Monitoring: Equipment and Computers
Background and Goal of Study: Bispectral Index (BIS) and Spectral Entropy (E) are proposed to monitor the depth of anaesthesia. This study compared the effect of ketamine on BIS and E during sevoflurane anaesthesia.
Methods: After IEC approval, 23 consenting women undergoing gynaecological surgery were enrolled in this double blind study. Anaesthesia was induced with propofol, sufentanil and rocuronium, and maintained with 1 MAC sevoflurane in air/O2. Patients were randomly assigned to receive either ketamine 0.5 mg/kg (GK) or normal saline (NS) (GC) under stable surgical conditions. Blood pressure, heart rate, BIS, response (RE) and state (SE) entropy were recorded from 10 min. before ketamine or NS administration (baseline) and every 2.5min thereafter (T2.5, T5, T7.5 T10, T12.5 and T15). Data (mean ± SD) were analysed using twoway mixed-design ANOVA's. Maximum relative increases of BIS, RE or SE (% of baseline) were compared using Bonferroni corrected Wilcoxon. P < 0.05 was considered statistically significant.
Results: Demographic data and ASA status were similar in both groups. Ketamine compared to NS provoked a significant increase in BIS, RE and SE from T2.5 (RE and SE) or T5 (BIS), and thereafter. BIS, RE and SE were also significantly higher in GK than in GC from T5 to T15. The maximum relative increase of RE (42.2 ± 10.4%) and SE (41.6 ± 10.9%) was significantly higher than that of BIS (29.4 ± 10.4%). Haemodynamic parameters did not change and were similar in both groups.
Conclusions: Under sevoflurane anaesthesia, ketamine causes a significant increase in RE and SE, proportionally greater than the increase in BIS. Ketamine may confound BIS, RE and SE as indices of the depth of anaesthesia.