Alternative Placement of Bispectral Index Electrode for Monitoring Depth of Anesthesia during Neurosurgery

In neurosurgery in particular, the recommended placement of electrodes for monitoring depth of anesthesia during surgery sometimes conflicts with the surgical site or patient positioning. Therefore, we proposed this study to evaluate the agreement and correlation of bispectral index values recorded from the usual frontal area and the alternate, post-auricular areas in neurosurgery patients. Thirty-four patients scheduled for neurosurgery under general anesthesia were included. Bispectral index (BIS) sensors were placed at both the frontal and post-auricular areas. The anesthesia given was clinically adjusted according to the frontal (standard) BIS reading. The BIS values and impedance were recorded;Pearsonʼs correlation and Bland-Altman plots were analyzed. The bias ± 2SD for the electrode placement before, during, and post-anesthesia were 0±23.32, 1.5±10.69, and 2.1±13.52, while the limits of agreement were －23.3 to 23.3, －12.2 to 9.2, and －17.7 to 13.5, respectively. The correlation coefficient between frontal- and post-auricular-area electrodes was 0.74 with a p-value ＜0.001.The post-auricular placement of a BIS electrode is a practical alternative to frontal lobe placement. Nevertheless, proper electrode location is important to minimize error.