Model-based automatic feedback control versus human control of end-tidal isoflurane concentration using low-flow anaesthesia

We studied the clinical use of an automatic feedback control system to adju st the end-tidal anaesthetic concentration with a low-flow method. The end- tidal controller uses two input signals (the end-tidal and inspiratory conc entrations) to control the isoflurane concentration in the fresh gas flow, using a model-based algorithm. We studied 22 ASA I-III patients during elec tive surgery lasting more than 2 h. The anaesthetist was asked to make four step changes of the target end-tidal concentration (+0.3, +0.6, -0.3, -0.6 vol%), either manually (Group A) or by setting the target value for the fe edback controller (Group B), and then the control was changed and the step changes were repeated, in a crossover design. Eighty step changes with each control method were compared in terms of response time, maximal overshoot and stability. The automatic control system was more accurate and stable th an the human controller for step increases and step decreases, with less ov ershoot/undershoot and greater stability [e.g. maximal overshoot 14.7 (SD 3 .7)% and 18 (8.1)% respectively for +0.6 vol% step changes, and 19.8 (3.7)% and 30.7 (13.2)% respectively for +0.3 vol% step changes]. However, the au tomatic control system showed a faster response time than the manual method only with large increasing steps (e.g. 149 (32) s and 205 (57) s respectiv ely for +0.6 vol% step changes) and was not different from manual control f or decreasing steps. Automatic control of the end-tidal isoflurane concentr ation can be better than human control in a clinical setting, and this task could be done automatically