Steady-state trumps accuracy: target-controlled infusion as a gain switch.

Target-controlled infusion (TCI) is a mature technology that enables the delivery of intravenous anaesthetics in the concentration domain. The accuracy of the pharmacologic models used by TCI systems is imperfect, especially regarding pharmacodynamic predictions. This shortcoming of TCI devices is not critical. That TCI systems produce steady-state effect-site concentrations at or near a specified target is a more important attribute than a high level of accuracy because anaesthesiologists titrate to a stable level of drug effect whatever the actual concentration is. In this sense, TCI functions as a 'gain switch'. Achieving a steady state is more important than perfect accuracy.