Closed-circle systems--a new direction in the practice of anaesthesia.

Closed-circuit anaesthesia is the most scientific method of anaesthesology. A fresh gas flow of at least 10 l/min is used for wash-in of the desired alveolar concentrations. In practice, maintenance of closed-circuit anaesthesia has been found remarkably easy, safe and logistically feasible by the administration of constant quantities of approximately 240 ml oxygen, 100 ml of nitrous oxide, 12 ml halothan evapour or 23 ml enflurane vapour per minute (both with 70% nitrous oxide). The only prerequisites are low-flow rotameters, an oxygen monitor and a ventilator designed specifically for anaesthetic use in the operating theatre.

Inspired O2 and N2O concentrations in essentially closed circuits.

By simulation of the FRC (functional residual capacity), washin of N2O was conceptually separated from uptake at the alveolar-capillary membrane. Data collected by mass spectrometry and pneumotachography during a period of 2 years showed a near-constant body uptake of N2O which makes clinical application of closed circuit N2O--O2 anesthesia simple and safe. The data failed to confirm accelerated washin by higher concentrations of N2O ("the concentration effect"), although there was a concentrating effect according to the law of mass action.

Closed circle systems. A new direction in the practice of anesthesia.

On the basis of the near-constant uptake of inhalation anesthetics at the alveolar membrane, the large closed-circuit volume which acts as a buffer, and the effect of concentration on uptake, we have evolved a simple and practical technique of closed-circuit anesthesia. After initial high flow wash-in of anesthetic gases into the FRC, a nearly constant slow fresh-gas flow can be maintained.

[The closed system (author's transl)]. / Das geschlossene System.

It is appropriate to consider dose-regulated or quantitative closed-circuit anesthesia in terms of uptake of inhalation anesthetics from a reservoir bag which is never allowed to fill completely. The method which the authors have used routinely for 8 years requires reduction of the dose as a function of the inverse square root of elapsed anaesthesia time. Nitrogen does not accumulate because there is an inevitable loss of 20 to 100 ml/min between cuff and trachea. A simpler method, which has been well received, is that of "minimal flow" anesthesia (Virtue) with a constant fresh gas flow of 300 ml/min O2 and 200 ml/min N2O. "Low-flow" techniques typically entail use of 500 ml each of O2 and N2O per min.