Plasminogen and Plasmin Increase after Polytrauma Shifting the Thrombin/Plasmin Activity Ratio Toward Fibrinolysis

Background: Hemostasis is regulated by a dynamic balance between thrombin and plasmin activity, and the supply of prothrombin and plasminogen in the general circulation. This dynamic balance is important so that the formation of clots occurs at the site of injury, but does not spread to distal sites where thrombus formation can block blood flow and lead to organ dysfunction. Procoagulant and fibrinolytic activity is regulated by the supply of both active enzymes, and their respective zymogens. Disruption in the plasma levels of the enzymes and zymogens can lead to coagulopathies that have serious effects on morbidity and mortality. Polytrauma and hemorrhage can lead to an acute coagulopathy in rats that mimics the clinical hypocoagulation in both civilian and military patients with severe trauma and hemorrhage. This coagulopathy could be due an imbalance between the pro-coagulant or fibrinolytic systems.

Hypothesis: We hypothesized that the dynamic balance between thrombin and plasmin activities would be altered after polytrauma and hemorrhage, favoring fibrinolysis.

Method: Sprague-Dawley rats (n=8) were anesthetized with Isoflurane. Polytrauma was induced by damage to the small intestines, the left and medial liver lobes, the right leg skeletal muscle, and by fracturing the right femur. Rats were then bled to a mean arterial pressure of 40mmHg and held there until 40% of the blood volume was removed. No fluid resuscitation was given. Blood samples were taken before (time 0) and at 30, 60, 120 and 240min. Rat plasmin and thrombin activity were measured by enzymatic assay. Plasminogen and prothrombin were measured after conversion to their respective active enzymes with tissue plasminogen activator or factor Xa respectively. Active enzyme was subtracted from the total to give the levels of zymogens. D-dimers were measured by ELISA.

Results: There was no significant change in plasma thrombin activity after polytrauma and hemorrhage. Prothrombin levels fell slightly, but significantly over time. In contrast, plasmin activity rose significantly by 30min and continued to be elevated through the next 4hrs (235%, p<0.05), and was paralleled by an increase in D-dimers (10±2.6 to 59±16ng/ml, P<0.05). Surprisingly, plasminogen also rose in the first 30min and continued to be significantly elevated by 4hrs paralleling the rise in plasmin (100±2.7 to 119±3.9ug/ml, P<0.05). Prothrombin and plasminogen levels were 40 to 100 times higher than their respective active enzymes at each time point, suggesting that the zymogen pool is not depleted even in this severe trauma model. The thrombin/plasmin activity ratio was calculated and found to fall rapidly 30 min after polytrauma and was low over the next 4hrs.

Conclusion: These data suggest that polytrauma and hemorrhage lead to dynamic changes in procoagulant and fibrinolytic systems. There was a rise in plasmin activity, and the fall in the thrombin/plasmin ratio suggesting that the fibrinolytic component dominates over time. The fall in prothrombin suggests consumption of the zymogen. Surprisingly, plasminogen levels rose quickly, suggesting that this acute phase protein is actively secreted after polytrauma. The high levels of prothrombin and plasminogen in comparison to thrombin and plasmin, suggests that the precursor pool for the active enzymes is not depleted. Polytrauma and hemorrhage led to a rapid decrease in the thrombin/plasmin activity ratio and suggests that fibrinolysis is rapidly activated. These results provide mechanistic support to clinical data showing that early treatment with anti-fibrinolytic drugs reduces bleeding. This project was funded by US Army Medical Research and Materiel Command.