Narcotrend® does not adequately detect the transition between awareness and unconsciousness in surgical patients

Background: The Narcotrend® index (MonitorTechnik, Bad Bramstedt, Germany) is a dimensionless number between 0 and 100 that is calculated from the electroencephalogram and inversely correlates with depth of hypnosis. The current study evaluates the capability of the Narcotrend® to separate awareness from unconsciousness at the transition between these levels. Methods: Electroencephalographic recordings of 40 unpremedicated patients undergoing elective surgery were analyzed. Patients were randomly assigned to receive (1) sevoflurane-remifentanil (≤ 0.1 μg · kg^-1 · min^-1), (2) sevoflurane-remifentanil (≥ 0.2 μg · kg ^-1 · min^-1), (3) propofol-remifentanil (≤ 0.1 μg · kg^-1 · min^-1), or (4) propofol-remifentanil (≥ 0.2 μg · kg^-11 · min^-1). Remifentanil and sevoflurane or propofol were given until loss of consciousness. After tracheal intubation, propofol or sevoflurane was stopped until return of consciousness and then restarted to induce loss of consciousness. After surgery, drugs were discontinued. Narcotrend® values at loss and return of consciousness were compared with each other, and anesthetic groups were compared. Prediction probability was calculated from values at the last command before and at loss and return of consciousness. Results: At 105 of 316 analyzed time points, the Narcotrend® did not calculate an index, and the closest calculated value was analyzed. No significant differences between loss and return of consciousness were found. In group 1, Narcotrend® values were significantly higher than in group 3. Prediction probability was 0.501. Conclusions: In these challenging data, the Narcotrend® did not differentiate between awareness and unconsciousness. In addition, Narcotrend® values were not independent from the anesthetic regimen.