Bilateral bispectral index differences in asymptomatic internal carotid stenosis

Abstract

Editor, An unexpected bispectral index (BIS) decrease may indicate inadequate cerebral perfusion.1 Further, BIS may be useful for monitoring carotid artery disease.2,3 Differences in BIS values from two electrodes applied simultaneously to the same patient have been reported.4 We describe a patient who, while under simultaneous bilateral BIS monitoring, suffered a right-side fall in BIS values. A hypertensive, 72-year-old male patient underwent bilateral inguinal hernia repair under local anaesthesia and deep sedation. We obtained the patient's explicit consent to publish his data in this case report. After standard monitoring, continuous three-lead ECG, continuous SpO2 and 5 min interval blood pressure determinations were recorded. Two BIS quattro sensors (Covidien, Medical, Boulder, CO, USA) were applied to each side of the forehead (right and left) and connected to two BIS Vista monitors (Covidien) as part of a clinical trial. Clocks on the two monitors were synchronised just before monitoring the patient. Bilateral BIS readings were recorded every minute. A unique bolus of 0.1 mg fentanyl and 160 mg propofol was intravenously administered, followed by a 6 mg kg−1 h−1 continuous propofol infusion. No additional medication was given. A laryngeal mask was placed and 50% oxygen in air spontaneous breathing was allowed. Field block was performed using 20 ml 1% mepivacaine. After 10 min, the suppression ratio began to increase progressively, especially on the right side. No mean blood pressure change was observed. After 20 min, BIS values from the right electrode suddenly decreased and the suppression ratio of the right electrode increased simultaneously (Fig. 1). A manually started blood pressure determination showed hypotensive blood pressure values; thus, propofol infusion was stopped and 100% oxygen was administered and blood pressure was increased using volume expanders. There were no concomitant changes in the other standard monitoring values. Eight minutes later, blood pressure had returned to previous values despite the right-side BIS value remaining lower than the left one for almost 20 min. A correlation between blood pressure and BIS difference could be observed.Fig. 1: no caption available.By the time the patient emerged from deep sedation, the two BIS values were not significantly different. Postoperative course was uneventful. No intraoperative awareness was found. As the neurological examination was normal (done by the anaesthesiologist), unilateral transient ischaemia was suspected. No further cerebral image diagnostic tests were made. As carotid echo Doppler test is highly available, it showed a right carotid stenosis. The arteriography revealed over 70% right stenosis (Fig. 2). The patient was scheduled for right carotid stent placement.Fig. 2: no caption available.The asymmetric BIS values alerted us to a possible vascular incident with concomitant cerebral ischaemia leading to permanent low BIS values of the right-sided electrode. BIS value changes can be observed during periods at risk of cerebral hypoperfusion, such as the carotid cross-clamping period.2,3 These findings need further studies to be conclusive. A 70-year-old patient with bilateral internal carotid artery stenosis was reported to have shown a sudden BIS value asymmetry after temporary bypass during a left carotid endarterectomy, as a sign of reperfusion. In the second operation for the right side, no marked discrepancy between the two sides was observed.5 In cases of internal carotid stenosis, discrepancies in BIS and state entropy values have also been seen during general surgery.6,7 The decrease in the BIS signal observed in our patient could indicate a low right frontal cortical activity, assumably due to cerebral hypoperfusion caused by a considerable decrease in blood pressure. However, blood pressure returned to previous levels, whereas the BIS asymmetry remained until the end of surgery. The large and unexpected decrease in BIS values made us think about an ischaemic event or inadequate cerebral perfusion as a result of a carotid stenosis, as it has been reported previously during carotid surgery.5–7 Although small discrepancies in BIS values from sensors placed on each side of the forehead are recognised as normal, a unilateral decrease in BIS values should be considered important and investigated, especially in patients with cardiovascular risk factors, as it can be an indicator of asymptomatic internal carotid stenosis. The timely diagnosis of cerebral hypoperfusion during general anaesthesia is not only limited to an EEG specialist, but may be achieved by the attendant anaesthesiologist as well. Acknowledgement There are no financial supports or sponsorships or conflicts of interest or assistance with the study to declare.