title: Evaluation of auditory evoked potentials to predict depth of anaesthesia during fentanyl/fluanisone-midazolam anaesthesia in rats.
abstract: OBJECTIVE
  To assess a method for monitoring depth of anaesthesia using components of middle latency auditory evoked potential (AEP) waveforms during anaesthesia with fentanyl/fluanisone and midazolam.
  
  
  METHODS
  Prospective observational study.
  
  
  METHODS
  Five female Wistar rats weighing between 210 and 250 g.
  
  
  METHODS
  Implanted electrodes were used to record AEPs in animals receiving five doses of anaesthetic. Recordings were made at 5 minutes post-injection (deep anaesthesia; no pedal withdrawal response, PWR) and then at 25 minutes (light anaesthesia; strong PWR). Responses showed five characteristic peaks occurring at 11, 14, 23, 42 and 68 ms that were measured for latency of occurrence and peak amplitude.
  
  
  RESULTS
  Auditory evoked potential peaks P14, N23 and P42 were increased significantly in latency with successive anaesthetic injections [avg. F(1,4) = 12.53, p < 0.001; avg. F(1,4) = 10.6, p < 0.001; avg. F(1,4) = 3.9, p = 0.02, respectively]. Peak N23 showed a significant reduction in latency during the 20 minute recovery period following both the first and second anaesthetic injections (t(3) = 7.52, p = 0.005; t(4) = 5.17, p = 0.007, respectively). Peak P42 occurred significantly earlier 20 minutes following the second anaesthetic injection (t(4) = 4.75, p = 0.009). The mean overall depth of anaesthesia assessed using PWR scores was significantly correlated with the mean latency of peak N23, such that as the strength of PWR increased, N23 occurred significantly earlier (r = -0.99, p = 0.01). The amplitude difference between peaks N23 and P42 increased after the second and third drug administrations [avg. F(1,4) = 10.65, p = 0.031 and avg. F(1,4) = 11.24, p = 0.028, respectively].
  
  
  CONCLUSIONS
  The characteristics of these peaks, and in particular latency of peak N23, may provide a useful tool for assessing depth of anaesthesia produced by this, and possibly other anaesthetic agents.
authors.affiliation: Comparative Biology Centre, Medical School, University of Newcastle upon Tyne, UK.
authors.forename: L M
authors.initials: LM
authors.lastname: Antunes
authors.affiliation: Comparative Biology Centre, Medical School, University of Newcastle upon Tyne, UK.
authors.forename: J V
authors.initials: JV
authors.lastname: Roughan
authors.affiliation: Comparative Biology Centre, Medical School, University of Newcastle upon Tyne, UK.
authors.forename: P A
authors.initials: PA
authors.lastname: Flecknell
keywords: anaesthesia; auditory evoked potential; fentanyl; fluanisone; midazolam; rat