Increased Heparin Clearance in Experimental Pulmonary Embolism

Patients with pulmonary embolism (PE) have been reported to have a shortened heparin half-life (t½). Heparin clearance was investigated in rabbits to explore the possible contribution of platelets in increased heparin clearance in acute RE. Heparin clearance was measured by a protamine titration (heparin activity) assay and by ³⁵S radioactivity. Both assay methods gave comparable results and showed a shortened heparin t½ in PE. Heparin t½ was significantly shorter in PE (12.0 min) than in venous thrombosis (VT) (18.2 min) and in control rabbits (20.0 min) when a dose of heparin 50 U/kg was injected. Heparin t½ was not reduced in animals with PE that were treated with heparin 200 U/kg and in animals that were embolized with polyether urethane coated-glass pellets. No significant absorbance of ³⁵S heparin to the surface of PE or VT was found. The heparin t½ was shortened to the same extent in severely thrombocytopenic animals (platelet count <30,000/mm³) with PE as in non-thrombocytopenic animals with PE (12.8 min compared to 12.0 min) and was significantly shorter than in thrombocytopenic, VT and control animals (p<0.005). In addition, platelet survival was uninfluenced by PE and there was no increase in antiheparin activity in plasma after PE. It is concluded that the mechanism for the shortened heparin t½ is not platelet mediated, that it is influenced by the nature of thromboembolic material and that this effect on heparin t½ appears to be dose dependent.

 

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