The influence of the click stimulus of the Alaris AEP monitor on the depth of anesthesia.

Abstract

To the Editor: Middle-latency auditory evoked potentials (MLAEP) are in use to quantify the pharmacodynamic action of anesthetic drugs (1–4). In addition, auditory evoked potentials have even been proposed to be better able to detect the transition from unconsciousness to consciousness than the BIS (5). The Alaris AEP monitor (Alaris, Hampshire, UK, version 1.4) is the first commercially available auditory evoked potential (AEP) monitor designed to measure the depth of anesthesia based on MLAEP. It generates an Alaris AEP monitor index (AAI), which is a dimensionless number scaled from 100 (awake) to 0. The MLAEP is elicited with a bilateral click stimulus of 70 dB intensity and 2 ms duration. Current investigations are comparing the applicability of bispectral index (BIS, Aspect Medical Systems Inc., Newton, MA) and the AAI for quantification of pharmacodynamic drug effect (6,7). However, at the moment it remains unclear whether the 70 dB click stimulus of the AEP monitor itself potentially influences the depth of anesthesia as indicated by BIS monitoring or by calculated propofol effect-site concentrations when targeting at a predefined BIS value. With institutional review board approval and written informed consent, 60 adult female patients scheduled for gynecological surgery were randomized to receive a propofol-remifentanil anesthetic with BIS and AAI monitoring or only with BIS monitoring in place. Exclusion criteria were deafness, a history of any disabling central nervous or cerebrovascular disease, hypersensitivity to opioids or substance abuse, or a treatment with opioids or any psychoactive medication. In all patients, BIS electrodes were applied to the patient’s head as recommended by the manufacturer, and BIS values were continuously assessed using an Aspect A-2000 BIS XP monitor. Furthermore, in a 5:1 ratio the Alaris AEP monitor headphone for the application of click sounds was additionally applied to the patient’s head. Induction of anesthesia was started with a remifentanil infusion at 0.4 μg/kg/min; 5 min later, propofol was given for hypnosis using a target-controlled infusion (TCI, Diprifusor™, AstraZeneca, Wedel, Germany), initially started at a propofol target concentration of 3.5 μg/mL. After loss of consciousness, the patients received 0.5 mg/kg of atracurium. Immediately after the trachea had been intubated, remifentanil was infused at a constant rate of 0.2 μg/kg/min, whereas propofol TCI was adjusted to BIS target values of 30, 40, 50 and 60 without further stimulation. BIS values were recorded and matched with the respective propofol effect-site concentrations as displayed by the Diprifusor software. After the assessment of study parameters had been finished, subsequent anesthesia was delivered according to individual clinical needs, and the surgical procedure was started. Data were compared by Student’s t-test and P < 0.05 was defined as statistically significant. The two groups were comparable for demographic data and mean remifentanil dosages. The BIS values and the calculated propofol effect-site concentrations were not significantly different between the two groups at the various time points of this study (Table 1).Table 1: BIS Values and Propofol Effect-Site Concentrations at Various Time PointsOur results indicate that during propofol/remifentanil anesthesia the 70 dB click stimulus of the Alaris AEP monitor does not influence the depth of anesthesia as indicated by BIS monitoring or by calculated propofol effect-site concentrations when targeting at a predefined BIS value. S. Kreuer, MD W. Wilhelm, MD, DEAA J. Bruhn, MD