Surgical resection guided by intraoperative electrocorticography (iECoG) has been in clinical use for many decades. The use of the bispectral index (BIS) for monitoring depth of anesthesia during different types of surgery, including epilepsy surgery, is increasing nowadays. The BIS is an EEG-derived variable indicating cortical electrical activity. However, the correlation between the BIS score and the iECoG score, with the purpose of optimizing the quality and time of the iECoG recordings in epilepsy surgery is unknown. The goal of this study was to evaluate the correlation between BIS values and iECoG parameters during the epilepsy surgery under anesthesia with propofol and fentanyl. This is a prospective study that included patients with epilepsy who underwent epilepsy surgery guided by BIS and iECoG (September 2008 to October 2013). Clinical, physiological, and sociodemographic characteristics are shown. We correlated the iECoG parameters (presence of burst suppressions [BS], suppression time [seconds], background frequency [Hz], and type of iECoG score by Mathern et al) with BIS values. We included 28 patients, 15/28 (53.5%) female, general mean age of 30.5 years (range 13-56 years). Patients underwent epilepsy surgery: 22/28 (79%) temporal and 6/28 (21%) extratemporal. We found a nonlinear polynomial cubic relationship between the mentioned variables noting that a BIS range of 40 to 60 gave the following results: iECoG BS periods <5 seconds, background frequency 10 to 17 Hz, and iECoG score 2 characterized by lack of >20-Hz background frequencies. No BS were observed with a BIS > 60. In conclusion BIS values and iECoG parameters during the epilepsy surgery under anesthesia with propofol and fentanyl have a nonlinear correlation. BS patterns were not found with a BIS > 60. These findings show that BIS is a nonlinear multidimensional measure, which possesses high variability with the iECoG parameters. BS patterns are not found with BIS > 60.
Keywords: bispectral index; electrocorticography; general anesthesia; propofol; temporal epilepsy surgery.