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Burst suppression ratio is the only determinant for BIS values below 30

¹ Bonn, Germany

To the Editor:

I read the article by Mérat et al. about bispectral index (BIS) monitoring and severe cerebral ischemia¹ with interest.

The BIS monitor is still a kind of "black box". Descriptions of the algorithm are sparse and, in their details, contradictory.^2,3 Recently a part of the BIS algorithm was made public:⁴ burst suppression ratios > 40% are invariably and linearly correlated with the BIS (r = -1), according to the equation: BIS = 50 – burst suppression ratio / 2.

Conversely, BIS values below 30 are linearly correlated with the burst suppression ratio. Therefore, the reported BIS value of 8 can be directly translated into a burst suppression ratio of 84% according to the above equation.

A BIS value of 8 is not related to phase coupling, nor to bispectral analysis, but is just an effect of the burst suppression ratio.

Thus, the observation of Mérat et al. is not BIS-specific but merely secondary to the occurrence of a burst suppression pattern associated with cerebral ischemia. In principle, such extensive burst suppression pattern can easily be identified by visual inspection of the electroencephalography and does not require processed monitoring like the BIS.

References

1 Mérat S, Lévecque JP, Le Gulluche Y, Diraison Y, Brinquin L, Hoffmann JJ. BIS monitoring may allow the detection of severe cerebral ischemia (French). Can J Anesth 2001; 48: 1066–9.[Abstract/Free Full Text]

2 Glass PS, Bloom M, Kearse L, Rosow C, Sebel P, Manberg P. Bispectral analysis measures sedation and memory effects of propofol, midazolam, isoflurane, and alfentanil in healthy volunteers. Anesthesiology 1997; 86: 836–47.[Medline]

3 Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology 1998; 89: 980–1002.[Medline]

4 Bruhn J, Bouillon TW, Shafer SL. Bispectral index (BIS) and burst suppression: revealing a part of the BIS algorithm. J Clin Monit Comp 2001; 16: 593–6.

REPLY:

² Paris, France

We agree with Dr. J. Bruhn concerning the poor interest of the bispectral index (BIS) for values below 30. However our purpose wasn’t to say that BIS always reflects the depth of anesthesia. We solely think that an unexpected modification of the BIS value, without modifications of anesthesia, is abnormal. In such a case, when the BIS decrease is unrelated to anesthesia, we suggest that the BIS may be useful to detect severe cerebral ischemia, whatever the BIS value.

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