EFFECT OF STOCHASTIC HETEROGENEITY ON LUNG IMPEDANCE DURING ACUTE BRONCHOCONSTRICTION - A MODEL ANALYSIS

In a previous study (J. H. T. Bates, A. M. Lauzon, G. S. Dechman, G. N . Maksym, and T. F. Schuessler. J. Appl. Physiol. 76: 616-626, 1994), we investigated the acute changes in isovolume lung mechanics immediat ely after a bolus injection of histamine. We found that dynamic resist ance and elastance increased progressively in the 80-s period after in jection, whereas the estimated tissue hysteresivity reached a stable p lateau after similar to 25 s. In the present study, are developed a co mputer model of the lung to investigate the mechanisms responsible for these observations. The model conforms to Horsfield's morphometry, wi th the addition of compliant airways and structural damping tissue uni ts. Using this model, we simulated the time course of acute bronchocon striction after intravenous administration of a bolus of bronchial ago nist. Heterogeneity was induced by randomly varying the values of the maximal airway smooth muscle contraction and the tissue response to th e agonist. Our results demonstrate that much of the increase in lung i mpedance observed in our previous study can be produced purely by the effects of airway heterogeneity. However, we were only able to reprodu ce the plateauing of hysteresivity by assigning a minimum radius to ea ch airway, beyond which it would immediately snap completely shut. We propose that airway closure played an important role in our experiment al observations.