Computer-assisted EEG monitoring during carotid endarterectomy

The revised brain symmetry index

OBJECTIVE

Recently, the extended brain symmetry index (BSI) was introduced to assist the visual interpretation of the EEG, in particular to quantify both the spatial (left-right) and the temporal spectral characteristics. The BSI has found application in monitoring during carotid endarterectomy, acute stroke and focal seizure detection. Here, we present additional relevant characteristics and a slightly modified version of this index, simulating its behavior as may occur in various clinical conditions, with an emphasis on the detection of cerebral ischaemia.

METHODS

The behavior of the revised and standard sBSI and tBSI is illustrated using random noise signals to simulate various changes in the EEG. The indices are evaluated as a function of spatial and temporal changes, and as a function of the number of channels.

RESULTS

The r-sBSI and the r-tBSI are normalized in the range [0-1] with sensitivities of about 0.05 for a 10% difference in signal amplitude, either spatial or temporal. The baseline value of the sBSI shows a modest dependence on the number of channels used.

CONCLUSIONS

The revised BSI has an improved sensitivity (about two times) to detect interhemispheric asymmetry and diffuse changes. The modified expression of the tBSI is more compact and allows a more intuitive understanding than previously proposed.

SIGNIFICANCE

qEEG assists in a more objective interpretation of the EEG, and is relevant in neuromonitoring.

Wake-Up Test After Carotid Endarterectomy for Combined Carotid–Coronary Artery Surgery: A Case Series

OBJECTIVE

In combined carotid-coronary artery surgery, it is important to determine patients' neurologic status after carotid endarterectomy (CEA). An initial stroke could be exacerbated by cardiopulmonary bypass required for coronary artery bypass graft (CABG) surgery. Various monitoring methods (eg, electroencephalogram) have been used to reduce neurologic deficits during CEA under general anesthesia. However, none of the methods of determining neurologic status of patients are ideal during the time between the end of CEA and the beginning of CABG surgery. In this study, patient's neurologic status was assessed after CEA with a wake-up test to identify stroke before CABG surgery.

DESIGN

A prospective nonrandomized case series.

SETTING

Single institution, university hospital.

PARTICIPANTS

Forty-four patients with carotid artery stenosis and coronary artery disease underwent combined carotid-coronary artery surgery.

INTERVENTIONS

After CEA, propofol and remifentanil anesthesia was discontinued, the wake-up test was performed, and then anesthesia was reinstituted for CABG surgery.

MEASUREMENTS AND RESULTS

A total of 48 wake-up tests were performed in 43 patients. Two wake-up tests were performed in each of the 5 patients who underwent bilateral CEA. Postoperative stroke were seen in 2 patients. In the first patient, despite a normal wake-up test, the stroke occurred in the cerebral hemisphere contralateral to the CEA, and the patient recovered within 12 days. In the second patient, there was a positive wake-up test after CEA, and he recovered within 3 days. One patient died postoperatively because of ventricular failure.

CONCLUSIONS

Although this is a case series, the authors believe that performing a neurologic examination using a wake-up test may make a contribution and increases the safety of combined surgical procedures in patients with coronary and carotid artery disease.

Experience of 500 cases of neurophysiological monitoring in carotid endarterectomy

BACKGROUND

Experience with Intraoperative monitoring using neurophysiological and haemodynamic indices in 500 operations for carotid endarterectomy is reported.

METHODS

Transcranial Doppler technique (TCD), electroencephalogram (EEG) and bilateral median somatosensory evoked potentials (SEP) were performed. Latency and amplitude of SEP, spectral analysis of EEG signal and blood flow velocity in the medial cerebral artery (MCA) were continuously measured.

FINDINGS

After two consecutive drops of N20/P25 complex of more than 50%, a warning was given, and when the decrease continued, an the alarm raised. Abnormal EEG changes, if any, appeared after a significant decrease in the N20/P25 amplitude. A mean blood flow velocity drop below 40% of the reference value after cross clamping was rated as a significant warning event.A warning as a result of a decrease in N20/P25 amplitude occurred in 80 operations (16.0%), after an spectral edge frequency decrease in 2 cases (0.4%) and after a V(mean) decrease in 21 cases (4.2%). False negative results were experienced in 2 patients (0.4%). A shunt was inserted in 2.8% of the operations. The overall mortality/morbidity rate was 2.4%.

