Abstract:

PURPOSE: There is controversy about relevant EEG signal changes indicating adequate or inadequate anaesthesia. Differences of drug-induced and nociceptive mediated signal changes have not been studied in detail. The present study investigates whether signal changes during decreases of depth of anaesthesia due to surgical stimulation depend on different isoflurane concentrations during sufentanil anaesthesia. METHODS: Following IRB approval and written informed consent 28 patients (ASA: I; age 43 +/- 11 y) scheduled for elective abdominal surgery were included in the study. Anaesthesia: propofol (2.0 mg/kg) and sufentanil (1.0 micrograms/kg). Following endotracheal intubation (vecuronium 0.1 mg/kg) patients were normoventilated (P(ET)CO2: 36-38 mmHg). Randomly assigned to steady-state anaesthesia (group 1: P(ET)Isoflurane 0.2%, (14n); group 2: P(ET)Isoflurane 0.6%, (14n) during the start of surgery. Monitoring: heart rate (HF), mean arterial blood pressure (MAP), P(ET)CO2, arterial oxygen saturation and rectal temperature. EEG (16 channels referenced to Cz; CATEEM, Medisyst, Linden) recorded 5 min before until 10 min after the start of surgery. EEG-analysis (FFT: 4s, 256/s, 0.45-35.0 Hz): topographical distribution of power spectral densities (delta, theta, alpha 1, and alpha 2). Artifact control: ECG and EOG. RESULTS: Surgical stimulation resulted in increases of MAP in both groups (p < 0.05 vs BL), whereas HR was only slightly affected in group 2 when compared with BL. Other variables except of EEG data did not change over time. In group 1 (0.2% isoflurane) surgical stimulation resulted in decreases of delta over the whole cortex (F2, C3, P3, O1) and in marked increases of alpha predominantly at central leads (C3)(p < 0.05 vs BL). In group 2 (0.6% isoflurane) nociceptive stimulation was associated with decreases of faster waves (alpha: F3)(p < 0.05 vs BL) and increases in delta at fronto-central areas (F3, C3)(p < 0.05 vs BL). CONCLUSIONS: EEG recordings are useful in assessing pharmacodynamic drug effects. In contrast, intraoperative EEG recordings have a low correlation to clinical signs of changes in the anaesthetic state. Previous studies demonstrate paradoxical EEG-arousal reactions during isoflurane anaesthesia. The present data suggest that classical or even paradoxical EEG arousal due to nociceptive stimulation may depend on the isoflurane concentration. It seems reasonable that the ascending reticular formation is functionally blocked by isoflurane in a dose-dependent manner.

Abstract:

BACKGROUND: Previous studies suggest that auditory evoked potentials (AEP) may be used to monitor anaesthetic depth. However, during surgery and anaesthesia, the quality of AEP recordings may be reduced by artefacts. This can affect the interpretation of the data and complicate the use of the method. We assessed differences in expert ratings of the signal quality of perioperatively recorded AEPs. METHODS: Signal quality of 180 randomly selected AEP, recorded perioperatively during a European multicentre study, was rated independently by five experts as 'invalid' (0), 'poor' (1), or 'good' (2). Average (n=5) quality rating was calculated for each signal. Differences between quality ratings of the five experts were calculated for each AEP: inter-rater variability (IRV) was calculated as the difference between the worst and best classification of a signal. RESULTS: Average signal quality of 57% of the AEPs was rated as 'invalid', 39% as 'poor', and only 4% as 'good'. IRV was 0 in only 6%, 1 in 62%, and 2 in 32% of the AEP, that is in 32% one expert said signal quality was good, whereas a different expert thought the identical signal was invalid. CONCLUSIONS: There is poor agreement between experts regarding the signal quality of perioperatively recorded AEPs and, as a consequence, results obtained by one expert may not easily be reproduced by a different expert. This limits the use of visual AEP analysis to indicate anaesthetic depth and may affect the comparability of AEP studies, where waveforms were analysed by different experts. An objective automated method for AEP analysis could solve this problem.

Abstract:

UNLABELLED: The electroencephalogram (EEG) and middle latency auditory evoked responses (MLAER) have been proposed for assessment of the depth of anesthesia. However, a reliable monitor of the adequacy of anesthesia has not yet been defined. In a multicenter study, we tested whether changes in the EEG and MLAER after a tetanic stimulus applied to the wrist could be used to predict subsequent movement in response to skin incision in patients anesthetized with 1 minimum alveolar anesthetic concentration (MAC) isoflurane in N2O. We also investigated whether the absolute values of any of these variables before skin incision was able to predict subsequent movement. After the induction of anesthesia with propofol and facilitation of tracheal intubation with succinylcholine, 82 patients received 1 MAC isoflurane (0.6%) in N2O 50% without an opioid or muscle relaxant. Spontaneous EEG and MLAER to auditory click-stimulation were recorded from a single frontoparietal electrode pair. MLAER were severely depressed at 1 MAC isoflurane. At least 20 min before skin incision, a 5-s tetanic stimulus was applied at the wrist, and the changes in EEG and MLAER were recorded. EEG and MLAER values were evaluated before and after skin incision for patients who did not move in response to tetanic stimulation. Twenty patients (24%) moved after tetanic stimulation. The changes in the EEG or MLAER variables were unable to predict which patients would move in response to skin incision. Preincision values were not different between patients who did and did not move in response to skin incision for any of the variables. MLAER amplitude increased after skin incision. We conclude that it is unlikely that linear EEG measures or MLAER variables can be of practical use in titrating isoflurane anesthesia to prevent movement in response to noxious stimulation. IMPLICATIONS: Reliable estimation of anesthetic adequacy remains a challenge. Changes in spontaneous or auditory evoked brain activity after a brief electrical stimulus at the wrist could not be used to predict whether anesthetized patients would subsequently move at the time of surgical incision.