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Depressive effect of isoflurane on motor evoked potentials in the Nubian goat

H. Andel, MD^*, G.S. Bayer, MD, R. Ciovica, MD, J. Monsivais, MD, M. Basco, M. Zimpfer, MD^*, and E. Turkof, MD

From the Departments of Plastic and Reconstructive Surgery and Ludwig Boltzmann Institute for Experimental Plastic Surgery,
^* Anesthesiology and Intensive Care,
Ludwig Boltzmann Institute for Clinical Anesthesiology and Intensive Care,
University of Vienna, Austria, Europe, and the Hand and Microsurgical Center and University of Texas at El Paso, El-Paso, Texas, USA.

Address correspondence to: Univ. Prof. Dr. Edvin Turkof, Abteilung für Plastische und Rekonstruktive Chirurgie, Universitätsklinik für Chirurgie, Allgemeines Krankenhaus der Stadt Wien, Währinger Gürtel 18 - 20, 1090 Wien, Austria, Europe. Phone: 43-1-40400/5620; Fax: 43-1-40400/6988; E-mail: edvin-turkof{at}via.at

Purpose: To determine the effect of isoflurane on motor evoked potentials (MEP) in a new animal model designed to verify the applicability of MEPs in brachial plexus surgery, and to compare the results with previous reports in other animals.

Methods: In seven goats, anesthesia was induced with 3 mgkg^–1 ketamine iv and maintained with nitrous oxide 40% in oxygen and 2 µgkg^–1hr^–1 fentanyl iv. The MEP were performed with two subcutaneous needle electrodes placed over the occiput (cathode) and the nasion (anode), with their plugs connected to the power output of a Digitimer D 180 electrical stimulator, connected to the trigger input of an electromyograph (model 8400, Cadwell Laboratories, Inc., Kennwick, Washington). Activation of the Digitimer caused central stimulation of the motor cortex, evoking baseline compound muscle action potentials (CMAPs) which were recorded from the left triceps muscle. Subsequently, isoflurane 2% was administered together with repeated central stimulation at 30 sec intervals.

Results: Onset of I- (indirect) waves increased from median 15,8 msec to median 26,8 msec P = 0,018 (latency increase ranged from: 9 to 11.5 msec), while peak-to-peak amplitudes decreased and subsequently disappeared. D- (direct) waves showed no latency increase, and finally disappeared as well. After disappearance of CMAPs, isoflurane administration was stopped and MEP repeated. The CMAPs reappeared (range: 210-360 sec) and regained initial peak-to-peak amplitudes and latencies.

Conclusion: These animal studies suggest that isoflurane should not be used during the recording of MEPs.