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

Background. Bispectral index (BIS) and state entropy (SE) monitor hypnosis. We evaluated the correlation and the agreement between those parameters during propofol anaesthesia and laryngoscopy with and without muscle relaxation. Methods. A total of 25 patients were anaesthetized with propofol. At steady state (SS: BIS 40-50), they randomly received rocuronium (R) or saline (S); 3 min thereafter, a 20 s laryngoscopy was performed. Correlation (regression analysis) and agreement (Bland-Altman analysis) were evaluated before induction (baseline), at loss of eyelash reflex (LER), at SS and during the 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 limits of 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 (6.0 to 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 [-19.0 to 23.3]; SS, Group S: 0.36 (-0.07 to 0.68), 1.9 (-2.5 to 6.3), and [-25.0 to 28.8]; SS, Group R: 0.63 (0.32 to 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, Group R: 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 SS in Group S. The MD was significantly different from 0 for overall recordings, during baseline and LER, but not for the other conditions. The agreement was poor except for baseline, and SS and L in Group R. Conclusions. BIS and SE are globally well correlated. In contrast, agreement is poor as differences of more than 20 units are frequently observed, except for awake and paralysed patients.