The effect of fluid volume loading on exclusion of interstitial albumin and lymph flow in the dog lung.

The excluded volume fraction for interstitial albumin (FE) was estimated in the lungs of seven mongrel dogs during steady state conditions following intravenous infusions of Ringer's solution, amounting to 0, 5, 10, and 15% of body weight (BW). We estimated the tissue blood volume with 51Cr red cells, extracellular space with 99mTc-diethylenetriaminepentaacetic acid (99mTc-DTPA), and the albumin pool with 125I human serum albumin. A prenodal tracheobronchial lymphatic was cannulated for recording lymph flow (QL), total protein (CL), and albumin [CL(A)] concentrations. From these measurements, we calculated the extravascular albumin content (QA) and 99mTc-DTPA space (VI) of lung tissue samples collected at the successive volume expansions. The apparent tissue concentration of albumin (CApp = QA/VI) decreased from a control value (mean +/- SE) of 0.89 +/- 0.06 to 0.46 +/- 0.04 g/dl following the 15% BW infusion, whereas CL(A) decreased from 1.43 +/- 0.16 to 0.50 +/- 0.07 g/dl for the same volume expansion. By assuming that pulmonary lymph represented tissue fluid, we calculated a control FE of 0.38 +/- 0.02 using the equation, FE = 1 - [CApp/CL(A)]. FE decreased following successive infusions to 0.28 +/- 0.03, 0.16 +/- 0.02, and 0.10 +/- 0.02. These data indicate a significant contribution by the decrease in FE to the total decrease in tissue albumin concentration as interstitial fluid volume increased. Somewhat unexpectedly, the mean steady state QL increased by only 2.1-fold following the 5% BW expansion, but did not further increase following subsequent volume expansions. This has been attributed to a nonlinear interstitial compliance, sequestration of interstitial fluid, or possible deterioration of the experimental preparation.