Performance of Closed-loop Anesthesia System Assessed during Orthopedic Surgery. Absalom et al. (page 67)

In this study of 10 patients undergoing elective hip or knee replacement, Absalom et al.  evaluated the performance of custom-made software to monitor and to automatically control propofol infusion in a closed-loop anesthesia system. Using the Bispectral Index (BIS) as the control variable, the system provides data management in the “monitor” mode, requesting electroencephalographic data at 5-s intervals and providing a graphical display of current and trend values. In “automatic” mode, the system automatically controls the propofol infusion after the operator has entered a target BIS value, a minimum propofol concentration, and the American Society of Anesthesiologists physical status of the patient. The system will not operate in automatic mode if the American Society of Anesthesiologists status is IV or V.

Patients received 0.5% epidural bupivacaine at T8 to provide preinduction anesthesia. General anesthesia was induced using the propofol target-controlled infusion system under manual control in monitor mode. When anesthesia was deemed to be clinically adequate and surgery had begun, the system was switched to automatic mode using BIS as the control variable. The adequacy of anesthesia was then assessed throughout the remainder of the procedure, both clinically and by calculating the median performance error, the median absolute performance error, and the mean offset of the control variable.

One anesthesiologist was in charge of clinical management of the patient, and another monitored the research equipment while also manually recording BIS, physiologic data, and blood and effect-site propofol concentrations at 5-min intervals. The target-controlled infusion system was switched back to manual mode when the surgeon began final closing, and the target propofol concentration was set to zero. Patients remained in the operating room until they had regained consciousness, laryngeal mask airways had been removed, and they could correctly state their date of birth. After surgery, none of the patients reported explicit recall of intraoperative events.

Operating conditions were adequate in 9 of the 10 patients. One began moving after hip manipulation by the surgeon, after about 45 min of stable anesthesia. In three patients, the research team found oscillation of the measured BIS around the set point. The median performance error and median absolute performance error were 2.2 and 8.0%, respectively, and the mean offset of the BIS from the set point was 0.9. The researchers urge further study to determine whether control performance could be improved by alterations to the gain factors or by using an effect site–targeted, target-controlled infusion propofol system.