Correlation beween AAI-index and the BIS-index during propofol hypnosis: a clinical study.

Objective: To determine the degree of linearity and correlation between the anaesthetic depth indices BIS and AAI over a wide range of hypnotic depth using propofol.

Methods: 20 ASA I patients were studied during propofol induction. Co-induction with 0.05 mg fentanyl and 30 mg propofol iv before initiation of the study. Thereafter repeated doses of propofol 0.5 mg/kg iv. every minute until BIS < 30. Loss of responsiveness to verbal command was determined by repeated loud commands to the patient. BIS (Aspect 2000 XP, BIS algorithm 4.0, system rev. 3.12, Aspect Medical Systems; Natick, MA, U.S.A.) and AAI-index (A-Line Auditory Evoked Potential Monitor, version 1.4, Danmeter A/S; Odense, Denmark) were determined simultaneously (n = 15). BIS alone without acoustical stimulation was studied in a control group (n = 5).

Main Results: Both indices decreased with increasing dose, and there was a high correlation between the two (r2 = 0.82). The indices showed however different values and while BIS were quite linear, the AAI-index had a more on-offb ehaviour.

Conclusion: The AAI-index correlates with the BIS-index during propofol hypnosis in the absence of surgical stimulation. Neither the BIS-index, heart rate, nor systemic blood pressure were influenced by the acoustical stimulation from the A-line monitor. Both indices decreased in relation to increasing doses of propofol, but the AAI-index was lower both before becoming unconscious, during transition to unconsciousness, and during the deeper levels of sedation. The AAI-index lacks linearity at both very low and higher levels of propofol sedation with a nearly on-off behaviour for wakefulness vs hypnosis.