Influence of nitrous oxide on motor-evoked potentials

Study Design. Rabbits were used as an experimental model in the study of motor-evoked potentials. Objectives. To evaluate the effect of nitrous oxide on motor-evoked potentials while monitoring direct muscle and spinal cord responses. Summary of Background Data. Motor-evoked potential monitoring provides a promising tool for intraoperative assessment of descending pathways function. However, to date, this technique is still al an experimental stage, since its routine use is mainly limited because of intraoperative recording difficulties caused by the influence of anesthesia. Methods. Eight male rabbits weighing between 3000 g and 3500 g were studied. Motor-evoked potentials were recorded from the extremity muscles and from the epidural space of the thoracic cord in response to electrical stimulation of the motor cortex at baseline conditions and at increasing nitrous oxide concentrations (10-70 vol%). Results. The authors found a major suppressive effect of high nitrous oxide concentrations on the electromyographic responses. With 50 vol% nitrous oxide, electromyographic amplitudes were suppressed to 46% (fore leg) and 14% (hind leg) of the baseline values, whereas latencies did not change significantly. In contrast to muscular activity, spinal evoked responses representing neural activity were not affected by any concentration of nitrous oxide. Conclusions. Intraoperative monitoring of descending pathways by means of motor-evoked potentials during anesthesia of the rabbits based on nitrous oxide is feasible when neural activity is evaluated. Higher doses of nitrous oxide, however, are not compatible with recording of muscular activity.