1 Department of General Anesthesiology, Cleveland Clinic Foundation, Cleveland, OH, USA. 2 Division of Neurosurgery, Department of Surgery, 3 Department of Anaesthetics and 4 Regional Medical Physics Department, Newcastle General Hospital, Newcastle upon Tyne, UK
* Corresponding author: Department of Neurosurgery, Newcastle General Hospital, Westgate Road, Newcastle upon Tyne NE4 6BE, UK. E-mail: a.d.mendelow{at}ncl.ac.uk
Accepted for publication February 3, 2005.
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Abstract |
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Methods. We monitored 52 patients continuously with the BIS monitor together with assessment of neurological function (contralateral upper and lower limb strength and the verbal component of the Glasgow Coma Scale for speech) in patients undergoing awake CEA.
Results. Overall mean BIS value in all patients was 96 (SD 2.9). In five patients who showed clinical evidence of cortical ischaemia during carotid cross-clamping, there was no change in the original range of BIS values throughout the procedure (96.7 [3.2]). In one patient BIS values decreased to 38 about 5 min after the incision and recovered within the next 10 min. The mean BIS value in the remaining 46 patients who did not develop clinical signs of ischaemia was 95.4 (2.6). Three cases are presented which demonstrate the inability of the BIS monitor to detect cerebral ischaemia.
Conclusions. Lack of correlation of BIS with the signs of cerebral ischaemia during CEA makes it unreliable for detection of cerebrovascular insufficiency. We conclude that awake neurological testing is the preferred method of monitoring in these patients.
Keywords: brain, ischaemia ; monitoring, bispectral index ; surgery, endarterectomy
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Introduction |
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The bispectral index (BIS) is a processed EEG parameter which is statistically derived from an empirical database using a proprietary methodology to calculate indices of the EEG power spectrum, burst synchronization and phase coupling. The stepwise development of BIS has been detailed by Rampil.17 It has been suggested previously that the value of BIS may decrease as a result of cerebral ischaemia.18 In yet another case report BIS was the earliest indicator of acute perioperative stroke during removal of a left ventricular assist device.19 The development of clinical ischaemia in awake CEA patients presents a unique opportunity to evaluate utility of BIS as an indicator of cerebral ischaemia. We hypothesized that BIS will correlate with clinical signs of cerebral ischaemia during carotid cross-clamping for CEA in awake patients.
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Subjects and methods |
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Five-lead ECG, pulse oximetry for oxygen saturation and invasive measurement of mean arterial blood pressure were performed for all patients. For local/regional anaesthesia a mixture of lidocaine 1% with epinephrine and bupivacaine 0.25% was infiltrated around the wound edges, into the wound and the carotid sheath as required. During the procedure each patient received oxygen at 48 litre min1 via a facemask or nasal prongs. One or more intravenous access lines were inserted and normal saline was given intravenously. Blood pressure was allowed to fluctuate spontaneously unless ischaemia developed, in which case it was elevated to a systolic pressure of 180 mm Hg using volume expansion. During the operation, the patients' neurological functions were continuously assessed. Contralateral upper and lower limb function and strength was assessed as normal, reduced or absent. Speech was assessed using the verbal component of the Glasgow Coma Scale. Neurological examination was conducted every 30 s when carotid clamps were applied. An ischaemic deficit was defined as any decrease in lower or upper limb power, confusion or dysphasia. If a severe neurological deficit occurred, a Javid shunt was inserted. The Bispectral Index A-2000 Monitor (Aspect Medical Systems, MA) was used in all the patients during carotid cross-clamping. A BIS Zip prep sensor was placed on the patient's forehead as recommended in the manufacturer's guidelines. All patients were continuously monitored before, during and after the cross-clamping. During the procedure the BIS monitor was connected to a laptop computer which recorded data every minute in real time.
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Results |
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Patient 2
A male patient aged 66 yr with a history of hypertension and angina on effort and >90% stenosis of right ICA underwent a right-sided CEA under local anaesthesia. Intraoperative BIS values were in the range 8694. The patient was responding correctly to all questions and obeying commands. This patient developed a left-sided hemiparesis and dysphasia immediately after cross-clamping but was fully conscious. At this point BIS values showed no change. A shunt was inserted in this patient with recovery of hemiparesis and speech over next 15 min. The shunt was removed 25 min later. BIS values remained in the original range of 8694 throughout the procedure. The patient was kept in recovery for 2 h for close neurological and haemodynamic assessment, after which he was sent to the ward. He was discharged on the third postoperative day.
