Effects of varying remifentanil concentrations on Analgesia Nociception Index® under propofol: an observational study.

Analgesia Nociception Index monitor provides a measurement of the nociception and anti-nociception balance based on heart-rate variability. The aim was to assess the ability of Analgesia Nociception Index (ANI) to detect standard noxious stimulation during anesthesia at different opioid concentrations in comparison to hemodynamic and Bispectral Index parameters. Sixteen patients undergoing general anesthesia with propofol and remifentanil. Standardized tetanic electrical stimulation was applied without any other concurrent stimuli, while different effect site concentrations of remifentanil were increased step-by-step (0.5, 1.5, 3.0, 5 and 7 ng/ml). For each tetanic stimulus, values of the different variables 60 s before and 120 s following the stimulus were analyzed. ANI values decreased significantly 120 s after the stimulus when compared to the mean ANI 60 s before the stimulus (P < 0.01). At lower remifentanil concentrations (0.5 ng/ml), all variables except electromyogram from the Bispectral Index significantly changed after the stimulus but in the higher concentrations (5.0 and 7.0 ng/ml) there was not a significant change. The other variables did not change significantly after the noxious stimulation. ANI was the only variable that positively correlated with the different remifentanil concentrations (R = 0.959, P = 0.01). Our study showed that there was a significant decrease in Analgesia Nociception Index after a tetanic stimulation while hemodynamic and BIS parameters did not change, suggesting that the Analgesia Nociception Index may perform better than traditional hemodynamic parameters at reflecting noxious stimulation. Analgesia Nociception Index significantly changed at lower, but not at higher, remifentanil concentrations.