Isoflurane anaesthetic depth in goats monitored using the bispectral indexof the electroencephalogram

The bispectral index (BIS) of the electroencephalogram has recently been us ed to monitor the depth of anaesthesia in humans. The BIS is a dimensionles s number that varies between 0 and 100. We hypothesized that the BIS could also be used to monitor depth of isoflurane anaesthesia in goats. Needle el ectrodes were placed over the frontal region of the scalp of goats and 5% i soflurane was administered via a mask. The BIS number was determined at cli nically relevant end-points. The BIS number did not change when the animals became recumbent (95 +/- 5 to 94 +/- 7, n = 15), but decreased to 65 +/- 1 3 and 64 +/- 15 when the corneal reflex and withdrawal response to a noxiou s stimulus, respectively, were lost (p < 0.001, n = 12). Direct laryngoscop y and intubation increased the BIS (56 +/- 7 to 83 +/- 11; p < 0.05, n = 10 ), as did a noxious pinch to the dew-claw (57 +/- 9 to 76 +/- 9; p < 0.05, n = 10). The spectral edge (frequency below which 95% of the total power re sided) paralleled the change in BIS. We conclude that the depth of isoflura ne anaesthesia in goats can be monitored using the BIS, although further wo rk is needed to determine its sensitivity and specificity.