Objectives: 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.
Study design: Prospective observational study.
Animals: 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].
Conclusion: 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.
Keywords: anaesthesia; auditory evoked potential; fentanyl; fluanisone; midazolam; rat.
Copyright © 2001 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved.