Effect of age on the comparability of bispectral and state entropy indices during the maintenance of propofol-sufentanil anaesthesia.

BACKGROUND

Manufacturers recommend maintaining anaesthesia at a bispectral index (BIS) or state entropy (SE) index value between 40 and 60.

METHODS

We prospectively studied 102 patients receiving propofol-sufentanil anaesthesia administered by anaesthetists blinded to these indices. The main endpoint was crude agreement (P(0)), defined as the proportion of agreement between BIS and SE index among three categories: <40, between 40 and 60, and >60. Discrepancies in recommendation (DR) were also considered. A DR is type 1 if BIS or SE is <40, while the other is simultaneously >60. A DR is type 2 when BIS and SE index values are on different sides of a threshold (40 or 60) with three subtypes according to the magnitude of their difference. A linear multiple regression was performed to identify covariates that are independently associated with P(0).

RESULTS

In total, 12 147 pairs of values were studied. P(0) was 59.9 (24.5%) [mean (sd)]. Thirty-three patients presented more than 50% discordant pairs and only seven patients presented more than 95% concordant pairs. Type 1 DR occurred in only 1.1% of all the pairs. The median (inter-quartile range) number of type 2 DR varied from 5 (3-8) to 2 (1-3) according to the degree of difference. Multivariate analysis showed that age (P=0.0004) and electrode position (P=0.0084) were independently associated with P(0). An increase in the age of 10 yr decreases P(0) by 5%.

CONCLUSIONS

The agreement between BIS and SE indices is moderate and deteriorates as patients' age increases. This study cannot determine which index is best adapted for elderly patients. Additional work comparing both indices with raw EEG traces is warranted.