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TECHNOLOGY, COMPUTING, AND SIMULATION

Artifact in the Bispectral Index in a Patient with Severe Ischemic Brain Injury

From the Department of Anaesthesia & Pain Management, Alfred Hospital, and the Departments of Anaesthesia, Epidemiology, and Preventive Medicine, Monash University, Melbourne, Victoria, Australia

Address correspondence and reprint requests to A/Prof. Paul S. Myles, Department of Anesthesia & Pain Management, Alfred Hospital, PO Box 315, Melbourne, 3004, Australia. Address email to p.myles{at}alfred.org.au

	Abstract

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The electroencephalogram (EEG) has been used to predict neurological outcome in patients with anoxic-ischemic brain injury. The bispectral index (BIS) may be a useful alternative. A persistently low BIS associated with burst-suppression of the raw EEG in the setting of minimal hypnotic drug administration may indicate severe cerebral ischemia. We report a case where a patient with presumed ischemic brain injury and an extremely low BIS had an unexplained increase in BIS that could be attributed to electrocardiogram artifact. Care should be taken when interpreting BIS in the setting of anoxic-ischemic brain injury or brain death.

IMPLICATIONS: The bispectral index (BIS) can be prone to artifact. In this report we found that electrocardiogram artifact led to an apparent normal BIS in a patient with complete burst-suppression associated with severe brain injury.

	Introduction

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A number of clinical signs and neurophysiological tests, including the electroencephalogram (EEG), have been used to predict neurological outcome in patients after cardiopulmonary resuscitation (1–5). EEG recordings with an isoelectric or burst-suppression pattern are highly specific for severe irreversible ischemic brain injury or death (1,2).

The bispectral index (BIS) (Aspect Medical Systems Inc., Newton, MA) is an EEG-derived variable that measures the hypnotic depth of anesthesia (6–8). BIS is affected by changes in cerebral function and hypoperfusion (9–14). Thus, a persistently low BIS may indicate severe brain injury. We report a case where a patient with ischemic brain injury and EEG burst suppression had an unexplained increase in BIS after cardiopulmonary bypass (CPB).

	Case Report

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A 47-yr-old 88-kg male with primary pulmonary hypertension underwent bilateral sequential lung transplantation. His postoperative course in the intensive care unit (ICU) was complicated by lung allograft failure, for which treatment included mechanical ventilation and inotropic support. On the 14th postoperative day in the ICU he complained of a painful left calf; 30 min later he had a cardiac arrest. Immediate resuscitation was commenced and transesophageal echocardiography confirmed pulmonary embolism. After 60 min of resuscitation, including re-thoracotomy and direct cardiac compression, percutaneous femoral extracorporeal membrane oxygenation (ECMO) was established. The patient was transferred to the operating room for emergency pulmonary embolectomy and on arrival he had fixed, dilated pupils; his arterial blood gas results were as follows: pH, 6.54; PO₂, 71 mm Hg; PCO₂, 92 mm Hg; base excess, -29.

BIS monitoring (version 3.4) was used to guide anesthetic drug administration. The initial BIS readings varied between 2–10, indicating probable global cerebral injury. Drug administration consisted of IV fentanyl, 22 µg/kg, and pancuronium, 8 mg; no hypnotic was used. After administration of muscle relaxant, the BIS reading varied between 0–5, with a suppression ratio (SR) of 100%. Nevertheless, in view of the uncertain ability of the BIS to predict neurological outcome, we elected to proceed with surgery using CPB at normothermia. After embolectomy, weaning to ECMO was achieved with epinephrine and norepinephrine, both at 400 ng · kg^-1 · min^-1. A sinus tachycardia (110–120 bpm) developed. The BIS increased to 38 with a SR of 0%–2%. The raw EEG trace was unusual; it was mostly isoelectric but with a regular slow wave occurring synchronously with the patient’s pulse. The pattern did not waver at all over the remaining 40 min of surgery and was associated with a fixed BIS reading of 38. The signal quality index was 90%–100%, there was no muscle activity detected, no pacemaker, and no external source of interference. The electrocardiogram (ECG) electrodes were repositioned without affecting the BIS.

Postoperatively, in the ICU, the patient’s pupils remained fixed and dilated. Brain death was confirmed with cerebral angiography on the morning after surgery and life support was withdrawn. This case was reported to an Aspect representative and instructions were received that enabled the BIS data from the monitor to be downloaded and converted to a graphics file (Fig. 1) (15).

View larger version (37K): [in this window] [in a new window]   Figure 1. Bispectral index (BIS) trend data from commencement of surgery until transfer from the operating room.

	Discussion

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Vivien et al. (13) studied 56 severely comatose patients in the ICU; BIS was recorded continuously and, if indicated, clinical brain death was confirmed by EEG or cerebral angiography. Twelve patients were brain dead at the time of ICU admission. Their individual BIS values were 0, and in each case brain death was subsequently confirmed. The remaining 44 patients were not brain dead at the time of admission, and their BIS values were between 20 and 79. Of these, 27 patients became brain dead and their BIS values decreased to 0 within a few hours to days. The remaining 17 patients who did not become brain dead had persistent EEG activity and their average BIS values remained above 35.

Normally, ECG and electromyographic artifact are filtered by the BIS monitor’s proprietary algorithm. In our case, we suspect the patient’s ECG or arterial pulsation was detected by the BIS monitor and not recognized as artifact. The fixed BIS noted after CPB implies detection of a stable signal pattern that was probably ECG or pulse related. The manufacturers of the BIS monitor have suggested that because the retrieved EEG data had a smooth pattern, with no peak in the power spectrum, it was consistent with pulse artifact.

It must be stressed that the BIS monitor was not designed, nor has it been validated, for detecting ischemic brain injury. Also, there have been some reports of artifact or technical difficulties preventing its interpretation in some circumstances (16–19). Our case had an unexplained BIS of 38 despite probable severe brain injury. Care should be taken when interpreting BIS in the setting of anoxic-ischemic brain injury or brain death.

	Footnotes

 
Disclaimer: Dr Myles is a principal investigator of the B-Aware Trial (www.b-aware-trial.org.au), an investigator-driven study part-funded by an unrestricted grant from Aspect Medical Systems (Newton, MA).

	References

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