significant ischemia of the brain related to various surgical manipulations. Numerous studies have demonstrated that synaptic transmission is abolished when cerebral blood flow (CBF) decreases below 15 ml/100 g/min, and the ability of the neuron to maintain its membrane potential fails when flow drops below 10 ml/ 100 g/min. At this level, permanent neurological damage may ensue within minutes if blood flow remains reduced. Unilateral hemispheric ischemia will abrogate EEG activity within about 20 seconds. However, permanent neuronal damage, which can be detected by potassium efflux, does not begin to occur until 5 minutes post ischemia. Surgical trials in humans have demonstrated that major EEG changes occur with drops in CBF below 10 ml/100 g/min, while more minor changes occur with flows of 10-18 ml/100 g/min. The EEG pattern generally remains stable at flows of 25 ml/100 g/min or greater. Zampella et al. demonstrated that only 5% of patients will have demonstrable EEG changes at flows of 20 ml/100 g/min, while 31% of patients showed these changes at flows of less than 13 ml/100 g/min [1]. The crucial role of EEG lies in identifying the "ischemic penumbra", which is the pathophysiological state of acute ischemia in which neurons are nonfunctional but still alive and salvageable by reperfusion. As EEG is capable of detecting this state of cerebral ischemia prior to the development of permanent damage, it can be an extremely valuable technique in monitoring procedures that may result in reduced blood flow. In general, EEG has been shown to be more sensitive and to show more rapid changes than the recording of somatosensory-evoked potentials for alerting the surgeon to potentially harmful manipulations, although the rate of false-positives is higher.

Importantly, EEG is particularly sensitive to anesthesia. Most anesthetic agents, including the halogenated gases, thiopental, midazolam, etomidate and propofol, cause similar EEG changes. At doses below the minimum alveolar concentration (MAC), widespread, frontally predominant, fast rhythms appear. Increasing doses are generally characterized by a disappearance of these alpha rhythms with concurrent appearance of a beta rhythm, followed by a progressive slowing towards theta and delta rhythms. Deep anesthesia is associated with a burst-suppression pattern, and at the deepest levels measurable potentials may disappear altogether. Although the effects of different anesthetics differ slightly at lower doses in terms of the particular patterns seen, slowing with burst-suppression is the common feature of all of these drugs. Anesthetic-induced EEG changes should be seen bilaterally and symmetrically over both cerebral cortices.

Intraoperative use of EEG during neuro-surgery has primarily been used in the monitoring of ischemic changes associated with carotid endarterectomy (CEA). Recordings are typically obtained pre-operatively, at induction, intermittently during dissection, and continuously during cross-clamp occlusion of the internal carotid artery (ICA). Indications of significant ischemia may potentially mandate use of a shunt during the period of cross-clamping. Considering that the use of a shunt increases operative risk, related to either the potential for embolism or prolongation of operative time, the surgeon is required to balance these concerns with the benefits of shunting. A retrospective study by Salvian et al. compared a large cohort of patients who had undergone CEA either with routine shunting (n = 92) or with selective shunting using EEG changes as the indicator (n = 213). Of the selectively shunted group, only 16% had EEG changes that led to shunting. Postoperatively, 4 of the 92 patients who were routinely shunted had major stroke. In contrast, the selectively shunted group had only one case of post-operative stroke, suggesting that the use of EEG in determining the need for shunting may significantly reduce the risk of postoperative neurological deficit [2].

The two major EEG changes predictive of cerebral ischemia are: (1) slowing with decreased amplitude in the ischemic hemisphere, and (2) attenuation of the anesthetic-induced fast rhythms. EEG changes may be classified as major or moderate, with total or near-complete attenuation of 8-15 Hz activity and/or at least a doubling of delta activity of 1 Hz or less representing major changes. These alterations typically involve the ipsilateral hemisphere but can also be seen bilaterally or exclusively in the contralateral hemisphere. Moderate changes include amplitude attenuation of at least 50% or an increase in delta activity of 1 Hz or greater. When alterations of the recorded activity occur, whether major or moderate, they generally begin within minutes

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