GASEOUS HOMEOSTASIS AND THE CIRCLE SYSTEM Validation of a Model

A model of a subject breathing from a circle system has been described in the previous paper (Conway, 1986). Whilst this model has been designed to study the pharmacokinetics of inhaled anaesthetic agents, it handles all inert gases in an identical manner. As an initial validation, the present paper describes the performance of the model in relation to two inert gases—nitrogen and helium. These gases are widely used in studies of respiratory physiology. Four studies are described: the ability of the model to maintain a nitrogen equilibrium when breathing air, the time course of nitrogen elimination from the subject model on oxygen breathing, the ability of the model to achieve a new equilibrium after perturbation of an existing steady state, and the measurement of functional residual capacity (FRC) using helium as an indicator. MATERIALS AND METHODS Except where otherwise stated, the values of the various subject and circle system variables were as shown in table I. In all the studies on nitrogen homeostasis, the subject model was initially in a state of inert gas equilibrium breathing air. The ability of the model to maintain a nitrogen equilibrium was assessed both with the subject model breathing air under non-rebreathing conditions, and breathing from a circle system initially filled with air and supplied with a basal flow of oxygen (that is, a flow equal to oxygen uptake (Fo,) when corrected from ATPD to BTPS). Elimination of nitrogen from the subject model was studied under non-rebreathing conditions with the subject model breathing oxygen, and wit