Infusing liquid anaesthetics into a closed circuit

Closed circuit anaesthesia offers the user many advantages but practical problems impede its widespread use. When conventional vaporizers are employed adequate amounts of agent cannot be delivered into a totally closed circuit during the early stages of an anaesthetic. Direct injection, or infusion, of liquid anaesthetic into the circuit overcomes this problem. The standard method for injecting agents directly into circuits is that described by Lowe and Ernst. Their system can be approximated to a series of constant rate infusions, as is frequently done for propofol, and forms the basis of our technique. For anaesthesia without nitrous oxide, liquid isoflurane should initially be infused into the circuit at a rate in ml/hr of 14 + 0.4 x weight in kg. After five minutes the infusion rate is reduced to 20% of this value. These rates are altered in the light of measured concentrations and clinical responses.