Anesthesia for Neurosurgery - Electrophysiologic Monitoring
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Electroencephalography (EEG) measures electrical activity of the neurons of the cerebral cortex and is thus used as a threshold marker for detecting ischemia due to inadequate CBF. It is used frequently during procedures that jeopardize cerebral perfusion, such as carotid endarterectomy, or to ensure electrical silence before circulatory arrest.
- Normal CBF in gray and white matter averages 50 mL/100 g/min. With most anesthetic techniques, the EEG starts to become abnormal when the CBF decreases to 20 mL/100 g/min. Isoflurane is distinct as the EEG becomes abnormal when CBF is much lower at 8 to 10 mL/100 g/min. Cellular survival is endangered when CBF decreases to 12 mL/100 g/min (lower with isoflurane). Thus, EEG changes can warn of ischemia before CBF becomes insufficient to maintain tissue viability. Prompt detection of EEG changes may be treated with increases in perfusion pressure or shunting to restore CBF to prevent infarction.
- The EEG may exhibit changes intraoperatively with no demonstrable neurologic deficit during postoperative examination. Cerebral ischemia can produce electrical dysfunction without causing neuronal cell damage because the blood flow threshold for electrical failure is higher than that needed to maintain cellular integrity.
- Factors other than anesthetics that may affect the EEG include hypothermia (which may limit the usefulness of EEG during cardiopulmonary bypass), hypotension, hypoglycemia, hypoxia, tumors, vascular abnormalities, and epilepsy. An abnormal EEG in patients with preexisting neurologic deficits, strokes in evolution, and recent reversible ischemic neurologic deficits can also make it difficult to interpret new changes.
- Anesthetic effects on the EEG are generally global, which often helps distinguish them from the focal changes of ischemia. A predominance of slow activity is seen as the anesthetic depth increases. “Deep” anesthesia may cause marked EEG slowing, making detection of superimposed ischemic changes during critical periods difficult to interpret. Maintaining a constant level of anesthesia during critical periods (e.g., carotid clamping) facilitates EEG interpretation.