Noninvasive measurements of albumin flux into lung interstitium with increased microvascular pressure.

The purpose of this study was to determine the effect of increasing left atrial pressure on noninvasive measurements of radiolabeled albumin normalized slope index (NSI). Using portable gamma scintillation detectors, we monitored radioactivities of 131I-labeled albumin and 51Cr-labeled red blood cells in the blood and over the lung of six anesthetized sheep before and 2 h after a 9- to 14-Torr increase in left atrial pressure. Measurements of NSI for 131I-albumin decreased > 50% after a step increase in left atrial pressure. We interpreted the data using a model that has been used to successfully describe unsteady-state lymph flow and protein concentrations after vascular pressure increases in sheep. Model predictions strongly suggest that the reduction in NSI is due to rapid fluid and solute removal from the interstitium via the lymphatics. The theoretical model was able to predict external scan data and lung lymph protein concentrations only when a change in lymphatic conductance (LI) or initial lymphatic pressure (P0) was imposed at the time of increased pressure. On average, model-predicted increases in LI were sevenfold, whereas predicted decreases in P0 were four- to fivefold. Imposed changes in LI and P0 opposed increases in interstitial fluid volume after increased pressure. This was consistent with normal-to-low postmortem measurements of bloodless wet-to-dry lung weight ratios. In summary, these results indicate that changes in the rate of fluid removal from the interstitium can significantly alter NSI, and in this case, NSI does not reflect pulmonary microvascular permeability. In sheep, increases in the lymphatics' ability to remove interstitial fluid may occur with relatively small increases in microvascular pressure.