Model-based administration of inhalation anaesthesia 1. Developing a system model

This paper is the first of a series of reports on a system model for the ad ministration of inhalation anaesthesia. We present the development and basi c testing of the model. it is a multiple-gas model; it covers fresh-gas flo w rates from basal to more than total ventilation and includes an actual, n ot an idealized, circle-absorber breathing system featuring a standing bell ows ventilator. Kinetics of nitrogen, oxygen, carbon dioxide, nitrous oxide , inhaled anaesthetic agents and helium are described. Their partial pressu res sum to the total pressure. Ventilation and cardiac output are treated a s continuous, not cyclical. The model of the breathing system was empirical ly matched to the chosen one (a GMS absorber and 7850 ventilator (Datex-Ohm eda)). Predictions for the wash-in of isoflurane and the uptake of desflura ne and isoflurane agree well with observed data. The results obtained by co ntinuously checking total gas,pressures, calculating mass balances and simu lating the measurement of alveolar space by the closed-circuit helium dilut ion method support the mathematical credibility of the model. it thus merit s further exploration.