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example, changes in heart rate related to both brain stem and vagal stimulation. However, the comparative ability to evaluate the nervous system by either clinical means or by the commonly used physiological monitoring tools available to the anesthesiologists is limited during anesthesia.
Depending on the surgical procedure, we routinely measure electrical activity dependent on the functioning of the brain stem (brain stem auditory evoked potentials and brain stem somatosensory evoked potentials), the cortex (the electroencephalogram, somatosensory evoked potentials, and visual evoked potentials), and cranial nerves II, III, IV, V, VI, VII, VIII, X, XI, and XII. It is imperative that the measures utilized are both specific to the neural tissue being manipulated and sensitive to changes in the functioning of the neural tissue produced by the surgical manipulations. Our conceptual approach requires that the neurophysiologist understand the nature of the patient's pathology, the operative strategy of the surgeon, and the anesthesiologist's approach to the management of that particular patient. It requires that the surgeon understand the level of information that the neurophysiologist can provide as the operative procedure is evolving, and it requires that the neuroanesthesiologist understand the effects of the pharmacological manipulations on the monitoring tools available to the neurophysiologist. Thus the keystone of the Skull Based Surgeons approach to intraoperative monitoring is the close and continuous interchange of information between all the members of the surgical team.
EEG
The neurophysiological measures routinely used in this center provide a functional map of much of the entire neuraxis. These include: the electroencephalogram (EEG), an unstimulated measure of cortical function suitable for providing information concerning the degree of cortical activation related to either metabolic process (e.g., hypoxia) or to pharmacological manipulation (e.g., pentobarbital-induced burst suppression to protect the patient's cortical function); the somatosensory and visual cortical potentials (SSEPs and VEPs), which provide additional measures of cortical function specific to certain pathways and vasculature; the auditory and somatosensory brain stem potentials (BAEPs and BSEPs), which provide information about the functioning of the brain stem again specific to certain pathways; and finally, EMGs produced by muscles innervated by the various cranial nerves, which provide information about both the cranial nerves themselves and their underlying brain stem nuclei. The key to effective CNS monitoring is to detect changes that are potentially damaging during a time period when intervention can alter the course of the ischemic insult. Besides preventing the insult itself, the most opportune time for intervention ranges from the event to the occurrence of cellular (Continued on page 103)
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