Control of end-tidal halothane concentration. Part A: Anaesthesia breathing system and feedback control of gas delivery.

Conventional anaesthetic breathing systems are not designed to control end-tidal gas concentrations, nor can they be used to measure accurately the uptake of oxygen or of anaesthetic agent. We built and tested a leak-tight closed-loop anaesthetic breathing system with low solubility to volatile anaesthetic agents and with efficient gas mixing. The system included a water-sealed spirometer, a small carbon dioxide absorber, a coaxial tube to the patient, a circulating pump and feedback controllers for system volume and anaesthetic concentration. Feedback control was implemented to adjust and control automatically the end-tidal anaesthetic concentration and the volume of the system with oxygen supplied through a mass flow controller and with halothane supplied by a titrating syringe. Controller gains, as a function of body weight, were found using a nine-compartment tissue uptake model. Stability was maintained with +/- 50% changes in alveolar ventilation and cardiac output. During subsequent investigations in an animal model, arterial, mixed venous and cerebral venous blood halothane concentrations were measured to show that the feedback-controlled halothane induction was optimized. We conclude that feedback control appears to be clinically applicable for adjusting the end-tidal halothane concentration and system volume to provide a rapid and optimized induction of anaesthesia.