Objectives of anesthesia ventilation include maintenance of hormoxemia, normocapnia, airway climatization and minimum alveolar concentration (MAC)-based dosage of volatile anesthetic agents. If these goals are to be achieved automatically, e.g., by means of feedback control, regulated and controlled variables need to be defined. Clinically relevant controlled variables are oxygen saturation (sO(2)), end-tidal carbon dioxide concentration (etCO(2)), water content of respiratory gases, and end-tidal concentration of volatile anesthetic agents (etVA). Corresponding regulated variables include inspiratory oxygen concentration (FIO2), water content of inspiratory gas (FIH2O), inspiratory concentration of volatile anesthetic agents and - for all variables - ventilatory minute volume. In particular if gases other than oxygen are used, e.g., N2O or N-2, the system volume also needs to be controlled. Obviously, the fresh gas flow (FGF) rate is neither a controlled nor a regulated variable for any of the qualities pertinent to anesthesia ventilation; rather the contrary, FGF adds noise to the system and any change of FGF adversely affects equilibria of minute ventilation, FIO2, and volatile agent concentration. However, most conventional anesthesia workstations (per ISO 5358 or EN 740) allow user-adjustable FGF, adding a source of error to all control systems. We have designed an anesthetic workstation, complying with EN 740, which has eliminated FGF as an independent variable and which allows closed-loop feedback control of relevant variables. It is a closed system without valves, offering further advantages with respect to work of breathing, airway climatization, cost, environmental protection, and quantitative anesthesia. The PhysioFlex is a computer-controlled high-flaw closed-circuit workstation, relying on three hierarchically arranged levels of feedback control: oxygen, volume, and agent. Controlled variables are FIO2, system volume and etVA, corresponding regulated variables are the volumes of oxygen, N2O/N-2, and liquid VA, digitally fed into the system. All variables are adjusted automatically (with reference to the computer's physiology and pharmacokinetics databank) to the respective set-point selected by the user. Integration of an activated charcoal filter allows reduction of VA concentration without opening the system. High flow within the system provides lag-free equilibrium of all concentrations, allowing feedback control without oscillation. A backup safety system protects against wrong dosage resulting from erroneous measurements (single fault condition). The digital administration of oxygen allows online monitoring of oxygen uptake, reflecting the patients (V) over dot O-2 (oxygen uptake). Everyone has learnt to trust feedback control systems in daily life and sometimes considers them superior to manual control, e.g., the autopilot in aviation. Applying the same philosophy to anesthesia ventilation relieves the clinician from some needless and distracting routines, giving him more time to take care of his patient.
