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Department of Anaesthesiology; Sree Chitra Tirunal Institute for Medical Sciences and Technology; Trivandrum; Kerala, India; pksinha{at}sctimst.ac.in
To the Editor:
Bispectral index (BIS), a value derived from the processed EEG, is used as a measure of anesthetic depth. An abrupt decrease in BIS is regarded as either increased depth of anesthesia or possibly ongoing cerebral ischemia (1,2). We report a transient, marked reduction in BIS immediately following an IV injection of a single dose of metoprolol during anesthesia in a neurosurgical patient where neither of the above causes was present.
A 28-yr-old male, 60 kg weight, ASA PS III patient, was scheduled for craniotomy and decompression of a large left frontal glioma. Preoperatively, the patient had symptoms of decreased memory, mild motor dysphasia, and generalized tonic–clonic seizures. The patient received oral diazepam 5 mg and IM glycopyrrolate 0.2 mg 1 h before scheduled surgery.
After establishing the IV line, a BIS electrode (Bispectral Index Monitor A – 2000, Aspect Medical Systems Inc., Newton, MA) was attached to the right side of the forehead. Preinduction BIS value was 96. Anesthesia was induced with propofol (110 mg) and fentanyl (200 µg) and muscle relaxation and orotracheal intubation was achieved with pancuronium. Anesthesia was maintained with 50% O2 in N2O with 1% isoflurane.
During retraction of the scalp flap, there was an episode of hypertension and tachycardia. Because of concern about possible excessive bleeding in the surgical field, the isoflurane in the fresh gas flow was briefly increased to 3% and propofol (50 mg) was administered. The blood pressure normalized to 105/ 49 mm Hg; however, the heart rate remained increased at 126 bpm. Isoflurane was then decreased to 1%. During the hemodynamic changes described, the BIS was between 35 and 45. In an attempt to relieve the tachycardia a single bolus dose of IV metoprolol 3 mg was administered. Within 45 s of the injection, the BIS abruptly decreased to 3 with a spectral edge frequency of 6.5. At this time, signal quality index, blood pressure, and heart rate were 88%, 100/45 mm Hg, and 102 bpm, respectively. This BIS value persisted for about 3 min and returned to 45 over the next 3 min. The further intraoperative course was unremarkable. The patient’ trachea was extubated after full recovery of consciousness with no sign of neurological injury.
There have been several reports regarding the effects of adrenergic agonists and antagonists on BIS, with studies reporting that adrenergic agonists such as ephedrine (3) and isoproterenol (4) may increase BIS. On the other hand, there have been conflicting reports regarding the effects of the adrenergic antagonists, atenolol, esmolol, and landiolol on BIS values (5–8). ß-Adrenoceptor antagonists not only block cardiovascular stress responses after noxious stimulation, but also increase the antinociceptive component of anesthesia (9,10). The mechanism for this effect remains unclear. The rapid decrease in BIS following the metoprolol injection might be explained by the fact that metoprolol crosses the blood–brain barrier to a greater extent than the more hydrophilic agents such as esmolol and atenolol (7) and is thus more likely to cause sudden decrease in BIS than other ß-adrenoreceptor antagonists.
Cerebral ischemia has also been reported as a cause of sudden reduction in BIS values (1,2). In this case, cerebral ischemia was not present as the surgeon was engaged in reflecting the scalp flap during the time of decline in BIS values. Also, the arterial blood pressure and heart rate were not excessively decreased following the injection of metoprolol.
REFERENCES
This article has been cited by other articles:
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  J. K. Panni and M. K. Panni Increased Anesthetic Depth, Not Metoprolol, May Have Caused the Change in Bispectral Index Anesth. Analg., February 1, 2008; 106(2): 670 - 670. [Full Text] [PDF]
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P. K. Sinha, D. Jacob, and T. Koshy Increased Anesthetic Depth, Not Metoprolol, May Have Caused the Change in Bispectral Index Anesth. Analg., February 1, 2008; 106(2): 670 - 671. [Full Text] [PDF]
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