Relationship between hemodynamic and electroencephalographic changes during general anesthesia

Anesthesiologists routinely use clinical signs to make decisions regarding the amount of anesthetic medication to administer during an operation. Difficulty in judging depth of anesthesia and concerns about awareness during 'balanced anesthesia' have renewed interest in cerebral function monitors. The availability of a simple, reliable, and sensitive monitor for determining the 'depth of anesthesia' would be extremely valuable in assessing the anesthetic and analgesic needs of the paralyzed patient during surgery. While anesthesiologists rely on autonomic responses (e.g., blood pressure, heart rate, diaphoresis) to decide whether or not a paralyzed patient is adequately anesthetized, the relationship between these responses and cerebral acitivity has not been well-characterized during an operation. The availability of EEG monitors that process the electrical activity of the cerebral cortex have simplified the recognition and interpretation of EEG changes during anesthesia. In a recent study, Rampil and Matteo reported that the electroencephalogram (EEG) was a sensitive and convenient measure of the depth of anesthesia during induction with thiopental. These investigators were able to demonstrate a correlation between the EEG spectral edge frequency (SEF) and the hemodynamic responses to laryngoscopy. Others have suggested that the EEG reflects the depth of anesthesia when a barbiturate or high-dose narcotic technique is used. In this report, the relationship between changes in hemodynamic variables, anesthetic drug concentrations and compressed EEG activity, using the Life-scan(TM), an acceptable alternative to the conventional 16-lead EEG for detection of cerebral ischemia during carotid endarterectomy, was examined in six patients anesthetized with propofol and nitrous oxide. Since previous studies have demonstrated a good correlation between blood propofol levels and emergence times from anesthesia when propofol was administered for induction and maintenance of general anesthesia, we evaluated the relationship between EEG changes and the rate of emergence from anesthesia.