Uptake and Elimination of Inhalation Anaesthetics during Outpatient Anaesthesia.

Outpatients expect to be able to walk into hospital and out again. They are usually unsedated and often apprehensive. When they are anaesthetized, induction must be rapid, depth of anaesthesia must be deep enough for the operative procedures performed—yet not so deep as to obtund protective reflexes—and prolonged recovery must be avoided. Many anaesthetic techniques are used with the aim of providing these conditions. Most of them make use of inhalational drugs. Whatever the technique, outpatient anaesthesia is associated with rapid changes in the depth of anaesthesia and in the respiratory and cardiovascular state of the patient. The so called "steady state" is unlikely even to be approached. These circumstances place the pharmacokinetics of outpatient inhalational anaesthesia into a special category. It is not known how anaesthetics work. It can be assumed that for a given anaesthetic, depth of anaesthesia depends upon the number of anaesthetic molecules acting on cells in the brain. This in turn depends upon the partial pressure of the anaesthetic in the brain tissue fluid. An understanding of the factors affecting the partial pressure of an inhalational anaesthetic in this tissue fluid, therefore, is the key to an understanding of the control of inhalational anaesthesia. Two factors which are related to the partial pressure of an inhalational anaesthetic in nervous tissue are its uptake and elimination. The relationship is not a simple one. It cannot be appreciated without an understanding of the pharmacokinetics of inhalational anaesthesia, at least in general terms. That is why the first part of this article is taken up with a general description of inhalational anaesthesia. No measurements of the uptake and elimination of inhalational anaesthetics during outpatient anaesthesia have been made, so the remainder of the article is concerned only with a discussion of pos180 sible approaches to the problem and with an outline of some preliminary laboratory measurements. If methods of measurement can be evolved, which are suitable for the special and difficult circumstances of outpatient anaesthesia, they may well find more general application.