Variability comparison of the composite auditory evoked potential index and the bispectral index during...

Monitors of hypnotic depth help anesthesiologists to guide the anesthetic. The performance of different monitors depends on several factors, index variability at a steady state of hypnotic depth being one. We compared the recently introduced AAI1.6 with the established bispectral index (BIS), regarding index variability during stable values of propofol effect-site concentration.

After ethics committee approval and written informed consent, anesthesia was performed in 40 patients with propofol as the target controlled infusion and fentanyl. Variability of BIS and AAI1.6 was calculated during periods of constant predicted propofol effect compartment concentration and constant levels of surgical stimulation as the median absolute deviation (MAD) from the median value. A variability index was calculated as 1.48*MAD/(threshold - median value), with threshold being the division line between awake and asleep. Threshold crossing time was used to evaluate the performance in predicting return of consciousness.

Variability index, however, was significantly larger for the AAI1.6, despite similar absolute variability measured as MAD. Lightening of anesthesia before recovery could be noticed earlier using the BIS than the AAI1.6, although consciousness was detected with a significantly higher Pk-value by the AAI1.6.

Variability in relation to the difference between the median index value during anesthesia and the threshold necessary to detect consciousness with high sensitivity is higher for the AAI1.6 than for the BIS. This, as well as the steeper concentration-response function found for AAI1.6, impairs the performance of the AAI1.6 in predicting imminent return of consciousness during decreasing propofol concentrations. However, it makes AAI1.6 well suited to detect consciousness when it has occurred.