Simultaneously extracted transient and steady-state evoked responses during general anesthesia: Variability of different rates

Unintended intraoperative awareness occurs in one to two individuals out of every one thousand treated with general anesthesia. Patients that experience intraoperative awareness have significant post-operative psychological sequelae. The ability to detect intraoperative awareness is currently suboptimal because the mechanism employed by anesthetic drugs to impair consciousness remains poorly understood. Studies have suggested that evoked potentials (EP) may be used to monitor the depth of anesthesia. Both transient and steady state responses can be simultaneously extracted using the Continuous Loop Averaging Deconvolution (CLAD) method with specially designed CLAD sequences. 20 Hz and 30 Hz jittered CLAD sequences in addition to 5 Hz isochronic and 40 Hz jittered CLAD sequences were applied in baseline awake and general anesthesia conditions. A qualitative method to assess the extracted EPs was developed in this study, termed Randomized Split Set Average (RSSA). The results showed that EPs extracted during general anesthesia require a greater number of sweeps to obtain a signal-to-noise ratio comparable to that observed in EPs extracted during the awake state. Therefore, the development of a real time or quasi real time EP monitoring system for anesthesia provides an increased challenge. The RSSA employed in this study is a useful method for assessing the signal quality of EP responses.