Effects of anesthetics on somatosensory and motor evoked potentials in humans

Abstract

Somatosensory evoked potential (SEP) monitoring has been used to intraoperatively evaluate central nervous system (CNS) integrity during selected surgical procedures, such as carotid endoarterectomy, intracranial vascular surgery, thoracic aortic surgery, and spinal cord procedure (Fisher et al., 1995; Nuwer et al., 1995; Cunningham et al., 1987; Gugino and Chabot, 1990). Several intraoperative factors including hypothermia, cooling of the stimulated extremities, hypoxia, hypercapnia, hypotension, and anesthetic agents, can influence the outcome of intraoperative SEP monitoring (Gugino and Chabot, 1990). Although the influence of factors other than anesthetic agents can be largely eliminated by careful control of body temperature and cardio-respiratory status, alteration in SEPs by anesthetic agents is unavoidable in patients undergoing procedures that require general anesthesia. Therefore, effects of anesthetics on SEPs should be considered when interpreting changes in SEPs. Different anesthetics have disparate effects, and different components of evoked potentials have distinct sensitivities to a given anesthetic. In general, early components (subcortical SEPs) are less sensitive to anesthetics than are late components (cortical SEPs). This is probably because anesthetics affect synaptic transmission more than axonal conduction — early components are due more to axonal conduction, whereas late components depend primarily on multisynaptic transmission. For intraoperative cortical SEP monitoring, special attention is required to ensure that the agents and dose levels selected favorably preserve late waves.