What errors are associated with BIS monitoring?

Earlier versions of the BIS monitor were very sensitive to electrocautery and the noise from electrocauteries would sometimes be incorporated into the BIS index value.  Earlier versions had difficulty recognizing and rejecting muscle activity and on occasion burst suppression on the EEG would be interpretated as a high BIS index value (1).  These problems appear to have been corrected.  There are however recurring reports related to failure of the BIS value to correlate with the clinical situation.  Patient movement despite BIS values in the range accepted as indicating adequate general anesthesia is commonly reported.  At least one case report has described a patient who was awake and responding despite a BIS level in the 50s (2).  Another paper reported recall in a patient with a BIS level in the 40s (3).  Another case report found changes in the BIS level with administration of muscle relaxation to lightly anesthetized patients (4).  Others have found changes in the BIS level with the administration of intravenous adrenergic agonists such as ephedrine (5).  On the other hand clinicians reported cases in which the BIS monitor identified the otherwise unrecognized onset of inadequate cerebral perfusion and in at least one case this allowed correction before brain injury occurred (6, 7, 8).  BIS has also been found to correlate with the severity of traumatic brain injury.  One unsolved problem is a dramatic fall in BIS values as potent inhalational agent concentration drops during emergence from nitrous oxide assisted general anesthesia.  This phenomenon, called paradoxical slowing is seen in the EEG so it is not an artifact.  Its mechanism is unclear but it is clearly not a deepening of the anesthetic.  Yet the current BIS algorithm interprets it as such.  Dr. Kofke has queried the scientific personnel at Aspect Medical and at present they have no solution.

1) Johansen JW: Development and clinical application of electroencephalographic bispectrum monitoring.  Anesthesiology 2000;93:1336-14

2) Kakinohana M et al: Emergence from propofol anesthesia in a nonagenarian at a bispectral index of 52. Anesth Analg 2005;101:169-70

3) Mychaskiw G, et al: Explicit intraoperative recall at a bispectral index of 47. Anesth Analg 2001;92:808-9

4) Liu N et al: The influence of a muscle relaxant bolus on bispectral and datex-ohmeda entropy values during propofol-remifentanil induced loss of consciousness. Anesth Analg 2005;101:1713-8 

5) Andrzejowski J et al: The effect of intravenous epinephrine on the bispectral index and sedation. Anaesthesia 2000;55:761-763

6) England MR: The changes in bispectral index during a hypovolemic cardiac arrest.  Anesthesiology 1999;91:1947-9

7) Rath GP and Singh D: Zero bispectral index during coil embolization of an intracranial aneurysm.  Anesth Analg 2007;105:887-888

8) Morimoto Y et al: The detection of cerebral hypoperfusion with bispectral index monitoring during general anesthesia. Anesth Analg 2005;100:158-61

David S. Smith, M.D., Ph.D.