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after restoration of blood flow, hyperemia begins. This hyperemic blood flow allows delivery of deleterious materials, such as leukocytes producing toxic mediators in the ischemic tissue. The initial hyperemia is followed by subsequent sustained hypoperfusion. Intracellular pH frequently returns to normal, but this is associated with continued depression of ATP stores. The EEG, which initially returns to a normal state, subsequently becomes abnormal over the long-term basis. In the future it may be possible to actually monitor the energy store of cerebral tissue. Until more sensitive monitors are available, probably the best monitor will continue to be changes in the brain's electrical activity. Ischemic changes on the EEG are evident both in the analog and processed signals. Almost routinely, the analog EEG signal is analyzed using some form of power spectral analysis. Many changes occur in the EEG before isoelectricity occurs, and these changes seem related to the underlying energy state of the brain. With the advent of implantable defibrillators, a clinical model of global ischemia in humans has become available, and EEG changes occurring in this state have been fairly well described. The EEG changes within 8-16 sec of ischemia. EEG changes include slowing, loss of high-frequency activity, increased amplitude of low-frequency activity, and overall decreases in amplitude. However, other patterns do occur, including loss of beta activity or increase in theta activity. The same general patterns seem to occur with focal ischemia, but with localized extent and more variable timing.
The principals underlying uses of intraoperative neurophysiologic recordings to reduce neurologic complications are (1) that it is possible to detect, before they become permanent, changes in function of the systems being surgically manipulated by recording electrical potentials and (2) that reversal of the surgical manipulation can prevent the induced change from resulting in permanent neurologic deficits. One might assume that the results of intraoperative neurophysiologic monitoring would be used to warn the surgeon when manipulations have caused changes that could result in permanent deficits. Such monitoring can be of wider usefulness, however. For example, when any change in the recorded potentials greater than the normal variations is communicated to the surgeon, the surgeon can use that information to help carry out the operation so the risk of injury is kept at a minimum. This places considerably greater responsibility on the personnel who perform intraoperative monitoring compared with those who perform clinical testing for diagnostic purposes. The person responsible for interpreting intraoperative monitoring must be able to interpret the results immediately and must have enough experience to make judgments about the nature and implications of observed changes in sensory evoked potentials and electromyographic (EMG) potentials. To make such judgments, the neurophysiologic monitoring personnel must have basic knowledge about how neuroelectrical potentials are generated and must understand the physiology of the (Continued on page 105)
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