title: 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 awake 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 microV. The MLAEP amplitude exceeded 2 microV 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.
authors.affiliation: Department of Anaesthesiology and Intensive Care, University of Bonn Bonn, Germany. wenningman@uni-bonn.de
authors.forename: I
authors.initials: I
authors.lastname: Wenningmann
authors.forename: S
authors.initials: S
authors.lastname: Paprotny
authors.forename: S
authors.initials: S
authors.lastname: Strassmann
authors.forename: R K
authors.initials: RK
authors.lastname: Ellerkmann
authors.forename: B
authors.initials: B
authors.lastname: Rehberg
authors.forename: M
authors.initials: M
authors.lastname: Soehle
authors.forename: B W
authors.initials: BW
authors.lastname: Urban
mesh_terms: D000161:Acoustic Stimulation / Q000379:methods; D000328:Adult; D016477:Artifacts; D001364:Awareness / Q000502:physiology; D004569:Electroencephalography; D005072:Evoked Potentials, Auditory; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results; D012815:Signal Processing, Computer-Assisted