Use of the bispectral index to predict a positive awareness reaction to laryngeal mask airway-Fastrach insertion and intubation

Abstract

Aim: To determine whether the bispectral index (BIS) can be monitored to predict and indicate an awareness reaction to laryngeal mask airway-Fastrach (LMA-Fastrach) insertion and intubation at BIS values between 40 and 60.

Methods: Fifty-one American Society of Anesthesiologists' (ASA) class I or II status patients aged over 20 years were included in this study. Midazolam 0.1 mg/kg was given for pre-medication, 30 min before induction. For induction, a 0.1-microg/kg bolus injection of remifentanil was followed by infusion, and propofol was administered until the eyelash reflex disappeared; the infusion rate was adjusted to maintain BIS values between 40 and 60. Loss of the eyelash reflex, loss of response to verbal commands, yawning and total propofol consumption were recorded. Patients were tested for awareness twice at 1-min intervals using the isolated forearm technique. The test was considered to be positive if the patient squeezed a hand when asked; after muscle relaxation, the patient was intubated and the test was repeated. In the recovery room and ward, patients were asked whether they could recall this event.

Results: Seven patients tested positive: two following LMA-Fastrach insertion and the remaining five following intubation. None of the patients had recall.

Conclusion: Awareness during anaesthesia may occur at BIS levels that indicate adequate anaesthesia, but this is not associated with recall of the events later.