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BJA Advance Access originally published online on August 23, 2006
British Journal of Anaesthesia 2006 97(5):666-675; doi:10.1093/bja/ael223
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© The Board of Management and Trustees of the British Journal of Anaesthesia 2006. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Correlation of the A-LineTM ARX index with acoustically evoked potential amplitude{dagger}

I. Wenningmann1,*, S. Paprotny1, S. Strassmann1, R. K. Ellerkmann1, B. Rehberg2, M. Soehle1 and B.W. Urban1

1 Department of Anaesthesiology and Intensive Care, University of Bonn Bonn, Germany
2 Department of Anaesthesiology, Charité Campus Mitte Berlin, Germany

*Corresponding author. E-mail: wenningman{at}uni-bonn.de

Background. Automated indices derived from mid-latency auditory evoked potentials (MLAEP) have been proposed for monitoring the state of anaesthesia. The A-LineTM ARX index (AAI) has been implemented in the A-LineTM monitor (Danmeter, V1.4). Several studies have reported variable and, in awake patients, sometimes surprisingly low AAI values. The purpose of this study was to reproduce these findings under steady-state conditions and to investigate their causes.

Methods. Ten awake unmedicated volunteers were studied under steady-state conditions. For each subject, the raw EEG and the AAI were recorded with an A-LineTM monitor (V1.4) during three separate sessions of 45.0 (1.6) min duration each. MATLABTM (Mathworks) routines were used to derive MLAEP responses from EEG data and to calculate maximal MLAEP amplitudes.

Results. The AAI values ranged from 15 to 99, while 11.4% fell below levels which, according to the manufacturer, indicate an anaesthetic depth suitable for surgery. Inter-individual and intra-individual variation was observed despite stable recording conditions. The amplitudes of the MLAEP varied from 0.8 to 42.0 µV. The MLAEP amplitude exceeded 2 µV in 75.3% of readings. The Spearman's rank correlation coefficient between the MLAEP amplitude and the AAI value was r=0.89 (P<0.0001).

Conclusions. The version of the A-LineTM monitor used in this study does not exclude contaminated MLAEP signals. Previous publications involving this version of the A-LineTM monitor (as opposed to the newer A-Line/2TM monitor series) should be reassessed in the light of these findings. Before exclusively MLAEP-based monitors can be evaluated as suitable monitors of depth of anaesthesia, it is essential to ensure that inbuilt validity tests eliminate contaminated MLAEP signals.

{dagger}Presented in part at the annual meeting of the European Society of Anaesthesiologists, Lisbon, Portugal, June 5–7, 2004.


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