The exchange of small molecules as a measure of normal and abnormal lung microvascular function

Evidence exists for the utility of measures of small-molecule exchange in assessing the normal and abnormal transport status of the lung vasculature. The evidence for the usefulness of PS for 14C-urea is: 1. PSu compares to other small molecules as would be expected from free-diffusion coefficients. 2. The extraction of 14C-urea decreases with increase flow as would be expected of a diffusion-limited indicator. 3. PSu is unaffected by moderate pressure increase, it increases when lymph protein flow indicated permeability increase, and it decreases when surface area is reduced. 4. Lung injuries such as E. coli endotoxemia in sheep and ARDS in patients can both reduce PSu, presumably through surface area reduction, and increase PSu, through increased permeability. Two events that accompany clinically important vascular injury complicate the interpretation of lung MT curves. These events are surface area loss and heterogeneity of flow and transport properties in the lung. Additional work is needed to precisely measure these variables and assess their importance.