Evaluation of the electroencephalographic bispectral index during fentanyl-midazolam anaesthesia for cardiac surgery. Does it predict haemodynamic responses during endotracheal intubation and sternotomy ?

The bispectral index, a value derived from the electroencephalogram, has be en proposed as a measure of anaesthetic effect. The aim of the present stud y was to evaluate the bispectral index during midazolam-fentanyl anaesthesi a for cardiac surgery for its possible role as a predictor of increases in systolic blood pressure during endotracheal intubation and sternotomy. Afte r institutional approval 15 consenting patients, scheduled for elective car diac surgery, were selected for the study. Anaesthesia was induced in all p atients with a loading dose of fentanyl 7.5-10 mu g kg(-1), midazolam 0.15 mg kg(-1) and pancuronium 0.1 mg kg(-1). After a further bolus dose of fent anyl 10-12.5 mu g kg(-1) prior to the start of incision and sternotomy, mai ntenance infusion rates of fentanyl 4-6 mu g kg(-1) h(-1) and midazolam 0.1 mg kg(-1) h(-1) were started and continued through surgery at the discreti on of the anaesthetist and guided by the presenting clinical and haemodynam ic responses. The control of anaesthesia was never based on the value of th e bispectral index. The mean bispectral index value decreased from 95.7 (3. 1) at base-line to 59.5 (12.0) after induction of anaesthesia and then rema ined below 70 throughout surgery. However, there was an important interindi vidual variability in bispectral index values despite standardized dosages of fentanyl and midazolam. There was no significant correlation between the bispectral index values in the pre-intubation and pre-incision period and the changes in systolic blood pressure during endotracheal intubation and s ternotomy, respectively. In conclusion, the large intersubject variability in the bispectral index values should be investigated further in the light of the great variability in the clinical effects of midazolam and fentanyl. The lack of significant correlation between the bispectral index values an d the haemodynamic responses suggest that the bispectral index, which is a helpful monitor of anaesthetic depth, is not a very reliable monitor of glo bal anaesthetic adequacy during total intravenous anaesthesia with a combin ation of midazolam and fentanyl in cardiac surgical patients.