Correlation and agreement between bispectral index and state entropy of the electroencephalogram during propofol anaesthesia

Background. Bispectral index (BIS) and state entropy (SE) monitorhypnosis. We evaluated the correlation and the agreement betweenthose parameters during propofol anaesthesia and laryngoscopywith and without muscle relaxation. Methods. A total of 25 patients were anaesthetized with propofol.At steady state (SS: BIS 40–50), they randomly receivedrocuronium (R) or saline (S); 3 min thereafter, a 20 s laryngoscopywas performed. Correlation (regression analysis) and agreement(Bland–Altman analysis) were evaluated before induction(baseline), at loss of eyelash reflex (LER), at SS and duringthe first 3 min after laryngoscopy (L). Results. The correlation coefficient r (95% CI), the mean difference(MD) (95% CI), and the limits of agreement [lower-upper limitsof 95% CI of MD (SD 1.96)] between BIS and SE were as follows.Overall recordings: 0.87 (0.83 to 0.90), 2.5 (1.2 to 3.0), and[–19.5 to 24.6]; Baseline: 0.45 (0.06 to 0.72), 7.6 (6.0to 9.2), and [–2.7 to 17.9]; LER: 0.74 (0.47 to 0.88),8.3 (3.5 to 13.2), and [–22.6 to 39.3]; SS, all patients:0.41 (0.14 to 0.63), 2.0 (–0.5 to 4.6), and [–19.0to 23.3]; SS, Group S: 0.36 (–0.07 to 0.68), 1.9 (–2.5to 6.3), and [–25.0 to 28.8]; SS, Group R: 0.63 (0.32to 0.82), 0.2 (–2.0 to 2.3), and [–14.0 to 14.4];L, all patients: 0.49 (0.32 to 0.63), 0.7 (–1.6 to 3.0),and [–25.6 to 27.1]; L, Group S: 0.41 (0.13 to 0.63),2.3 (–2.4 to 7.1), and [–36.7 to 41.3]; L, GroupR: 0.72 (0.56 to 0.83), –0.6 (–2.2 to 1.0), and[–14.3 to 13.1]. The correlation was good except for SSin Group S. The MD was significantly different from 0 for overallrecordings, during baseline and LER, but not for the other conditions.The agreement was poor except for baseline, and SS and L inGroup R. Conclusions. BIS and SE are globally well correlated. In contrast,agreement is poor as differences of more than 20 units are frequentlyobserved, except for awake and paralysed patients.