Computerized monitoring of the EMG and EEG during anesthesia : an evaluation of the anesthesia and brain activity monitor (ABM®)

An intraoperative evaluation was made of the electroencephalographic (EEG) and electromyographic (EMG) functions of the Anesthesia and Brain activity Monitor (ABM). This device derives both these measures from a single electrode pair that is typically placed on the mid-forehead and mastoid process. The evaluation consisted of 1) quantifying the zero-crossing frequency (ZXF) of the EEG and mean integrated voltage of both measures (MIVEEG and MIVEMG) that occurred during induction and emergence from general anesthesia in 17 patients and 2) case reports sampled from an additional 41 patients. Alone or combined, variations in these parameters did not consistently accompany changes in the depth or adequacy of anesthesia as determined by standard clinical signs (e.g. heart rate, blood pressure, movement). Interpatient variability in the EEG measures during recovery from anesthesia was so large that neither the absolute value of ZXF nor that of MIVEEG could discriminate between moderate (i.e., maintenance) and light (i.e., emergence) anesthesia. Although MIVEMG uniformly decreased in anesthetized, unparalyzed patients (compared to the pre-operative awake state), noticeable increases during recovery often did not occur until limb movement was observed. Additionally, the common use of neuromuscular blockers made interpretation of low MIVEMG values quite difficult during anesthesia maintenance. However, selected individual case reports illustrated the potential benefit of routine intraoperative, microprocessor-based EEG/EMG monitoring. The single channel EEG/EMG display of the ABM seems sufficient to warn the anesthesiologist of pathologic decreases in cerebral electrical activity. Marked depression of cerebral function is associated with accidental anesthetic overdose, hypoxia or global ischemia. Additionally, the device should be useful for monitoring burst-suppression or isoelectric EEG patterns intentionally produced during barbiturate or isoflurane coma for cerebral protection.