CONCLUSION

A decrease of more than 50% in the amplitude of the thalamocortical somatosensory evoked potential complex N20/P25 proved to be the most reliable warning of danger of ischaemia during carotid endarterectomy.

New developments in cerebral monitoring

In past years, cerebral monitoring was mostly focused around global cerebral perfusion and metabolism monitoring, with the use of transcranial Doppler recordings, jugular bulb oximetry and near-infrared spectroscopy. Most of the recently introduced cerebral monitoring modalities, such as brain tissue partial oxygen tension monitoring and cerebral microdialysis, offer new opportunities by providing regional information on the specific brain area in which the probe is inserted. Ideally, these probes should be inserted in that area of the brain that is most vulnerable to ischaemia, but that may be salvageable with appropriate therapy. In this case, the combination of global and regional cerebral monitoring might offer the best information on which to base patient management. Also, the introduction of more clinically useful, functional neuroimaging techniques may be a valuable adjunct to future neurological critical care management.

A clinical evaluation of near-infrared cerebral oximetry in the awake patient to monitor cerebral perfusion during carotid endarterectomy

STUDY OBJECTIVE

To evaluate the relationship between continuous noninvasive monitoring of cerebral saturation (regional cerebral oxygen saturation [rSo2]) and occurrence of clinical and electroencephalographic (EEG) signs of cerebral ischemia during carotid cross-clamping.

DESIGN

Prospective clinical study.

SETTING

University hospital.

PATIENTS

Fifty ASA physical status II and III inpatients undergoing elective carotid endarterectomy with a cervical plexus block.

INTERVENTIONS

rSo2 was continuously monitored throughout surgery, while an independent neurologist evaluated the occurrence of both clinical and EEG signs of cerebral ischemia induced during carotid cross-clamping.

MEASUREMENTS AND MAIN RESULTS

rSo2 was recorded 1 and 3 minutes after clamping the carotid artery during a 3-minute clamping test. In 5 patients (10%), the carotid clamping test was associated with the occurrence of clinical and EEG signs of cerebral ischemia. All these patients were treated with the placement of a Javid shunt, which completely resolved the symptoms. In no patient was permanent neurological injury reported at hospital discharge. In 4 of these patients, EEG signs of cerebral ischemia were present at both observation times, and in one of them, the duration of cerebral ischemia was less than 2 minutes. The percentage rSo2 reduction from baseline during the carotid clamping test was 17% +/- 4% in patients requiring shunt placement and only 8% +/- 6% in those who did not require it (P = .01). A decrease in rSo2 15% or greater during the carotid clamping test was associated with a 20-fold increase in the odd for developing severe cerebral ischemia (odds ratio, 20; 95% confidence interval, 6.7-59.2) (P = .001); however, this threshold had a 44% sensitivity and 82% specificity, with only 94% negative predictive value.

CONCLUSIONS

Continuous rSo2 monitoring is a simple and noninvasive method that correlates with the development of clinical and EEG signs of cerebral ischemia during carotid cross-clamping; however, we could not identify an rSo2 threshold that can be used alone to predict the need for shunt placement because of the low sensitivity and specificity.

QEEG changes during carotid clamping in carotid endarterectomy: spectral edge frequency parameters and relative band power parameters

Intraoperative monitoring is needed to identify accurately those patients in need of a shunt during carotid endarterectomy. EEG can be used for this purpose, but there is no consensus on the variables to use. Using a database consisting of 149 EEGs recorded from patients during carotid endarterectomy under isoflurane (n=61) or propofol (n=88) anesthesia and who did or did not receive a shunt, the authors investigated which of 16 derivations (common reference, Cz) and 12 parameters (relative and absolute powers and spectral edge frequencies [SEFs]) singly or in combination could best distinguish between the shunt and the nonshunt groups for the two anesthesia regimens. Receiver operating characteristic curves were used to select derivation/parameter combinations for three types of trend computation: (1) values of relative powers and SEFs during clamping (C) only, (2) clamp minus preclamp (baseline) differences (C-B), and (3) C-B differences in absolute logarithmic power (DeltalogP). For both anesthesia regimens, C-B computation distinguished best between the shunt and nonshunt groups. For isoflurane anesthesia, SEF parameters were the best, and for propofol anesthesia the relative power parameters. Discriminant analysis, in which additional derivation/parameter combinations were added, increased the discriminative power of the DeltalogP computation but not of the C or C-B computations. For isoflurane anesthesia, SEF 90% was the best single parameter for distinguishing between patients who did and did not need a shunt and the four best derivations were F3-Cz, P4-Cz, C4-Cz, and F7-Cz. For the propofol anesthesia, the relative power (C or C-B computations) of the delta band was the best and the four best derivations were F8-Cz, T4-Cz, C4-Cz, and F4-Cz.

