Correlation of the A-Line™ ARX index with acoustically evoked potential amplitude†  I. Wenningmann, S. Paprotny, S. Strassmann, R.K. Ellerkmann, B. Rehberg, M. Soehle, B.W. Urban  British Journal of Anaesthesia  Volume 97, Issue 5, Pages 666-675 (November 2006) DOI: 10.1093/bja/ael223 Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 1 AAI vs observation time. All 30 recording sessions are shown. Each row shows, in sequence, the first, second and final recording from the same subject, different rows contain recordings from different subjects. British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 2 The AAI as a function of time averaged over all 30 recording sessions. The decline of the average AAI as a function of recording time (in min) within a session could be fitted monoexponentially: AAI=60.8 [60.6–60.9]+18.9 [18.3–19.5]*exp{(−0.24) [−0.23 to −0.25]*time (in min)}. The 95% CIs are shown in square brackets. Within the first 2, 5 and 10 min the AAI declines by 11, 17 and 21 units, respectively. British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 3 Histogram of all AAI values (updated every second) pooled from all sessions of all 10 volunteers (30 sessions, n=80 993 AAI values). The total time of episodes with AAI values at or below 30 was ∼153 min (out of 1350 min), representing 11.4% of the overall recording time for all 10 subjects. The height of bin 99 is excessively large because the algorithm of the A-Line™ monitor truncates an intermediary index leading to the final AAI index that cannot exceed a value of 99 (see also Fig. 8). Signals rejected by the monitor as artifacts are shown as a separate bin; they occurred with a frequency of less than 2.7%. British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 4 (a) Comparison between subjects (represented by columns) and the three sessions for each subject (subdivisions of column) of the total time that AAI values did not exceed 30 within a single session. For subjects 1, 8 and 9 not a single AAI was recorded with a value of 30 or less. (b) Histogram of all episodes (n=338) with an AAI≤30 and at least 6 s duration. The longest episode lasted 264 s. The average episode length was 27.6 s. The total time of these episodes was 9212 s (∼153 min). British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 5 MLAEP for three different subjects (a/e and c/g were recorded from the same subject on different days). Note the different scaling of the y-axes. Data for subfigures on the same level on left and right were recorded from the same subject but averaged in different ways (left, consecutively; right, grouped according to AAI values). (a–d) EEG traces were averaged in blocks of 512 consecutive sweeps, generating between 40 and 51 MLAEP for each subject and session. Succeeding averages have been superimposed in each subfigure. The insets show the corresponding AAI values vs recording time. (e–h) EEG sweeps were grouped according to their simultaneously recorded AAI values (four bands: AAI <31, AAI 31–60, AAI 61–94 and AAI >95) and averaged in each band. British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 6 Histogram of peak-to-peak amplitudes (20–80 ms window) for all volunteers and all three sessions (1418 MLAEP responses in all). British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 7 Unprocessed raw EEG trace of volunteer ‘eight’. The post-auricular muscle reflex in this patient is extremely well pronounced and can be seen even in the unprocessed EEG. The nine peaks of the post-auricular muscle reflex represent the responses to the nine clicks during this period. British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions

Fig 8 Plot of peak-to-peak amplitudes of the 20–80 ms window of all 1418 MLAEP and their respective AAI values averaged over that period. The solid line is drawn at AAI=30. 135 MLAEP are characterized by an AAI value of 30 or less. The amplitudes of 132 of these MLAEP do not exceed 2.5 µV. For maximal amplitudes of around 10 µV and larger the AAI index no longer increases because the algorithm of the A-Line™ monitor truncates an intermediary index leading to the final AAI index that cannot exceed a value of 99 (see Fig. 3). The correlation (Spearman) between AAI values and AEP amplitudes was excellent (r=0.89) and highly significant with a P-value below 0.0001 (t=74.9). British Journal of Anaesthesia 2006 97, 666-675DOI: (10.1093/bja/ael223) Copyright © 2006 British Journal of Anaesthesia Terms and Conditions