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 toknow whether patients are sufficiently anaesthetized to toleratedirect laryngoscopy and endotracheal intubation. Because ofthe lack of an accurate objective method to determine the levelof general anaesthesia, under- or overdosing of anaestheticsmay occur. Auditory evoked potential (AEP) is one of severalphysiological parameters under investigation. We aimed to determinethe clinically required depth of anaesthesia, measured by theA-line^TM AEP Monitor and expressed as A-Line ARX Index^TM (AAI)for 90% probability of acceptable conditions for endotrachealintubation. Methods. We studied 108 patients anaesthetized by mask withincreasing concentration of sevoflurane in 30% oxygen and 70%nitrous oxide. Fentanyl 1.5 µg kg^–1 and glycopyrrolate0.2 mg were administered intravenously immediately before startinginduction of anaesthesia. The monitor was programmed to givean alarm at AAI 10, 15, 20, 25 or 30 according to randomization.When the alarm sounded, the end-expiratory sevoflurane concentrationwas registered and endotracheal intubation was attempted. Intubationconditions were assessed by an observer blinded to the AAI. Results. At AAI 10 we found acceptable conditions in 91% (confidenceinterval CI 72–99%]) of patients. The prediction probabilityvalue P_K of AAI was 0.69 (CI 0.59–0.79) and the P_K ofend-expiratory sevoflurane concentration was 0.93 (CI 0.87–0.99).ED₉₀ (the AAI with a 90% probability of acceptable intubationconditions) was calculated as 8.5 (CI 0–17.5). Conclusions. AAI indicates the depth of anaesthesia necessaryfor acceptable endotracheal intubation conditions. Under theconditions of the present study, end-expiratory sevofluraneconcentration was a better predictor and may turn out to bemore useful in the clinical setting.