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visual data along with simultaneous somatosensory data or interspersed with auditory data. If the visual data are obtained in conjunction with somatosensory data, the visual stimulus is delayed to occur 98 msec after the somatosensory data, and at least three channels of data are collected: one for the BSEPs, the second for the MSPs, and the third for the VEPs.
TRANSCRANIAL DOPPLER
TCD is, in a sense, the earliest Warning indicator of difficulty, because a change in environment usually occurs before any ischemic insult can occur. The ability of Doppler technology to provide a noninvasive measure of blood flow velocity and the recent interest in emboli detection have led to increasing interest in its potential perioperative application, and the following sections discuss the application in various clinical situations. The Doppler effect and its use in medicine have been well described, and are included in the appendix.
Carotid Surgery: It has been shown that intraoperative drops in middle cerebral artery (MCA) flow velocity, with carotid clamping to 60% or less, is associated with EEC flattening, and that drops in MCA flow velocity also correlate with changes in cerebral oximetry and SjO2. Thus, TCD is now being used in carotid surgery.
There is evidence that TCD detection of emboli can play an important role in the intraoperative surgical management of these patients. The presence of >10 microemboli during dissection has been found to have a statistically significant relationship with perioperative cerebral complications. Although no prospective studies have been done, the suggestion is that if surgical technique is modified in response to these emboli, then perhaps the perioperative stroke rate can be reduced. The significance of emboli occurring during shunting is less certain; some authors suggest that they are associated with perioperative stroke, whereas others have not been able to establish such a relationship.
TCD may also be of benefit in the early postoperative period. Large increases in MCA flow velocity and pulsatility index ipsilateral to the surgery are predictive of intracerebral hemorrhage and are more accurate indicators than the presence of headache or hypertension. The persistence of microemboli after declamping has also been found to be related to the development of carotid artery thrombosis.
Cardiopulmonary Bypass (CPB): As a clinical tool, TCD has yet to find its niche in cardiac surgery. There has been great interest lately in the significance of intraoperative emboli detected by TCD, as well as their potential use as a monitor of the adequacy of cerebral perfusion during CPB.
Emboli during cardiopulmonary bypass have been observed by a number of authors. One of the earliest reports is by Padayachee, comparing the incidence of microemboli with the use of a membrane versus a bubble oxygenator. They demonstrated that the use of a membrane oxygenator was associated with a marked decrease in the incidence of microemboli during CPB. Increasing gas (Continued on page 130)
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