A brain symmetry index (BSI) for online EEG monitoring in carotid endarterectomy

INTRODUCTION

Carotid endarterectomy is a common procedure as a secondary prevention of stroke, and one of the early controversies in carotid surgery is centered around whether a shunt should be used during this procedure. Although various EEG parameters have been proposed to determine if the brain is at risk during carotid artery clamping, the common procedure is still the visual assessment of the EEG. We propose a brain symmetry index (BSI), that has been implemented as an on-line quantitative EEG parameter, as an additional criterion for shunt need in carotid endarterectomy.

METHODS

The BSI captures a particular asymmetry in spectral power between the two cerebral hemispheres, and is normalized between 0 (perfect symmetry) and 1 (maximal asymmetry). The index was evaluated retrospectively in a group of 57 operations in which the EEG and the transcranial Doppler were used as criteria for shunt insertion. In addition, after online implementation of the algorithm, several patients have been evaluated prospectively.

RESULTS

If no visual EEG changes were detected, it was found that the change in BSI from baseline, DeltaBSI<or=0.03 in all patients. In none of these patients shunting was performed, except for 11 in whom shunting was advised based on changes in the transcranial Doppler signal. None of these patients suffered from neurological complications. In those operations with visual EEG changes during test-clamping and selective shunting, we found that DeltaBSI>or=0.06. In this group, one patient suffered from intraoperative stroke and one patient died, most likely from a hyperperfusion syndrome.

CONCLUSIONS

The BSI may assist in the visual EEG analysis during carotid endarterectomy and provides a quantitative measure for electroencephalographic asymmetry due to cerebral hypo-perfusion. In patients with a change in the BSI (DeltaBSI) smaller than 0.03 during test clamping, visual EEG analysis showed no changes, whereas if visual EEG analysis did warrant shunting, it was found that DeltaBSI>or=0.06.

Electroencephalography (EEG) and somatosensory evoked potentials (SEP) to prevent cerebral ischaemia in the operating room

We review the principal aspects of EEG and SEP to detect and prevent cerebral ischaemia in the operating room during interventions at risk. EEG and SEP are variables that indirectly reflect cerebral blood flow (CBF) provided that anaesthetic regimen, body temperature, and arterial blood pressure of the patient are stable. When CBF decreases and reaches the functional threshold, slowing and/or attenuation of EEG occurs while the amplitude and the latency of cortical SEP are, respectively decreased and lengthened. Based on these changes, numerous criteria corresponding to critical thresholds have been defined. A decrease in EEG amplitude greater than 30% or EEG changes lasting more than 30 s have been considered as significant by clinicians. The main criteria resulting from computerized EEG analysis were a reduction in total power and/or in spectral edge frequency. Regarding SEP, a more than 50% decrease in N20 amplitude and/or a more than 1 ms increase in central conduction time were the most frequently used criteria. According to the bulk of literature, it may be concluded that processed EEG analysis is more sensitive than visual EEG analysis to detect cerebral ischaemia, and that SEP are not less sensitive than conventional EEG. Moreover, literature shows that SEP are as specific as computerized EEG analysis to disclose ischaemia during carotid endarterectomy.

Intraoperative neuromonitoring

BACKGROUND

Intraoperative neuromonitoring (IONM) has been a valuable part of surgical procedures for over 25 years. Insight into the nervous system during surgery provides critical information to the surgeon allowing reversal or avoidance of neural insults.

REVIEW SUMMARY

Electrophysiological tests including electroencephalography, electromyography, and multiple types of evoked potentials (somatosensory, auditory, and motor) are monitored during surgeries that involve risk to the nervous system. Deterioration of signals suggests a surgical insult and is associated with an increased risk of postoperative deficit. Intraoperative identification of this risk allows corrective action. In addition, IONM teams make use of their armamentarium of tests to evaluate anatomy or function of the nervous system in response to specific questions posed by the surgical team.

CONCLUSIONS

Intraoperative recordings are now a routine part of many surgical procedures. Their correct application leads to improved surgical outcome.