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.