Alveolar gas relationships during use of the circle system with carbon dioxide absorption.

Expressions have been derived to show the dependence of alveolar oxygen and anaesthetic concentration on fresh gas flow to a circle system, the composition of fresh gas, ventilation and gas uptake. The form of these expressions is influenced by the degree of mixing of fresh and expired gases within the circle system. These expressions assume an equilibrium state within the circle system and the rate at which equilibrium will be approached has been quantified in terms of the time-constants of change of composition of gas within the system. Time-constants approach infinity as fresh gas flow approaches values which just satisfy gas uptake. Whilst simplifying assumptions made in the derivation of the various alveolar gas equations limits their accuracy, the expression can serve as a guide to the likely behaviour of circle systems under any given conditions of use.