Changes in rapidly extracted auditory evoked potentials during tracheal intubation

Background: One of the problems encountered in assessment of the hypnotic l evel during anesthesia is the extraction of a consistent and reliable measu re online and close to real time. Hemodynamic parameters such as heart rate and blood pressure are not, at least with the traditional single parameter versus time presentation, adequate for ensuring an optimal level of anesth esia, especially when using neuromuscular blocking agents (NMBA). In the Li terature, it has been demonstrated that auditory evoked potentials (AEP) ar e able to provide two aspects relevant to determining level of anesthesia: firstly they have identifiable anatomical significance and, secondly, their characteristics reflect the way the brain perceives a stimulus. Methods: The aim of this study was to evaluate the AEP index based on a sys tem identification model, the autoregressive model with exogenous input (AR X-model), and to compare it to the classical method, the moving time averag e (MTA). The ARX enables the extraction within 15-25 sweeps, depending on t he signal-to-noise ratio (SNR), whereas MTA typically needs 250-500 sweeps. The hypothesis of the present study was that since the ARX-model extracts the AEP faster than the MTA-model, the former should be able to detect chan ges during the brief, intense stimulus of endotracheal intubation. Twelve f emale patients scheduled for gynecological surgery were included in the stu dy. Anesthesia was initiated with thiopentone and maintained with isofluran e and alfentanil. The AEP was mapped into an index (AEP-index) normalized t o 100 when the individual was awake and decreasing to an average of 25 duri ng thiopentone induced anaesthesia. The results were compared to those obta ined by MTA-extracted AEP. Results: During tracheal intubation 9 patients showed an increase in the AR X-extracted AEP-index larger than 15, and 6 of these patients showed an inc rease larger than 25 (mean increase=33, SD=18). The MTA-extracted AEP-index showed only one patient with an increase larger than 15. The ARX-extracted AEP changed significantly faster than the MTA-extracted AEP. Conclusion: The ARX-extracted AEP-index increases during tracheal intubatio n. There is a significant difference between the ARX-extracted AEP and the traditional MTA-extracted AEP, in terms of response time. In order to trace short-lasting changes in the hypnotic level by AEP, the AEP should be extr acted by a method with a fast response such as the ARX-model. (C) Acta Anaesthesiologica Scandinavica 44 (2000).