Carotid Endarterectomy

Indications for CEA

(NASCET & ECET trials)
• TIA & ipsilateral lesion >70%
• Failure of medical mix with >40% stenosis
• Established stroke- should be perceptible benefit
• Asymptomatic with bruit & >70% stenosis

Neurological Monitoring- Why?
• 1° goal - Identify patients that would benefit from placement of a shunt.
• 2° goal - Identify patients who may benefit from BP augmentation.
• N.B.- The concept of selective shunting not accepted by all surgeons.
• Most intra-operative strokes are thomboembolic and most peri-operative strokes are postoperative.

Neurologic Monitoring- Types

Monitors of function
• Awake patient
• EEG- Standard Vs processed.
• Somatosensory evoked potentials

Monitor of O2 supply & demand
• Jugular SvO2/ Cerebral Oximetry


Neurologic monitoring_ Types

Monitors of blood flow
• Internal carotid artery stump pressures
• Transcranial doppler
• Radioactive Xe 133

Awake patient
• At first glance the simplest and most non-invasive methods is to maintain direct contact with the patient.

• Requires some form of loco-regional anaesthesia, a spare nurse and the ability to communicate well with the patient.

• 2-3% conversion rate to GA
Stump pressures
• Represents back pressure from collateral flow through the circle of Willis.

• Advantage of being cheap, continuous and little expertise is required.

• Criticisms
– Inaccurate
– Critical stump pressure unknown (? <50mmHg)

Standard EEG
• 16-20 channel continuous in real-time.
• No false negatives in a series of >2000 patients @ Mayo Clinic.
• Gold standard of EEG analysis but requires an experienced electroencephalographer.
• Minimum of 2 channels per hemisphere provided territory of MCA included.
• 100% sensitivity and specificity.

Normal a wave EEG
Processed EEG- what is it?
• Resurgence because of increased power of microprocessors to do the algorithms.
• 2 types - Power or Bispectral analysis.
• Power analysis uses Fourier transformation to convert raw EEG signal into its component sine waves of an identifiable frequency and amplitude.

Processed EEG- what is it?
• These amplitudes and frequencies are plotted for against time and displayed as either a compressed or density spectral array CSA or DSA.
• Bispectral is a recent development finding a niche as an anaesthetic depth monitor.
Technique to generate Compressed spectral array
CSA during hypotension

Processed EEG- Why use it?
• Cross-clamp during CEA is a unilateral time specific event involving a large part of the hemisphere.
• These are the best circumstances for interpreting the processed EEG.
• Allows ‘black box’ approach to EEG interpretation by novices. 91-98% sensitivity and specificity

Processed EEG
• Criticisms
– Electronically remote from original.
– Less information.
– Bilateral Vs Unilateral. Not 100% reliable.
– Data acquisition period. Not 100% real-time.

Limitations of EEG generally
• EEG not ischemia specific.
• May not detect subcortical or small cortical infarcts.
• Patients with pre-existing, bilateral or contralateral cerebrovascular disease.
• False negatives
• False positives

Somatosensory evoked potentials
• Sensory cortex is primarily supplied by the MCA
• Repetitive electrical stimuli applied to a peripheral nerve and the responses are averaged to produce the evoked response.
• Generally 4 channels are necessary
• A control site should be monitored as well to differentiate global factors- Anaesthetic & metabolic
SSEP
• Characteristic changes of ischemia are a decrease in amplitude and/or increase in latency.

• Validated in animals but not in humans intra-operatively.

• Usually median nerve at the wrist chosen for CEA- Short latency more resilient.
Short latency SSEP from median nerve at the wrist


Jugular venous Saturation & Cerebral Oximetry
• Both are measures of the balance between cerebral oxygen supply and demand.

• However, limited benefit in this setting because they are a measure of global oxygenation and regional variations would go undetected.
Transcranial Doppler
• Use of doppler shift phenomenon
• Temporal bone used as window (relatively thin) to measure flow velocity in the MCA.
• Assumptions
– Flow µ flow velocity only if diameter of vessel and measurement angle of doppler probe remain constant.
– CBF in basal arteries µ cortical CBF.
Transcranial doppler
• Not only allows continuous measurement of mean blood flow but also detection of embolic events.
• Postoperative TCD has been used in one centre as a basis to treat with dextrans.
Normal TCD trace

Transcranial doppler
• Criticisms
– Doesn’t allow for normal variation in flow velocities, which appears to be large.
– CBF and MCAv not well correlated.
– Cross clamping a relatively short period and most strokes occur at other times during or after surgery.

Radioactive Xenon
• Injected intravenously or into ipsilateral carotid typically pre, during and post clamping.
• Detectors placed over the ipsilateral area supplied by the MCA.
• Decay curves are then analysed.
• Expense and expertise required are the limiting factors.

Conclusion
• Each has its pros and cons.
• Use some form of neurologic monitor.
• One that suits you and your surgeon.