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Department of Neuroanaesthesiology; All India Institute of Medical Sciences; New Delhi, India; prabhakarhemanshu{at}rediffmail.com
To the Editor:
Bispectral index is used as a monitor of the depth of anesthesia. However, several factors unrelated to anesthesia can modify the index. Unusually low bispectral index has been reported during cerebral ischemia (1,2), intraventricular hemorrhage (3), in patients with dementia (4), and in those with genetically determined low-voltage electroencephalographic (EEG) signals (5). We report a case of sudden decrease in bispectral index to zero in a patient undergoing coil embolization of basilar top aneurysm.
A 55-yr-old man was admitted with a hemorrhage into the interpedencular cistern. Digital subtraction angiography revealed a 2.0 x 2.0 x 2.4 cm basilar top aneurysm with 1.5 cm wide neck and coil embolization was scheduled. In addition to routine monitors, a bispectral index monitor (Aspect Medical Systems, Newton, MA) was used. The bispectral index monitor sensor was applied to the forehead and left temporal area (Fig. 1). A bispectral index of 97 was observed in the awake state. General anesthesia was induced with fentanyl and thiopental and maintained with isoflurane (0.6%–1%) in oxygen and nitrous oxide (1:2). The anesthetic depth was maintained keeping the bispectral index between 40 and 60.
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The aneurysm had a wide neck and the radiologist planned to place stents in the right and left posterior cerebral arteries before coiling the aneurysm. The left posterior cerebral artery could be stented, whereas the right posterior cerebral artery stent could not be placed. During this period, the bispectral index remained between 40 and 60 with burst suppression ratio of 0%. While attempting to navigate the posterior cerebral artery, it was observed that the bispectral index decreased abruptly from 40 to 15. Contrast injection showed that the aneurysm had ruptured and there were no hemodynamic changes. Within 90 s, the bispectral index reached zero (Fig. 1). Immediate coiling of the aneurysm was done. The bispectral index started to increase but remained between 0 and 25. The burst suppression ratio was noted to be more than 90%, with a signal quality index greater than 90. During embolization at the time of detachment of coils, ventricular tachycardia developed, which was eventually converted to sinus rhythm. The bispectral index remained consistently low with a burst suppression ratio of greater than 90%. A CT scan showed an acute subarachnoid hemorrhage and hematoma in the right occipital region. Two hours later, the ECG showed electromechanical dissociation. Despite all efforts, the patient could not be revived.
In our patient, rupture of the aneurysm probably resulted in sudden increase in intracranial pressure resulting in decreased cerebral perfusion pressure and thereby global cerebral ischemia. This would have reduced brain electrical activity resulting in a suppressed or isoelectric EEG, which would have decreased the bispectral index. Increased intracranial pressure could have also caused the sudden cardiac dysrrhythmias and hemodynamic instability. While some suggest that bispectral index may not be a reliable indicator of focal cerebral ischemia (6), we believe that it may be useful with more global ischemia. A large prospective study is needed to justify the role of bispectral index monitoring in detecting intraoperative cerebral ischemia.
REFERENCES
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