The Bispectral Index in the Diagnosis of Perioperative Stroke: A Case Report and Discussion

We discuss a case where the bispectral index (BIS; Aspect Medical, Natick, MA) was the earliest indicator of acute perioperative stroke during the removal of an Abiomed BVS 5000® (Penn State Cardiovascular Center, Hershey, PA) left ventricular assist device (LVAD). Up to 3% of cardiac surgical patients suffer serious, typically embolic, neurological complications, with an associated 20% mortality rate (1). The opportunity to make an earlier diagnosis may help improve outcome in this group as novel therapeutic options become available. Case Report A 69-yr-old man underwent emergency coronary artery bypass grafting after acute myocardial infarction complicated by postinfarction angina and cardiogenic shock. Large-dose right ventricular inotropic support and an Abiomed BVS 5000® LVAD were required to help separation from cardiopulmonary bypass. Six days later, ventricular function had recovered sufficiently to attempt LVAD removal. The patient had been heparinized during this time to reduce the substantial risk of thrombus formation within the device. Abnormal laboratory results included a platelet count of 86 × 109/L, prothrombin time of 16.9 s, activated partial thromboplastin time of 63.2 s, serum urea nitrogen of 113 mg/dL, and creatinine of 2.9 mg/dL. Separation from cardiopulmonary bypass was successful on the first attempt with moderate inotropic support, but the BIS index precipitously decreased from its previously steady state of 50–60 to <10 and remained <15 until the end of the case. Despite heparinization, a cerebral embolus was suspected, and a computerized tomography of the brain was performed the next day, confirming extensive cerebral infarction (Fig. 1, A and B). The patient was managed conservatively and quickly succumbed to complications consistent with intracranial hypertension.Figure 1: (A) Computed tomography demonstrates extensive areas of hypodensity in the right parietal and temporal lobes consistent with acute infarct in the territory of the right middle cerebral artery (arrows). (B) There is also hypodensity in the left parietal occipital region with extension into the thalamic area consistent with acute left posterior cerebral artery infarction (arrows).Discussion This sudden and sustained change in the BIS index was clearly temporally related to a plausible source of cerebral embolization (onset of ejection); however, it has only been developed to monitor depth of anesthesia (2). The BIS index is a single number derived by multivariate analysis of rolling averages of 16-channel electroencephalogram (EEG) signals from more than 5000 anesthetized patients recorded in three frontal leads. However, during carotid endarterectomy, the EEG is established as a monitor of cerebral ischemia after carotid artery cross clamping (3,4). Similarly, there are characteristic EEG changes seen during acute cerebral ischemia that are related to functional outcome (5). Indeed, the BIS index has also been reported to change in response to cerebral ischemia (6) and during resuscitation from cardiac arrest (7,8). In this case, extensive cortical lesions were involved. Deeper or smaller infarcts may not register on the BIS index, limiting sensitivity, and BIS index changes are not specific for cerebral infarction, so the BIS index is not useful to screen for cerebral infarction. However, a sustained and marked decrease in the BIS index in a suspicious clinical context may merit further investigation. If cerebral embolization is suspected, then early diagnosis is both essential and feasible. Computerized tomography scanning, within the first five hours after the onset of symptoms, can demonstrate changes in 70%–80% of patients presenting with acute hemispheric stroke (9) and is essential to exclude cerebral hemorrhage. Definitive therapy involves reperfusion of ischemic brain tissue. Intraarterial thrombolysis within six hours of the cerebral event is the most effective method (10), and its use has been reported in the successful treatment of cerebral embolism associated with LVAD support (11). Successful intraarterial suction thrombectomy has also been described for internal carotid, but not more distal, thrombi (12), and a laser system is being developed (EndoVasix, Belmont, CA). Mechanical thrombolysis has the attraction of avoiding the bleeding complications associated with thrombolytic drugs. Supportive measures aim to avoid exacerbating brain injury by controlling the rate of rewarming (13), aggressively controlling fever (14), and treating increased intracranial pressure. In addition to hyperventilation, osmotherapy, and barbiturate therapy, moderate hypothermia (32°C–33°C) is effective in reducing intracranial pressure (15). In summary, this case report demonstrates that a change in intraoperative BIS index may be the first indication of a serious cerebral event. A sudden and sustained change in the BIS index may merit further investigation because large vessel thrombosis may be amenable to treatment. For postoperative cases, suction thrombectomy will obviously become the preferred intervention for carotid occlusion and for more distal lesions as this and other techniques develop. In the immediate postoperative period, thrombolytics are typically contraindicated. However, if an extensive infarction is developing, particularly including the dominant hemisphere, the benefits of attempted reperfusion may outweigh the risk of mediastinal hemorrhage. For example, the mortality associated with infarction of more than 50% of the middle cerebral artery territory is 85%(9), and although autologous retransfusion of mediastinal blood and reoperation would likely be required to control bleeding, thrombolysis may reduce the overall mortality. Should reperfusion therapy be judged as too risky, aggressive monitoring and treatment of intracranial hypertension, including sustained hypothermia, may improve outcome. Accurate definition of the value of the frequently used BIS index as an early warning of perioperative stroke is important to prepare us for the rapidly developing therapeutic options for this devastating complication.