摘要：

There are two types of heparin-induced thrombozytopenia (HIT): first the non-immunmediated, clinically inapparent HIT type I and second the immunmediated HIT type II which is associated with thromboembolic complications. The incidence of a HIT type I is about 25%. HIT type II occurs in about 3% of heparin-treated patients when heparin therapy is continued for more than 5 days. It is a disease which needs treatment in an intensive care unit and interdisciplinary co-operation. The mortality is about 20% and a further 20% of the patients suffer from permanent health damage such as limb-amputation, myocardial infarction or cerebral stroke. In most cases the antigen in HIT type II is a complex consisting of heparin and platelet factor 4 (PF 4), rarely a complex of heparin and a different protein. This complex of heparin and PF 4 binds to more than one antibody and forms immun-complexes which activate platelets by combining with the platelet Fc-receptor. Through repetitive platelet-activation followed by PF 4 sequestration, the resulting PF 4-overshoot combines with glucosaminoglycanes located on the surface on the endothelial cells. The HIT-antibody is not specific and combines also with complexes consisting of heparin and endothelial-standing glucoaminoglycanes. These immuncomplexes on the endothelial surface cause an immunmediated endothelial injury and lead to an increased thrombotic risk. Probably this is an important mechanism in the development of thromboembolic complications in HIT type II. Usually HIT type II occurs during the first exposition to heparin between the 6th and the 20th day of heparin-treatment, while during re-exposition to heparin it may re-occur within some hours. The platelet-count decreases beyond 100,000 μl. Typical signs are venous and arterial thromboembolic complications especially in the big venous vessels, limb arteries, cerebral arteries, and coronary vessels. Next to the typical clinical signs, evidence of HIT-antibodies is required for the diagnosis of a HIT type II. Established diagnostic methods are the heparin-induced platelet-activation-(hipa-)test and the immunoassay (PF 4/heparin-ELISA). Lepirudin (Refludan ), a recombinant hirudin recently developed for the treatment of HIT type II shows an immediate action which can be easily monitored by the activated partial thromboplastin time (aPTT). There is no risk of cross-reactivity to HIT-antibodies. To prevent a HIT type II it is important to check the platelet-count before initiation of heparin-therapy, and from the 5th day of heparin-treatment platelet-count should be checked daily. In case of re-exposition to heparin, platelet-count should be checked before and then daily during anticoagulation with heparin. Further, any unnecessary heparin-treatment should be avoided and if necessary it should be finished within 5 days. Oral anticoagulation with a dicumerol should be started as soon as possible. Patients who once suffered from a HIT type II should be given a rescue-certificate. These patients should not be treated with antithrombin III- or prothrombin-preparations because of the high heparin concentrations.

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