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GENERAL ARTICLES

Middle Latency Auditory Evoked Responses and Electroencephalographic Derived Variables Do Not Predict Movement to Noxious Stimulation During 1 Minimum Alveolar Anesthetic Concentration Isoflurane/Nitrous Oxide Anesthesia

Eberhard Kochs, MD, Cor J. Kalkman, MD, Christine Thornton, PhD, Douglas Newton, FRCA, Petra Bischoff, MD, Hermann Kuppe, MD, Jochen Abke, MSE, Ewald Konecny, PhD, Werner Nahm, PhD, and Gudrun Stockmanns, MSE

*Department of Anesthesiology, Technical University, Munich, Germany; Department of Anesthesiology, Academic Hospital University of Amsterdam, The Netherlands; Northwick Park Hospital, St. Mary's Medical School, London, United Kingdom; §Eppendorf Hospital, Hamburg, Germany; ||Academic Hospital and Departments of ¶Medical Technology and **Informatics, Technical University Duisburg, Germany

Address correspondence and reprint requests to Cor J. Kalkman, MD, Department of Anesthesiology H1-116, Academic Hospital University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. Address e-mail to c.j.kalkman{at}amc.uva.nl

The electroencephalogram (EEG) and middle latency auditory evoked responses (MLAER) have been proposed for assessment of the depth of anesthesia. However, a reliable monitor of the adequacy of anesthesia has not yet been defined. In a multicenter study, we tested whether changes in the EEG and MLAER after a tetanic stimulus applied to the wrist could be used to predict subsequent movement in response to skin incision in patients anesthetized with 1 minimum alveolar anesthetic concentration (MAC) isoflurane in N₂O. We also investigated whether the absolute values of any of these variables before skin incision was able to predict subsequent movement. After the induction of anesthesia with propofol and facilitation of tracheal intubation with succinylcholine, 82 patients received 1 MAC isoflurane (0.6%) in N₂O 50% without an opioid or muscle relaxant. Spontaneous EEG and MLAER to auditory click-stimulation were recorded from a single frontoparietal electrode pair. MLAER were severely depressed at 1 MAC isoflurane. At least 20 min before skin incision, a 5-s tetanic stimulus was applied at the wrist, and the changes in EEG and MLAER were recorded. EEG and MLAER values were evaluated before and after skin incision for patients who did not move in response to tetanic stimulation. Twenty patients (24%) moved after tetanic stimulation. The changes in the EEG or MLAER variables were unable to predict which patients would move in response to skin incision. Preincision values were not different between patients who did and did not move in response to skin incision for any of the variables. MLAER amplitude increased after skin incision. We conclude that it is unlikely that linear EEG measures or MLAER variables can be of practical use in titrating isoflurane anesthesia to prevent movement in response to noxious stimulation.

Implications: Reliable estimation of anesthetic adequacy remains a challenge. Changes in spontaneous or auditory evoked brain activity after a brief electrical stimulus at the wrist could not be used to predict whether anesthetized patients would subsequently move at the time of surgical incision.

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999