Specific Problems in Interpretation of Absolute Values of Spectral Edge Frequency (SEF) in comparison to Bispectral Index (BIS) for Assessing Depth of Anesthesia

Abstract

The spectral edge frequency (SEF) is a milestone in the computer-assisted EEG analysis for assessing the depth of anesthesia. The SEF as well as the bispectral index (BIS) are numerical descriptors of the EEG. For both, SEF and BIS, limits have been reported within which recall and hemodynamic or motor responses to surgical stimuli are unlikely. We studied the SEF at four different states of anesthesia based on individual observations. Further, we compared the SEF and BIS values in relation to the proposed limits at different endtidal concentrations of sevoflurane (1.2 and 2.4 Vol%et) and desflurane (3.0 and 6.0 Vol%et). The SEF and BIS values were assessed according to the proposed limits for SEF (13.65 and 12 Hz) and BIS (60). We found a high interindividual variability and low intrapersonal stability of the SEF values in the awake state. These findings impair the discrimination of awake and sleep state. The mean SEF was outside the target range at both endtidal concentrations of sevoflurane and desflurane. In contrast, mean BIS values at all measuring phases during anesthesia met the goal of < 60. The increased concentrations of anesthetics are not reflected adequately in the phases of excitation and burst suppression activity. Summarizing, a suitable, statistically calculated approximate target limit of SEF as a tool to guide the depth of anesthesia does not seem to be available at the present time.