Closed-Loop Control Of Anesthesia With Volatile Anesthetics

Objective: The Bispectral Index (BIS) is an electroencephalogram-derived measure of anesthetic depth. A closed-loop anesthesia system was built using BIS as the control variable, a proportional-integral-differential incorporated with adaptive control algorithm, and a target-controlled infusion system as the control actuator. Closed-loop performance was assessed with enflurane and compared with the traditional manual control inhalation anesthetic technique in the stability of hemodynamics, the quality of induction and emergence, and the quantity of enflurane per hour.Methods: Twenty-eight adult patients scheduled to undergo elective abdomen surgery were enrolled. General anesthesia was induced using the low-flow fresh gas technique with enflurane under spontaneous respiration. Fentanyl 0.4 μ g · kg~-1 and succinylcholine 100 mg were injected intravenously to facilitate endotracheal intubation after eyelid reflex disappeared. The both process of induction and maintenance was controlled by the closed-loop target-controlled infusion system. Meanwhile, in the manual control group, anesthesia processes were controlled with enflurane vaporizer with an intra-operative target bispectral index of 50 in all cases. Adequacy of anesthesia and change of hemodynamics and the concentration of enflurane during closed-loop control were assessed clinically and by calculating the median performance error, the median absolute performance error, and the mean offset of the control variable.Results: Besides the mean wobble is lower than manual group (4.26%: 10%; P=0.01), the median performance error (2.01%:4.16%), the median absolute performance error (5.48%:9.62%) and the mean offset of the BIS from the set point (4.52: 2.26) were not statistically difference, respectively. Although there was oscillation of the measured BIS around the set point, no patients reported awareness or recall of intraoperative events. During maintenance of anaesthesia, cardiovascular variables did not differ between closed-loop and manual group. Cardiovascular parameters were stable in both groups. The duration of induction, emergence and responses is not different with manual group. The concentration of enflurane increases gradually with time and the curve of end-tidal enflurane concentration is not match with the predicted value by the model.Conclusions: The system was able to provide clinically adequate anesthesia with enflurane and kept the hemodynamics stable. But closed-loop control did not show any decreases in induction, emergence and respond time than the manual group. The used quantity of enflurane was not decreased either. Further studies are required to determine whether control performance can be improved by providing adequate vaporization of enflurane.