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maintained with the anesthesiologists regarding any changes in blood pressure, temperature, heart rate, or muscle tone, since changes in any of these variables may alter the responses.
The N20 and P30 waves are sensitive to certain types of anesthetics. Barbiturates and inhalation anesthetics (e.g., isoflurane, ethrane, and halothane) will markedly decrease the amplitude of the cortically generated activity, including N20, in a dose-dependent, but individualized, manner. Of the inhalation anesthetics, isoflurane produces the weakest effects on cortical activity. Thus in those cases where a balanced narcotic technique may not be used, due to the need to measure EMG activity, isoflurane is used as the anesthetic. The balanced anesthetic (a combination of nitrous oxide, a muscle relaxant, and minor tranquilizer) is the much preferred method for recording SSEPs by themselves. In cases in which isoflurane is used to supplement another form of anesthesia, a concentration of under 0.4% should produce minimal effects on the cortical somatosensory activity in adults; however, these effects are very individualized and even low levels of inhalation agents may reduce the amplitudes of cortically generated activity in some patients. The somatosensory short latency potentials behave similarly to those from the auditory system and are unaffected by most anesthetic manipulation. Temperature changes significantly influence the SSEP latency. For each degree Celsius of local cooling, the nerve conduction velocity decreases by about 2.5 m/sec. During long operations, of this type, a drop in temperature around the nerve being stimulated can result in a progressive in crease in N20 latency, unrelated to surgical intervention Also latencies may be transiently affected when the surgeon irrigates with physiological solution at cooler temperatures; thus it is recommended that warm saline be used for irrigation.
For intraoperative monitoring of the function of neural systems to be successful, a suitable anesthesia regimen must be used. Therefore, the anesthesia team must be made aware of specific requirements related to neurophysiologic monitoring in individual cases. For example, when motor nerves are monitored by recording EMG potentials, the anesthesia regimen must exclude muscle relaxants; and the cortical components of, for example, somatosensory evoked potentials (SSEPs) are greatly affected by inhalation anesthesia. Potential problems with anesthesia, stem. Brain stem manipulations are traditionally detected through their effects on cardiovascular function, but we have found it of value to use BSAERs as well to monitor brain stem manipulations during operations to remove acoustic tumors.
Visual Evoked Potentials
Despite the pressing need for intraoperative monitoring of visual pathways, particularly during surgery at the anterior cranial base, visual evoked potential (VEP) monitoring has failed to gain wide intraoperative use. Intraoperative use is hindered by many (Continued on page 128)
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