Bispectral index and serum drug levels during TCI of remifentanil and propofol for cholecystectomy

S.C. Høymork, B. Grimsmo,J. Ræder. Department. of Anaesthesia, Ullevaal University Hospital, N‑0407, Oslo

Target control infusion (TCI) has been used to more accurately control intravenous drug infusions for general anaesthesia, whereas bispectral index (BIS) has been introduced as a new way of monitoring the level of anaesthesia in the individual patient. We wanted to study the accuracy of TCI systems for propofol and remifentanil, the inter-individual variation of measured serum drug levels and whether these values were correlated with BIS.

Methods

After Ethical Committee approval and informed consent, 20 adult outpatients, normal weight, age 20-65 years, due for laparoscopic cholecystectomy were enrolled in an open study. The patients received no medication except for TCI of propofol (Diprifusor©, 5 µg/ml during induction, 3 µg/ml thereafter) and remifentanil 7.5 ng/ml during the whole procedure. Endotracheal intubation and controlled ventilation was facilitated with rocuronium 0.6 mg/kg. By the end of the procedure, the infusions were stopped simultaneously as the patients received fentanyl 0.01 mg/kg. BIS values were monitored continuously (Aspect© 1050) during the procedure. An arterial line was established for repeated bloodsamples taken for analyses of serum propofol and remifentanil.

Results

The measured serum levels of remifentanil were generally lower than estimated from the TCI system, whereas the propofol values were generally higher (table 1). For both drugs there were large interindividual variations. Those with a high level of one drug tended to have a high level of the other drug as well, initially. In the individual patient, the level of remifentanil was stable throughout the procedure (p<0.005), for propofol no similar correlation was found. There were no correlations between measured serum drug levels and BIS values. During start of surgery high levels of remifentanil was correlated with low values of systolic blood pressure (p<0.01).The patients opened their eyes at 16±4.5 min after stop of the infusions.

Table 1: Estimated (=target) and measured serum drug levels (mean ± SD)

	Est. remifent. (ng/ml)	Measured remifent	Est. propofol (µg/ml)	Measured propofol
Before intubation	7,50 ± 0,00	5,68 ± 2,43	4,99 ± 0,05	7,69 ± 1,59
Start of surgery	7,50±0,00	5,94±2,07	3,07±0,13	3,79±1,09
Stable surgery	7,50±0,00	6,60±2,33	3,00±0,00	4,62±0,96
End of surgery 7,	7,17±0,94	6,11±1,97	3,00±0,00	4,43±1,05
Eyes opening	1,00±0,25	0,86±0,33	1,16±0,17	1,26±0,30

Discussion

Present TCI systems for propofol and remifentanil result in large interindividual variations in serum drug levels during laparoscopic cholecystectomy, and mean values are different from the estimated target. Within the serum levels measured during this surgical procedure, BIS registration was not able to pick out patients with highly deviating serum levels from the others. This may indicate that during cholecystectomy, BIS is not sensitive enough to detect differences in serum drug levels within those encountered in this study. This may be different if lower serum drug targets and a lighter level of anaesthesia are studied.