THE EFFECTS OF VOLATILE ANESTHETICS ON THE RELATIVE AMPLITUDES AND LATENCIES OF SPINAL AND MUSCLE POTENTIALS-EVOKED BY TRANSCRANIAL MAGNETIC STIMULATION

Study Design. The effects of halothane, enflurane, or isoflurane on mo tor-evoked potentials via transcranial magnetic stimulation were inves tigated in cats. Time and dose effects on muscle potentials and spinal potentials were determined by measuring relative changes in amplitude s and latencies. Methods. In 16 cats, muscle potentials and spinal pot entials were evoked transcranially using magnetic stimulation, Potenti als were recorded every 2 minutes for 10 minutes at end-tidal anesthet ic concentrations of 0.25%, 0.5%, 0.75%, or 1.0%, and for 10 minutes a fter agents were removed. Results. These anesthetics significantly att enuated the amplitude, but not the latency of muscle potentials. Effec ts were reversible and time and dose dependent. In contrast, these age nts had little or no effect on amplitudes or latencies of spinal poten tials. Conclusions. Monitoring intraoperative changes in spinal potent ials may provide useful information regarding motor pathway assessment , because anesthetics have minimal effects on spinal potentials, where as this is not so for muscle potentials.