Equilibration of intravascular albumin with lung lymph in unanesthetized sheep

In 16 unanesthetized sheep with chronic lung lymph fistulas we measured pulmonary vascular pressures, lymph flow, lymph and plasma total protein and albumin concentration. We determined the rate of equilibration of radioiodinated albumin between plasma and lung interstitial fluid (lung lymph) in three steady-state conditions; baseline (n = 14), increased pulmonary microvascular pressure (n = 9) and increased microvascular permeability (n = 4). The tracer protein equilibration proceeded according to single compartment wash-in kinetics in all experiments. Lung lymph flow averaged 5.3 +/- 2.8 (S.D.)ml/h under baseline conditions, 16.1 +/- 10.6 ml/h during increased pressure and 37.3 +/- 29.4 ml/h during increased permeability. The half time of equilibration averaged 2.9 +/- 1.0 h, 2.2 +/- 1.0 h and 0.7 +/- 0.2 h, respectively. Lung interstitial fluid equilibrates with plasma proteins more rapidly than most other organs. The marked difference between increased permeability and the other conditions demonstrates the sensitivity of this method. No evidence was obtained that any tracer protein entered lung lymph within the caudal mediastinal lymph node.