Quantitative Anaesthesia with the Help of Closed-Loop Control
Closed-loop control systems have been used since the time of the Babylonians, when irrigation systems were opened and closed in closed-loop control mode . Closed-loop control in anaesthesia began in 1950 when Bickford used the EEG signal to control the infusion of thiopental . A syringe with thiopental was attached to a stepper motor. Each time the EEG crossed zero, a bolus of thiopental was given. As the patient became anaesthesized the infusion rate slowed because the frequency of the EEG decreased. Closed-loop control has adjusted ventilation during anaesthesia by comparing the difference between the desired and a measured end-tidal carbon dioxide concentration . If the CO2 was high, the inspiratory airway pressure was incremented in 0.1-mm steps. When the end-tidal reached the desired level, the airway pressure was held constant. The most common application of closed-loop control is the maintenance of arterial blood pressure by the infusion of sodium nitroprusside . Experience has shown that a desired end-tidal anaesthetic orblood pressure is maintained more consistently andmore accurately under closed-loop control than is done manually . Closed-loop control can perform some very usedful and helpful tasks when it is made available in the anaesthesia machine design.
KeywordsBreathing System Circuit Volume Anaesthesia Workstation Inspiratory Airway Pressure Anaesthetic Uptake
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