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Excerpt
The rationale for the use of electroencephalogram (EEG) recordings for neuromonitoring is that the electrical function of the brain is driven by cellular metabolism which in turn is critically dependent on cerebral blood flow (CBF) for supply with oxygen and substrate. The CBF threshold for alterations of the EEG is below 30 ml 100g−1 min−1 with electrical silence occurring in the range of 15-20 ml 100g−1 min−1. Assessment of brain function by EEG recordings can be used for guidance of pharmacological and surgical treatment to ameliorate CBF and brain metabolism. The brain has no substantial stores for oxygen or glucose and EEG deterioration will become immediately apparent at ischaemic CBF. However, effects of anaesthetics may interfere with EEG changes induced by decreases in cerebral blood flow or metabolism. When EEG recording is used in the operating room or in the intensive care unit a thorough knowledge of the influence of anaesthetics with respect to functional changes is required.
EEG monitoring may help to prevent or reduce neuronal injury and neurological deficit associated with cerebral ischaemia. In addition, EEG recordings may help to determine changes in brain function in comatose patients and for detection of epileptiform activity.
Intraoperative monitoring of the raw EEG signals has to be performed by an experienced neurophysiologist. For monitoring of brain function during anaesthesia or in sedated or comatose ICU patients the raw EEG has to be processed. Recently, computerized EEG systems have been recommended because of their ease of application, clarity of display, and reported ability to identify ischaemic EEG changes. However, the sensitivity and specificity of EEG parameters for detection of cerebral ischaemic episodes on the background of confounding anaesthetic effects and a variety of artefacts still has to be defined. EEG changes may not only occur during anaesthesia or ischaemia but also with hypothermia, hypo- and hypercapnia, changes in haematocrit, etc. When EEG measures are used for quantification of anaesthetic effects or detection of cerebral ischaemia the native EEG signal should be displayed as a control of signal quality, detection of confounding artefacts and transient signals (i.e. burst-suppression, spikes, etc.) which may not be represented in calculated EEG derivatives.
Applications of raw and processed EEG monitoring include: detection of cerebral insults; monitoring cerebral perfusion during temporary vessel occlusion; monitoring of adequacy of cerebral oxygenation during cardiopulmonary bypass; monitoring of depth of anaesthesia/sedation; titration of anaesthetic/sedative drugs for achievement of maximal metabolic suppression (burst-suppression EEG); detection of seizure activity in anaesthetized and comatose patients; detection of brain death. Other less-well-documented situations for EEG monitoring are: prognosis of outcome in head-trauma patients; potential detection of anxiety and pain in paralysed patients; hepatic or septic encephalopathy.
Several EEG parameters calculated from spectral analysis such as median power frequency, spectral edge frequency, etc. have been proposed to quantify pharmacodynamic drug effects. Recent studies suggest that bispectral EEG analysis may be used for calculation of parameters which correlate to depth of anaesthesia.
Cerebral ischaemia may occur during carotid artery occlusion or with insertion of a temporary shunt. For EEG monitoring during carotid endarterectomy, the electrode montage must represent brain areas supplied by the carotid artery. Because ischaemia-induced EEG alterations may not be different from anaesthetic-induced EEG changes, rapid changes in anaesthetic drug delivery must be avoided during these situations. Following the onset of an ischaemic event initial EEG changes may be observed within 1 min. With persisting cerebral ischaemia, brain electrical silence may occur. There is controversy if EEG monitoring for carotid endarterectomy is superior to monitoring of evoked responses. Several studies indicate that stroke can be reduced 10-fold using selective shunting in carotid endarterectomy based on EEG monitoring.
