Efficacy of A-line™ AEP Monitor as a tool for predicting acceptable tracheal intubation conditions during sevoflurane anaesthesia

Background. It is essential for the clinical anaesthetist to know whether patients are sufficiently anaesthetized to tolerate direct laryngoscopy and endotracheal intubation. Because of the lack of an accurate objective method to determine the level of general anaesthesia, under- or overdosing of anaesthetics may occur. Auditory evoked potential (AEP) is one of several physiological parameters under investigation. We aimed to determine the clinically required depth of anaesthesia, measured by the A-line TM AEP Monitor and expressed as A-Line ARX Index TM (AAI) for 90% probability of acceptable conditions for endotracheal intubation. Methods. We studied 108 patients anaesthetized by mask with increasing concentration of sevoflurane in 30% oxygen and 70% nitrous oxide. Fentanyl 1.5 m gk g � 1 and glycopyrrolate 0.2 mg were administered intravenously immediately before starting induction of anaesthesia. The monitor was programmed to give an alarm at AAI 10, 15, 20, 25 or 30 according to randomization. When the alarm sounded, the end-expiratory sevoflurane concentration was registered and endotracheal intubation was attempted. Intubation conditions were assessed by an observer blinded to the AAI. Results. At AAI 10 we found acceptable conditions in 91% (confidence interval [CI 72‐99%]) of patients. The prediction probability value PK of AAI was 0.69 (CI 0.59‐0.79) and the PK of endexpiratory sevoflurane concentration was 0.93 (CI 0.87‐0.99). ED90 (the AAI with a 90% probability of acceptable intubation conditions) was calculated as 8.5 (CI 0‐17.5). Conclusions. AAI indicates the depth of anaesthesia necessary for acceptable endotracheal intubation conditions. Under the conditions of the present study, end-expiratory sevoflurane concentration was a better predictor and may turn out to be more useful in the clinical setting. Br J Anaesth 2005; 94: 601‐6