NONINVASIVE MEASUREMENTS OF ALBUMIN FLUX INTO LUNG INTERSTITIUM WITH INCREASED MICROVASCULAR PRESSURE

The purpose of this study was to determine the effect of increasing le ft atrial pressure on noninvasive measurements of radiolabeled albumin normalized slope index (NSI). Using portable gamma scintillation dete ctors, we monitored radioactivities of I-131-labeled albumin and Cr-51 -labeled red blood cells in the blood and over the lung of six anesthe tized sheep before and 2 h after a 9- to 14-Torr increase in left atri al pressure. Measurements of NSI for I-131-albumin decreased > 50% aft er a step increase in left atrial pressure. We interpreted the data us ing a model that has been used to successfully describe unsteady-state lymph flow and protein concentrations after vascular pressure increas es 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 sc an data and lung lymph protein concentrations only when a change in ly mphatic conductance (L(I)) or initial lymphatic pressure (P-o) was imp osed at the time of increased pressure. On average, model-predicted in creases in L(I) were sevenfold, whereas predicted decreases in P-o wer e four- to fivefold. Imposed changes in L(I) and P-o opposed increases in interstitial fluid volume after increased pressure. This was consi stent with normal-to-low postmortem measurements of bloodless wet-to-d ry 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 microvascu lar permeability. In sheep, increases in the lymphatics' ability to re move interstitial fluid may occur with relatively small increases in m icrovascular pressure.