Hypotension

In the normal, awake patient, significant levels of hypotension seem to be needed to cause the earliest of central nervous system (CNS) signs, as measured by discrimination tests such as the flicker-fusion test. This test examines the flicker rate at which the observer perceives the light to be continuous. In the early days of deliberate hypotension, this test was part of the preoperative evaluation to judge how far the pressure could be reduced for surgery. Clear signs of confusion and an inability to concentrate or respond properly to simple commands generally represent very low levels of cerebral perfusion when caused by hypotension because the normal cerebral circulation has a large capacity to vasodilate and maintain normal flow in the face of significant hypotension. The electroencephalographic changes associated with even this level of hypotension are not dramatic, although they are clear by comparison with a previously active recording. Herein lies the problem with using intraoperative EEG to determine whether a given level of hypotension has resulted in brain ischemia. Electroencephalographic changes are not very pronounced and are bilateral. These changes are also nearly identical to those caused by increasing doses of many anesthetic drugs. Electroencephalographic changes associated with hypotension can be detected, but when the hypotension is induced slowly and associated with changes in anesthetic drugs (e.g., use of isoflurane to reduce blood pressure), the changes are very difficult to interpret. Electroencephalographic changes associated with acute, severe hypotension, such as may be caused by sudden arrhythmias, are easier to read. A tracing of the effect of hypotension on the compressed spectral array is shown in Figure 38-8 . ^[53]

Many patients undergoing surgery do not have a normal cerebral circulation. In these individuals, even mild hypotension may result in significant cerebral ischemia, and monitoring the EEG during planned hypotension may be very helpful, provided other causes of similar electroencephalographic changes can be carefully controlled. Little literature supports the use of electroencephalographic monitoring during hypotension, but in our opinion, when the EEG is being monitored (e.g., during carotid surgery), the changes caused by hypotension do represent cerebral ischemia of a significant degree and should be considered an important finding.