| he system composed of the anesthesia machine, the circle-absorber breathing circuit, and patient has been modeled at different levels: (1) a pressure-flow model; (2) a continuous-flow model with fixed volumes and constant, mean-valued flows; (3) a mass-balance model to represent the circle-absorber system under steady-state or quasi-steady-state conditions. The pressure-flow model serves as a reference for comparison with the simpler models. The continuous-flow model is proposed for use in development of anesthesia machines with automatic control of inspired or expired gas composition. The mass-balance model is proposed for computing dosing schedules for low-flow anesthesia and is the basis of the continuous-flow model. Instrumentation was constructed to measure patient gas exchanges, inspiratory and expiratory limb flows, fresh gas inflows, circuit pressure, and excess gas (popoff) flows in surgical cases for the purpose of validating these models, principally the mass-balance model. Comparison of measured inspiratory and expiratory concentrations for 13 surgical cases with the mass-balance model shows a mean underprediction by the model of inspired oxygen of... |
