ELECTROENCEPHALOGRAPHIC MONITORING FOR SURGICAL PROCEDURES

Although the EEG has only recently achieved widespread use as a measure of the depth of anesthesia, its use as a monitor of the adequacy of CBF, especially during carotid endarterectomy, has been established for many years. Because anesthetic drugs do affect the EEG (see Table 38-3 ) and many of these electroencephalographic changes may mimic those associated with inadequate CBF, some guidelines about anesthetic management during electroencephalographic monitoring for cerebral ischemia may be helpful ( Table 38-4 ).

Cardiopulmonary Bypass

In humans, changes that occur with the institution of CPB may alter the EEG by multiple different mechanisms (see Chapter 50 and Chapter 51 ). For example, plasma and brain levels of anesthetic drugs may be altered by CPB or by anesthetic drugs commonly given at the institution of bypass; alterations in arterial carbon dioxide tension and blood pressure may occur; and hemodilution with

TABLE 38-4 -- Anesthetic guidelines for intraoperative monitoring for ischemia

No changes in anesthetic technique should be made during critical periods of monitoring (e.g., induced hypotension, carotid clamping, aneurysm clipping).

Avoid major changes in anesthetic gas levels or boluses of opiates, barbiturates, or benzodiazepines near times of increased ischemic risk.

If drugs must be given that create extreme electroencephalographic slowing or an isoelectric pattern (e.g., barbiturates), it is sometimes possible to monitor somatosensory evoked potentials, which may remain relatively unaffected.

hypothermic perfusate usually occurs. These effects, all of which may produce electroencephalographic changes similar to the pathologic changes seen with ischemia, make it difficult to interpret the changes occurring around the time of institution of CPB.

Levy and colleagues^{[42] [43]} tried to differentiate the normal effects of hypothermia from other events occurring at the institution of and conclusion of CPB. Initially, they concluded that only a qualitative relationship could be determined, but later, with the use of a much more sophisticated analysis technique (i.e., approximate entropy), electroencephalographic changes associated with changes in temperature could be quantified.

Gugino and colleagues^[44] in Boston and Edmonds and colleagues^[45] in Louisville attempted to use quantitative (i.e., processed, multiple channel) EEG during CPB to detect cerebral hypoperfusion and relate these changes to postoperative neurologic function. Some minimal work has been done with intervention after detection of cerebral hypoperfusion using quantitative EEG. Such monitoring techniques have not been widely adopted, because although the data appear promising, only a small number of patients have been studied, and there are very few corroborating studies. This type of monitoring is extremely costly in time, personnel, and equipment, and given the lack of convincing outcome data, the cost-benefit ratio is unclear at best. Other investigators have failed to demonstrate any convincing relationship between intraoperative electroencephalographic parameters and postoperative neurologic function, especially in small infants and children.^[46] Much work remains to be done studying the use of the processed, quantitative EEG to provide information for clinical management of patients during CPB. None of the currently available studies and recommendations would stand up to a close examination using evidence-based medicine.