Oxford Meeting - November 2002

Is Auditory Evoked Potential Monitoring during Anaesthesia of Predictive Value?

K Canavan¹, KJ O'Hare¹, G McGinn¹, GNC Kenny² & D Russell^{1, 2}. Departments of Anaesthesia, ¹South Glasgow University Hospitals NHS Trust, Glasgow G51 4TF and ²University of Glasgow, Glasgow G31 2ER.

Introduction: The Auditory Evoked Potential Index (AEP_index), derived from the auditory evoked response, has been investigated as a monitor of anaesthetic depth (1). It has been shown to change repeatedly with transitions between consciousness and unconsciousness, patients having an AEP_index in the range 38-98 when awake and 21-55 when anaesthetised (2). It has been used successfully as the input signal for closed loop propofol anaesthesia in spontaneously breathing patients (3), has been shown to predict movement in response to laryngeal mask airway insertion following induction of anaesthesia with propofol and alfentanil (4), and to predict movement in response to skin incision during sevoflurane anaesthesia (5). We investigated whether AEP_index predicts movement in response to skin incision in patients anaesthetised solely with propofol.

Method: Following local research ethics committee approval and with written consent, 26 unpremedicated ASA 1 or 2 patients undergoing surgical procedures involving a groin incision were studied. Median age was 43.5 years (range 17-68), median weight was 77.5 kg (range 55-96) and 8 patients were female. Anaesthesia was induced and maintained with propofol by ‘Diprifusor’ target controlled infusion, to a C_T of 6.8mcg.ml^-1; Cp50_CALC from the study by Stuart and colleagues (6). Patients breathed oxygen-enriched air, and surgical incision was made a minimum of 12 minutes after induction to allow equilibration between blood and brain propofol concentrations. The AEP_index was elicited as previously described (1), and was noted immediately prior to surgical incision. The surgeon who was blind to the AEPindex determined the presence or absence of gross purposeful movement in response to incision.

Results: Sixteen patients were deemed to have moved in response to the stimulus and 10 patients did not. The values for AEP_index immediately prior to incision in these patients are shown below. AEP_index values for the two groups were not significantly different when compared using the Mann-Whitney Test for non-parametric data (p=0.29).

Conclusions: In this preliminary investigation, auditory evoked potential monitoring did not predict gross purposeful movement in response to the initial surgical incision in unpremedicated patients anaesthetised solely with propofol.

References:

1.       Mantzaridis H, Kenny G.N.C. Auditory evoked potential index: a quantitative measure of changes in auditory evoked potentials during general anaesthesia. Anaesthesia 1997; 52:1030-1036.

2.       Gajraj RJ, Doi M, Mantzaridis H, Kenny GNC. Analysis of the EEG bispectrum, auditory evoked potentials and the EEG power spectrum during repeated transitions from consciousness to unconsciousness. British Journal of Anaesthesia 1998; 80:46-52.

3.       Kenny GNC, Mantzaridis H. Closed-loop control of propofol anaesthesia. British Journal of Anaesthesia 1999; 83:223-8.

4.       Doi M, Gajraj RJ, Mantzaridis H, Kenny GNC. Prediction of movement at laryngeal mask airway insertion: comparison of auditory evoked potential index, bispectral index, spectral edge frequency and median frequency. British Journal of Anaesthesia 1999; 82:203-207.

5.       Kurita T, Doi M, Katoh T, Sano H, Sato S, Mantzaridis H, Kenny GNC. Auditory evoked potential index predicts the depth of sedation and movement in response to skin incision during sevoflurane anesthesia. Anesthesiology 2001; 95:364-70.

6.       Stuart PC, Stott SM, Millar A, Kenny GNC, Russell D. Cp50 of propofol with and without nitrous oxide 67%. British Journal of Anaesthesia, 2000, Vol. 84, No. 5 638-639.