesthesia was induced with thiopental sodium and vecuronium bromide and maintained with a mixture of nitrous oxide (4 l·min^-1), oxygen (2 l·min^-1 and sevoflurane (1%). Mean arterial blood pressure (MAP), heart rate (HR), and either AEP values or BIS values were recorded every 2.5 min after intubation up to 30 min before surgery. Pentazocine (0.6 mg·kg^-1) was administered intravenously (IV) 15 min after intubation. Data were presented as mean±SD, and results were analyzed by using repeated-measures analysis of variance. Post hoc comparisons were done using Tukey's test. For comparison between AEP and BIS groups, Student's test or χ² test was used. P<0.05 was considered significant. Results : MAP and HR showed no significant differences between values before and after IV pentazocine, and between AEP and BIS groups. BIS values increased significantly (p<0.01) at 5 min after pentazocine IV, whereas AEP values showed no significant changes in response to pentazocine IV. Conclusions : IV pentazocine revealed different aspects of neural processing between AEP and BIS during nitrous oxide sevoflurane anesthesia The significant increase in BIS values, but not AEP values, elicited by pentazocine is paradoxical in that it is associated with a deepening level of hypnosis.