Correlation of the A-Line™ ARX index with acoustically evoked potential amplitude

Abstract

Background. Automated indices derived from mid-latency auditory evoked potentials (MLAEP) have been proposed for monitoring the state of anaesthesia. The A-Line™ ARX index (AAI) has been implemented in the A-Line™ 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 a wake unmedicated volunteers were studied under steady-state conditions. For each subject, the raw EEG and the AAI were recorded with an A-Line™ monitor (V1.4) during three separate sessions of 45.0 (1.6) min duration each. MATLAB™ (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-Line™ monitor used in this study does not exclude contaminated MLAEP signals. Previous publications involving this version of the A-Line™ monitor (as opposed to the newer A-Line/2™ 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. © 2006 Oxford University Press.