Fentanyl and midazolam anaesthesia for coronary bypass surgery: a clinicalstudy of bispectral electroencephalogram analysis, drug concentrations andrecall

Bispectral index (BIS) was assessed as a monitor of depth of anaesthesia du ring fentanyl and midazolam anaesthesia for coronary bypass surgery. In ID patients given morphine premedication, anaesthesia was induced with a combi nation of midazolam and fentanyl and thereafter maintained with a continuou s infusion of a mixture of midazolam and fentanyl 5 and 50 mu g kg(-1) h(-1 ), respectively. BIS was recorded continuously but not shown to the attendi ng anaesthetist. Plasma concentrations of midazolam and fentanyl were measu red five times during the procedure. An auditory stimulus was given during bypass. All patients were interviewed twice after operation for explicit an d implicit recall. No patient had any anaesthetic complications. BIS decrea sed during anaesthesia, but varied considerably during surgery (range 36-91 ) with eight patients having values >60. Midazolam and fentanyl drug concen trations did not correlate with BIS. No patient reported explicit or implic it recall. During clinically adequate anaesthesia with midazolam and fentan yl BIS varies considerably. The most likely reason is that BIS is not an ac curate measure of the depth of anaesthesia when using this combination of a gents.