Bilateral BIS variability during isoflurane anaesthesia

Background and Goal of Study: Bispectral Index (BIS) is usually measured using an electrode montage placed on one side of the head. We have recently reported a drug trial where significant bilateral asymmetry of BIS was observed [1]. Unilateral variability in BIS, measured as the standard deviation over 3 minutes (sBIS) may reflect the level of analgesia [2]. This was an observational pilot study to examine bilateral BIS and sBIS during routine isoflurane anaesthesia. Materials and Methods: We studied historical, anonymized BIS records from 9 female ASA I patients obtained as the randomized placebo component of a previous drug trial. Patients had given written informed consent to a protocol approved by our ethics committee. 2 channel BIS recordings were obtained using a referential frontal montage (A-1000, BIS v3.3, ZipPrep electrodes). 30 minutes of continuous BIS recording (5s updates) were identified during surgery for the left and right side of the head (LBIS and RBIS). The standard deviation for the bilateral BIS values was calculated based on 3 minute epochs (LsBIS and RsBIS). Paired non-parametric analysis (Wilcoxon Signed Rank) compared LBIS v RBIS and LsBIS v RsBIS. P<0.001 was considered significant. Results and Discussion: Combining all patients indicated a clinically small but statistically significant difference between paired bilateral BIS (RBIS median 38.4, range 14.8-92.7; LBIS median 36.2, range 14.5-94.6) and sBIS (RsBIS median 2.98, range 0.1-34.8; LsBIS median 2.87, range 0.2-33.6). Individual paired analysis indicated only one patient with no significant difference in bilateral BIS (RBIS median 38.6, range 22.2-46.1; LBIS median 38.1, range 24.3-44.4 P=0.068), and two other patients with no significant difference in bilateral sBIS. 4 patients demonstrated clinically obvious asymmetry in BIS when bilateral traces were overlaid. The relatively high incidence of bilateral asymmetry in BIS and sBIS was unexpected, and there was no obvious clinical cause. Surgical or sensory stimulation could not be discounted. Conclusion(s): Studies on the EEG during anaesthesia should consider that brain asymmetry may be present. Brain asymmetry is known in specialties other than anaesthesia [3], and additional clinically-relevant information may be present in bilateral indices such as BIS and sBIS. References: [1] Pomfrett CJD, Dolling S, Anders N et al. EJA 2009; in the press. [2] Bloom MJ, Bekker A, Sesgagiri CV et al. Anesthesiology 2008;109:A1303. [3] Davidson RJ, Hugdahl K (eds) Brain Asymmetry 1996; MIT Press.