Transport abnormalities from single-breath dynamics of Ar, CO2 and O2.

A model framework is developed for analyzing non-invasive measurements of Ar, CO2, O2, and lung volume during a single exhalation from healthy and diseased lungs. The objective is to distinguish ventilation inhomogeneity from gas-exchange limitations associated with alveolar-capillary transport and processes within the blood. Several practical, quantitative indices are compared to provide distinction and physiological interpretation in terms of rate-determining transport processes. Some indices arise from a model in which the composition in the alveolar space differs from that of the exhaled gas. Other indices are computed from the effective dead-space volumes for Ar, CO2, and O2. Ventilation inhomogeneity is accounted for by the Ar dynamics. Values of these indices were computed from experiments with human subjects having normal lungs, restrictive pulmonary disease, or chronic obstructive pulmonary disease. These three groups are clearly distinguished by a pair of indices: one reflects ventilation inhomogeneity, while the other reflects dysfunction associated with the alveolar-capillary transport and capillary blood flow and distribution.