Objective: Comparison of two versions of bispectral index (BIS) derived from the electroencephalogram in mechanically ventilated and continuously sedated patients after major abdominal surgery.
Design: Prospective, cohort, observational, unblinded study.
Setting: Surgical intensive care unit of a university hospital.
Patients: Forty-six patients undergoing major abdominal surgery scheduled for postoperative mechanical ventilation and continuous sedation.
Interventions: Patients were continuously sedated using propofol and sufentanil.
Measurements and main results: Electrodes for determination of BIS were placed at the forehead of the patients according to the manufacturer's specifications immediately after arrival to the intensive care unit. The level of consciousness was assessed every 2 hrs by a clinical sedation scale (Ramsay sedation scale, levels 1-6). BIS, electromyographic activity, and signal quality index were recorded simultaneously at the same time as the Ramsay scale for 24 hrs or until patients were ready for extubation. BIS (version 2.10) and BIS XP (version 3.12) were determined at 2-hr intervals. BIS and BIS XP showed a high correlation of readings (rs = .79, p < .01). However, the methods did not yield identical results. Both variables were significantly influenced by electromyographic activity, especially at high values, whereas there was only a weak correlation with body temperature. Some of the deeply sedated patients (Ramsay 5 or 6) had BIS readings >80 (BIS, 16%; BIS XP, 13%; p = not significant).
Conclusions: The newer algorithm BIS XP did not perform better than the previous version BIS in patients after major surgery who were mechanically ventilated and sedated on an intensive care unit. This precludes the use of BIS or BIS XP for distinguishing, among deeply sedated ICU patients, those with and without preserved cerebral electrical activity.