Anesthesia providers have been using processed EEG parameters (brain monitors) to assess the depth of anesthesia for many years. The Bispectral Index (BIS®; Aspect Medical Systems, Inc.), one such brain monitor, provides a direct measure of the hypnotic state of the patient \[1, 2\]. These clinicians use the BIS value, a number ranging from 0 (very deep anesthesia) to 100 (awake state), to help optimize anesthetic dosing. Using BIS technology, anesthesia providers may adjust anesthesia doses to provide adequate sedation while avoiding over sedation, resulting in faster recovery \[3\] and a reduced incidence of awareness with recall \[4\].
While BIS technology helps anesthesia providers achieve desired levels of hypnosis, they currently rely primarily on monitoring hemodynamic (blood pressure and heart rate), autonomic (tearing, sweating), and somatic (moving) responses to noxious (painful) stimulation as a means to detect potential patient arousals. Additional analgesics (narcotics, NSAIDS) are often administered in order to suppress further response to noxious (painful) stimulation. Several studies have shown that noxious stimulation can also affect EEG signals, resulting in increased variability in the BIS index, suggesting that information in EEG signals could potentially help clinicians anticipate and detect patient response to noxious stimulation.
Ropcke et al. \[5\] showed that BIS values were higher with surgical stimulation than without any stimulation. Other reports have shown that focal noxious stimuli in volunteers and patients induce transient increases in BIS \[6-9\]. Many of these studies show that adding analgesics suppresses the BIS response to noxious stimulation, and the level of suppression achieved was related to the dose of the added drugs \[6, 8, 9\].
Based on these findings, it is expected that insufficient analgesia would likely result in transient increases in BIS due to ongoing surgical stimulation, increasing the overall variability of BIS. Recently reported findings confirm that overall variability of BIS increased prior to and following intraoperative somatic events \[10\]. These reports also identified similar increases in variability of the EMG, with the largest changes realized from a Composite Variability Index (CVI) which combined the BIS variability (sBIS) and EMG variability (sEMG) into a single value. Other studies have also shown an association between these variability measures and postoperative pain scores \[11, 12\]. These studies showed that sBIS, sEMG, and CVI computed over the entire surgical procedure were all higher in both adults and children with worse postoperative pain scores. However, the reliability and optimum method of displaying these variability scores has yet to be been determined.
The present study is designed to compare whether sBIS, sEMG, or CVI can be used to predict unwanted intraoperative somatic responses to stimulation, and to determine whether these values are related to patient-assessed postoperative pain scores. Our hypothesis is that increases in these variability measures are associated with increased probability of somatic responses. If our hypothesis is confirmed, these variability measures may help clinicians by highlighting periods of inadequate analgesia.
