Bispectral analysis of the electroencephalogram correlates with patient movement to skin incision during propofol/nitrous oxide anesthesia

Abstract

Background: Bispectral analysis is a signal-processing technique that determines the harmonic and phase relations among the various frequencies in the electroencephalogram. Our purpose was to compare the accuracy of a bispectral descriptor, the bispectral index, with that of three power spectral variables (95% spectral edge, median frequency, and relative δ power) in predicting patient movement in response to skin incision during propofol-nitrous oxide anesthesia. Methods: Forty-four adult patients scheduled for elective noncranial surgery were studied. Gold cup electroencephalographic electrodes were placed on each patient in a frontoparietal montage (F(p1), F(p2), P3, and P4) referred to C(z), and the electroencephalogram was recorded continuously and processed off-line. Conventional frequency bands were used to describe power spectrum variables. Anesthesia was induced with propofol (1.5-3.0 mg-1 · kg-1 and maintained with 60% nitrous oxide in oxygen and with propofol at one of three randomized infusion rates (100, 200, or 300 μg · kg-1 · min-1). Inadequate anesthetic depth was defined as patient movement in response to a 2-cm skin incision at the planned site of surgery. Plasma propofol concentrations were measured within 2 min after skin incision. Results: Complete data were available for 38 patients, of whom 17 moved in response to skin incision. Analysis of the area under the receiver operating characteristic curves showed that only for bispectral index and drug dose group was there a significant predictive relation (area > 0.5). Furthermore, the bispectrum was significantly predictive even after stratification by dose group. Conclusions: The bispectral index of the electroencephalogram is a more accurate predictor of patient movement in response to skin incision during propofol-nitrous oxide anesthesia than are standard power spectrum parameters or plasma propofol concentrations.