CEA requires temporary clamping of the carotid artery being worked on rendering the ipsilateral hemisphere dependent on collateral flow from the vertebral arteries and the contralateral carotid artery through the Circle of Willis. Neurologic monitoring is used to verify adequate perfusion of bilateral regions of the brain and to guide decision making in regards to shunting, BP control, and surgical technique. Also competing needs for increased BP vs. reducing myocardial workload, neurologic monitoring allows for aiming for lowest BP to maintain perfusion while reducing myocardial workload. Monitoring options include an awake patient under local anesthesia, EEG, SSEPs, and less often transcranial doppler(TCD), cerebral oximetry and stump pressures with reliability in that order.
Advantages of an awake patient: The most effective in detecting ischemic episodes, less post-op hypertension when done under field block, easy post-op neurologic exam.
Disadvantages of an awake patient: Requires very cooperative patient. Patient may panic, while draped in sterile field if he/she becomes aphasic or hemiplegic intraoperatively, and could require immediate GA and a secured AW. Anxious patients will have increased sympathetic response increasing risk for myocardial ischemia in patients already prone to cardiac events. And not all surgeons can work quickly enough to make a field block practical or tolerable for older arthritic patients.
EEG records spontaneous electrical activity of cortical surface cells, an area more prone to decreased perfusion. It is a sensitive parameter for ischemia since electrophysiologic activity accounts for 60% of cerebral metabolic demand. EEG changes occur in about 20% of patients during carotid occlusion and are indicative of potentially serious ischemia. Changes lasting more than 10 minutes correlate strongly with post-op neurologic deficits, and thus EEG changes of greater than a mild degree are an indication for shunt placement or induced hypertension. Typical regional cerebral blood flow is 50-55ml/min/100gm brain tissue. Ischemia typically occurs around 18-20 ml/min/100gm and tissue death at 8-10. EEG deterioration begins around 15-20ml/min/100gm, and manifests as frequency slowing or amplitude attenuation, severe ischemia may be isoelectric.
Limitations are that deep structures are not monitored, preexisting deficits or EEG changes reduce predictive value (may not show intraop changes), may miss regional ischemic events, especially if using only 4-channel, and are affected by changes in temperature, BP, PaCO2, and anesthetic depth, however, these are more likely to be b/l. Focal embolic events may also be missed. 16 lead EEG is the gold standard- responds quickly and detects regional changes, but requires a skilled technician and continuous observation, thus processed EEGs with fewer leads, 2-4 channels, are available and widely used. Need electrodes covering bilateral anterior and posterior regions of brain.
SSEPs are based on detection of cortical potentials after electrical stimuli are presented to a peripheral nerve.
Advantages: also evaluates deep brain structures vs. EEG and cortical function only, and may be better for patients with previous CVA and EEG changes.Disadvantages: Not felt to be as sensitive or specific for ischemic injury during CEA. Requires considerable expertise. Also effected by choice of anesthesia and need constant light plane to be maintained to accurately interpret changes in EPs.
Transcranial doppler is not a good sole intraoperative monitor. Measures mean blood flow velocity in MCA and detects emboli. Emboli account for up to 65% of postop deficits. Can detect acute thrombotic occlusion and microemboli and is much more useful in this aspect especially in helping surgeons modify their technique. Does not evaluate functional changes. Also useful for predicting postop hyperperfusion syndrome and help in reducing BP to avoid complications.
Carotid stump pressure estimates hemispheric blood flow by measuring pressure in the carotid stump distal to the clamp. Stump pressure is more often used to determine whether or not a shunt should be placed intraoperatively. The problem with this is that an adquate pressure doesn’t assure perfusion to all regions of the brain. Shunt thresholds vary between surgeons, anesthesiologists, and institutions but a threshold between 40 and 60 mmHg is typical. Nevertheless, in some patients this may not be adequate for compromised areas and in others perfusion is adequate at pressures well below this resulting in unnecessary shunting. On a scientific basis there is no correlation between stump pressure and regional or global blood flow.
None of these have been shown to improve outcome since postoperative emboli and not intraoperative hypoperfusion are most likely cause of periop stroke, but do aid in decision to shunt and BP maintenance.