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 administration of inhalation anaesthesia. We present the development and basic testing of the model. It is a multiple-gas model; it covers fresh-gas flow rates from basal to more than total ventilation and includes an actual, not an idealized, circle-absorber breathing system featuring a standing bellows ventilator. Kinetics of nitrogen, oxygen, carbon dioxide, nitrous oxide, inhaled anaesthetic agents and helium are described. Their partial pressures sum to the total pressure. Ventilation and cardiac output are treated as continuous, not cyclical. The model of the breathing system was empirically matched to the chosen one (a GMS absorber and 7850 ventilator (Datex-Ohmeda)). Predictions for the wash-in of isoflurane and the uptake of desflurane and isoflurane agree well with observed data. The results obtained by continuously checking total gas pressures, calculating mass balances and simulating the measurement of alveolar space by the closed-circuit helium dilution method support the mathematical credibility of the model. It thus merits further exploration.