Prediction of movement at laryngeal mask airway insertion: comparison of auditory evoked potential index, bispectral index, spectral edge frequency and median frequency.

We have studied 46 patients to compare the efficacy of the auditory evoked potential (AEP) index, bispectral index (BIS), 95% spectral edge frequency (SEF) and median frequency (MF) in predicting movement in response to insertion of the laryngeal mask airway (LMA). Anaesthesia was induced with target-controlled infusions of propofol and alfentanil. After loss of eyelash reflex and adequate jaw relaxation, the LMA was inserted without the assistance of a laryngoscope or neuromuscular blocker. Patients who showed any visible spontaneous muscle movement within 1 min of LMA insertion were defined as movers. Values in movers and non-movers at 30 s before LMA insertion were analysed. Only AEP index discriminated between movers and non-movers with a prediction probability of 0.872. BIS, SEF and MF could not predict movement at LMA insertion. AEP index was the most reliable predictor of movement in response to LMA insertion.