Changes in bispectral index values during lumbar arthrodesis.

EDITOR: Several authors have reported sudden increases of the bispectral index values during different procedures such as endoscopic shoulder surgery [1], deep hypothermic circulatory arrest [2] or after using an upper-body blanket for warming [3]. In addition, spurious bispectral index values can be observed when the electromyogram activity of the bispectral index monitor reaches a value of 50 [4]. In all these reports there is a general agreement that the bispectral index monitor shows an erroneous value, which must be interpreted carefully. In the present report we present four cases of patients who underwent general anaesthesia for lumbar arthrodesis, in which the depth of anaesthesia was assessed with the BIS A-2000TM bispectral index monitoring system (Aspect Medical Systems, Natick, MA, USA). In all patients, we observed a sudden increase in the bispectral index values coinciding with an intense surgical stimulation of the spinal roots.