Paradoxical Combinations of Bispectral Index and Burst Suppression Ratio

The relationship between the bispectral index (BIS) and the BIS monitor's burst suppression ratio (BSR) has been extensively researched, with the current consensus being that BIS is fully driven by the BSR for BSR ≥ 40%. For lower BSR, the BIS seems to be derived from an unknown combination of electroencephalogram (EEG) parameters. In general, the BIS and BSR are not linearly correlated. With limited knowledge about the indices and their interactions, BIS- and BSR-driven anesthesia navigation may lead to index combinations showing paradoxical information.

Using intraoperative recordings of BIS and BSR from 62 patients 80.9 ± 5.8 (mean ± standard deviation [SD]) years, we analyzed the distribution of BIS and BSR values as well as their relation to each other with a focus on paradoxical situations, ie, an adequate BIS of 40 to 60 together with BSR ≥ 5%. We quantified the incidence rate and duration of these situations as well as the distribution of these BSR events within the BIS = 40 to 60 range.

Only 56.9 [44-74.2]% (median [Q1-Q3]) BIS values fell inside the 40 to 60 range despite titration to this range. We found a disproportionately high incidence of BIS 41 to 42. BSR showed an exponentially declining, continuous distribution. We could observe paradoxical BIS and BSR values lasting for considerable stretches of time that could exceed 2 minutes.

BIS values are not continuously scaled, with some index values occurring distinctly more often. Paradoxical values of BIS between 40 and 60 and BSR≥5% can occur, potentially confusing anesthesia care providers.