Comparison of Alaris AEP index and bispectral index during propofol-remifentanil anaesthesia

Background. The Alaris AEP monitor(TM) (Alaris, UK, version 1.4) is the first commercially available auditory evoked potential (AEP) monitor designed to estimate the depth of anaesthesia. It generates an 'Alaris AEP index' (AAI), which is a dimensionless number scaled from 100 (awake) to 0. This study was designed to compare AAI and BIS(TM) (Aspect, USA, version XP) values at different levels of anaesthesia. Methods. Adult female patients were premedicated with diazepam 0.15 mg kg(-1) orally on the morning of surgery. Electrodes for BIS and Alaris AEP monitoring and a headphone to give auditory stimuli were applied as recommended by the manufacturers. Anaesthesia was induced with remifentanil (0.4 mug kg(-1) min(-1)) and a propofol target-controlled infusion (Diprifusor(TM) TCI, AstraZeneca, Germany) to obtain a predicted concentration of initially 3.5 mug ml(-1). After loss of consciousness the patients were given 0.5 mg kg(-1) of atracurium. After tracheal intubation, remifentanil was given at 0.2 mug kg(-1) min(-1) and the propofol infusion was adjusted to obtain BIS target values of 30, 40, 50, and 60. AAI and BIS values were recorded and matched with the predicted propofol effect-site concentrations. Prediction probability was calculated for consciousness vs unconsciousness. Values are mean (sd). Results. Fifty female patients, 53 (15), range 18-78 yr, ASA I or II were studied. Mean values before induction of anaesthesia were 95 (4), range 99-82 for BIS and 85 (12), range 99-55 for AAI. With loss of eyelash reflex both values were significantly reduced to 64 (13), range 83-39 for BIS (P<0.05) and 61 (22), range 99-15 for AAI (P<0.05). The prediction probability P-K for consciousness vs unconsciousness (i.e. loss of eyelash reflex) was better for BIS (P-K=0.99) than for AAI (P-K=0.79). At a BIS of 30, 40, 50, and 60 the corresponding AAI values were 15 (6), 20 (8), 28 (11), and 40 (16), and these were significantly different. Conclusions. During propofol-remifentanil anaesthesia a decrease of the depth of anaesthesia as indicated by BIS monitoring is accompanied by corresponding effects shown by the AAI. However, wide variation in the awake values and considerable overlap of AAI values between consciousness and unconsciousness, suggests further improvement of the AAI system is required.