The Effects of Volatile Anesthetics on the Relative Amplitudes and Latencies of Spinal and Muscle Potentials Evoked by Transcranial Magnetic Stimulation

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Abstract
The effects of halothane, enflurane, or isoflurane on motor-evoked potentials via transcranial magnetic stimulation were investigated in cats. Time and dose effects on muscle potentials and spinal potentials were determined by measuring relative changes in amplitudes and latencies. In 16 cats, muscle potentials and spinal potentials were evoked transcranially using magnetic stimulation. Potentials were recorded every 2 minutes for 10 minutes at end-tidal anesthetic concentrations of 0.25%, 0.5%, 0.75%, or 1.0%, and for 10 minutes after agents were removed. These anesthetics significantly attenuated the amplitude, but not the latency of muscle potentials. Effects were reversible and time and dose dependent. In contrast, these agents had little or no effect on amplitudes or latencies of spinal potentials. Monitoring intraoperative changes in spinal potentials may provide useful information regarding motor pathway assessment, because anesthetics have minimal effects on spinal potentials, whereas this is not so for muscle potentials.

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