Patient 3
A female patient aged 72 yr underwent awake CEA. She had a history of non-insulin-dependent diabetes mellitus and hypertension. She had 7080% stenosis of the left ICA and <40% stenosis of the right ICA. About 5 min after the incision, BIS values demonstrated a sharp fall from 9298 to the low sixties. The patient became confused and dysphasic while the BIS values dropped further to 3842. The patient gradually recovered over the next 10 min and it was decided to continue with the surgery. At this time BIS values returned to the normal range of 9098. The remaining procedure was uneventful. No shunt was required during carotid cross-clamping in this patient.
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Discussion |
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The correlation of standard EEG and a decreased cerebral blood flow (CBF) during CEA has been well established.24 The EEG changes become apparent when CBF drops below 20 ml (100 g)1 min1 although individual variations may be seen. While there are no large studies that document improved neurological outcome with EEG monitoring during CEA, there is convincing evidence that persistent EEG abnormalities that occur intraoperatively are sensitive in detecting intraoperative strokes.25 It is widely accepted that EEG attenuation occurs with progressive decrease in CBF with disappearance of and ß rhythms and increase in delta rhythm.26 In one study, the use of EEG monitoring and selective shunting was associated with a reduction in the frequency of carotid shunts (49% vs 12% in the selective shunting group) and a decline in combined major neurological morbidity and mortality (2.3% vs 1.1%).8 Thus EEG is believed to be a sensitive detector of cerebral ischaemia and a valuable tool for determining the need for shunting during CEA.2729 Similarly, processed EEG has been reported to detect cerebral ischaemia during CEA successfully.3032 The BIS specifically decomposes the EEG, quantifying the level of synchronization in the signal together with the traditional amplitude and frequency variables, thereby providing a more complete description of complex EEG patterns. It is a proprietary quantitative EEG variable that is derived using a complex algorithm from time, frequency and bispectral domains from a two-channel EEG. The BIS algorithm was designed specifically to quantify depth of anaesthesia in operating theatre settings. However it has been increasingly used in intensive therapy units to titrate sedatives and quantify the level of sedation in critically ill patients.3335 Furthermore, acute decreases of the BIS values have been shown to be related to severe cerebral ischaemia,18 specifically in carotid endarterectomy patients.36 In one case report, an acute decrease in BIS, which coincided with a decrease in cerebral haemoglobin saturation detected by near-infrared spectroscopy, suggesting a reduction in cerebral blood flow, was shown to be associated with acute slowing of the raw EEG waveforms.37 The same group used BIS monitoring combined with near-infrared spectroscopy to detect cerebral ischaemia during cardiac surgery in 65 children.38 Recently BIS has been reported as detecting cerebral hypoperfusion in a patient with a history of chronic renal failure undergoing construction of arteriovenous shunt who had had an intracranial haematoma evacuated 1 month earlier.39 A marked decrease in bispectral index by cervical haematoma reducing cerebral perfusion pressure has been reported in a patient with a ruptured abdominal aortic aneurysm undergoing aneurysmectomy.40 The authors suggest that the BIS monitor may be a simple and convenient monitor for cerebral ischaemia detection. However, our results show that there was no correlation between BIS values and neurological assessment in any of the patients who developed a clinically apparent ischaemic deficit during the clamping period. Although processed EEG has been shown to be an accurate monitor of neurological function and a reliable indicator of whether a shunt is required during CEA, there have been reports of its failure to detect ischaemic episodes during CEA.21 22 This also applies to the standard EEG41 42 and perhaps to BIS monitoring as seen by our results.
In our study, physical examination during awake carotid endarterectomy allowed prompt accurate identification of patients with cerebral ischaemia who would clearly benefit from placement of a shunt. Although monitoring of neurological function of the awake patient tests gross functions, it is easy to perform and more sensitive than both EEG and BIS monitoring as seen from our results.
In summary, our findings suggest that BIS monitoring during awake CEA is unreliable for the detection of cerebrovascular insufficiency. Awake neurological testing is the preferred method of monitoring in these patients.
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Acknowledgments |
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Footnotes |
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References |
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