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

Background. Automated indices derived from mid-latency auditoryevoked potentials (MLAEP) have been proposed for monitoringthe state of anaesthesia. The A-Line^TM ARX index (AAI) has beenimplemented in the A-Line^TM monitor (Danmeter, V1.4). Severalstudies have reported variable and, in awake patients, sometimessurprisingly low AAI values. The purpose of this study was toreproduce these findings under steady-state conditions and toinvestigate their causes. Methods. Ten awake unmedicated volunteers were studied understeady-state conditions. For each subject, the raw EEG and theAAI were recorded with an A-Line^TM monitor (V1.4) during threeseparate sessions of 45.0 (1.6) min duration each. MATLAB^TM(Mathworks) routines were used to derive MLAEP responses fromEEG data and to calculate maximal MLAEP amplitudes. Results. The AAI values ranged from 15 to 99, while 11.4% fellbelow levels which, according to the manufacturer, indicatean anaesthetic depth suitable for surgery. Inter-individualand intra-individual variation was observed despite stable recordingconditions. The amplitudes of the MLAEP varied from 0.8 to 42.0µV. The MLAEP amplitude exceeded 2 µV in 75.3% ofreadings. The Spearman's rank correlation coefficient betweenthe MLAEP amplitude and the AAI value was r=0.89 (P<0.0001). Conclusions. The version of the A-Line^TM monitor used in thisstudy does not exclude contaminated MLAEP signals. Previouspublications involving this version of the A-Line^TM monitor(as opposed to the newer A-Line/2^TM monitor series) should bereassessed in the light of these findings. Before exclusivelyMLAEP-based monitors can be evaluated as suitable monitors ofdepth of anaesthesia, it is essential to ensure that inbuiltvalidity tests eliminate contaminated MLAEP signals. Presented in part at the annual meeting of the European Societyof Anaesthesiologists, Lisbon, Portugal, June 5–7, 2004.