Intrapatient reproducibility of the BISxp® monitor

Background: The Bispectral Index (BIS) reportedly reflects anesthetic depth. It is recommended that anesthetic agents should be titrated to maintain the BIS between 40 and 60 arbitrary BIS units during anesthesia. For anesthesia providers to follow this recommendation, the monitor should be predictably affected by different anesthetic agents and have good interpatient and intrapatient reproducibility. The authors hypothesized that when two BISxp® devices (Aspect Medical Systems, Newton, MA) are placed concurrently on the same patient, their readings are concordant throughout the anesthetic period. Methods: Simultaneous BIS recordings from two BISxp® monitors were obtained during anesthesia at 5-s intervals from 12 participants. Results: In total 22,860 concurrent paired BIS readings were obtained. For 10.7% of the time, there were sustained periods of 30 s or greater where the readings suggested a different depth of anesthesia. For 6% of the time, there were sustained periods of 30 s or greater where the readings differed by 10 or more arbitrary BIS units. The regression coefficient (R ²) for the two devices was 0.65 (range, 0.35-0.92). There was zero bias between the devices, and the 95% limits of agreement ranged between -18 and +17. Conclusion: A conflicting anesthetic management was suggested by the simultaneous BIS readings 10.7% of the time. These results suggest that BISxp® does not always provide a reproducible single number. Anesthesia providers should not rely exclusively on the BIS reading when assessing depth of anesthesia.