title: Time delay of index calculation: analysis of cerebral state, bispectral, and narcotrend indices.
abstract: BACKGROUND
  On the basis of electroencephalographic analysis, several parameters have been proposed as a measure of the hypnotic component of anesthesia. All currently available indices have different time lags to react to a change in the level of anesthesia. The aim of this study was to determine the latency of three frequently used indices: the Cerebral State Index (Danmeter, Odense, Denmark), the Bispectral Index (Aspect Medical Systems Inc., Newton, MA), and the Narcotrend Index (MonitorTechnik, Bad Bramstedt, Germany).
  
  
  METHODS
  Artificially generated signals were used to produce up to 14 constant index values per monitor that indicate "awake state," "general anesthesia," and "deep anesthesia" and smaller steps in between. The authors simulated loss of and return to consciousness by changing between the artificial electroencephalographic signals in a full-step and two stepwise approaches and measured the time necessary to adapt the indices to the particular input signal.
  
  
  RESULTS
  Time delays between 14 and 155 s were found for all indices. These delays were not constant. Results were different for decreasing and increasing values and between the full-step and the stepwise approaches. Calculation time depended on the particular starting and target index value.
  
  
  CONCLUSIONS
  The time delays of the tested indices may limit their value in prevention of recall of intraoperative events. Furthermore, different latencies for decreasing and increasing values may indicate a limitation of these monitors for pharmacodynamic studies.
authors.affiliation: Department of Anesthesiology, Klinikum rechts der Isar, Technische Universität München, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Pilge
authors.forename: Robert
authors.initials: R
authors.lastname: Zanner
authors.forename: Gerhard
authors.initials: G
authors.lastname: Schneider
authors.forename: Jasmin
authors.initials: J
authors.lastname: Blum
authors.forename: Matthias
authors.initials: M
authors.lastname: Kreuzer
authors.forename: Eberhard F
authors.initials: EF
authors.lastname: Kochs
mesh_terms: D000758:Anesthesia; D004569:Electroencephalography; D006801:Humans; D011930:Reaction Time