At the cessation point of inspiratory flow on an airway pressure curve of constant-flow CMV (controlled mechanical ventilation), the pressure falls rapidly at first and then gradually tapers down to an endinspiratory plateau. By fitting these two pressure changes to the voltage changes in the respiratory circuit model, the resistance and compliances in the lung periphery and the resistance in the large airway can be estimated. We designed an algorithm estimating the values of circuit elements using a circuit simulator MicroCap Ⅲ. The validity of this method was examined on the mechanical lung model. The sensitivity came to within ±15%. Further validation was carried out on five Beagles treated with methacholine. The large airway resistance, the small airway resistance and compliance, and alveolar compliance demonstrated a two- to three-fold increase in resistances and a two thirds to one-half decrease in compliances, when compared with the base-line values before administration of methacholine. This method can be applied not only to patients during CMV, but also during surgery.
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