AIRWAY MECHANICS, GAS-EXCHANGE, AND BLOOD-FLOW IN A NONLINEAR MODEL OF THE NORMAL HUMAN LUNG

A model integrating airway/lung mechanics, pulmonary blood flow and ga s exchange for a normal human subject executing the forced vital capac ity (FVC) maneuver is presented. It requires as input the intrapleural pressure measured during the maneuver. Selected model-generated outpu t variables are compared against measured data (flow at the mouth, cha nge in lung volume, and expired O-2 and CO2 concentrations at the mout h). A nonlinear parameter-estimation algorithm is employed to vary sel ected sensitive model parameters to obtain reasonable least squares fi ts to the data. This study indicates that 1) all three components of t he respiratory model are necessary to characterize the FVC maneuver; 2 ) changes in pulmonary blood flow rate are associated with changes in alveolar and intrapleural pressures and affect gas exchange and the ti me course of expired gas concentrations; and 3) a collapsible midairwa y segment must be included to match airflow during a forced expiration . Model simulations suggest that the resistances to airflow offered by the collapsible segment and the small airways are significant through out forced expiration; their combined effect is needed to adequately m atch the inspiratory and expiratory flow-volume loops. Despite the lim itations of this lumped single-compartment model, a remarkable agreeme nt with airflow and expired gas concentration measurements is obtained for normal subjects. Furthermore, the model provides insight into the important dynamic interactions between ventilation and perfusion duri ng the FVC maneuver.