Lung fluid balance after pulmonary embolization: effects of thrombin vs. fibrin aggregates

We examined the relationship between the activation of fibrinolysis and the increase in lung vascular permeability after pulmonary microembolization (PM). Sheep were prepared with lung lymph fistulas to assess pulmonary transvascular fluid and protein dynamics. Studies were made in three groups: group I (n = 8) in which PM was induced by an iv infusion of thrombin (60 +/- 13 NIH U/kg); group II (n = 7) in which PM was induced by an iv infusion of 50-micrometers-diameter fibrin microaggregates (0.32 +/- 0.009 g/kg); and group III in which the left atrial pressure was increased by 10-15 Torr by inflation of a balloon catheter. Thrombin caused an increase in pulmonary lymph flow (Qlym) without a change in the lymph-to-plasma protein concentration ratio (L/P ratio) indicating an increase in the lung vascular permeability to proteins. Fibrin microaggregates also increased Qlym, but the increase was associated with a decrease in the L/P ratio. The results in the latter group were similar to those obtained after left atrial hypertension in normal sheep. The increase in permeability after PM induced with thrombin was associated with large increases in the plasma concentration of fibrin degradation products, as compared with PM induced by fibrin microaggregates. The process of intravascular coagulation with the resultant generation of fibrinolysis and fibrin degradation products may be required for the increase in lung vascular permeability to proteins after pulmonary microembolization.