Heparin-Induced Thrombocytopenia

Abstract

Unfractionated heparin (UFH) is the most commonly used anticoagulant during cardiac surgery. UFH can be easily monitored and its effect rapidly reversed by a low cost agent, protamine. However heparin use can be complicated by a rare and potentially fatal adverse immunological reaction namely heparin-induced thrombocytopenia (HIT). In HIT, heparin binds to platelet factor 4 (PF4), which is released by activated platelets. As a result, an immunological response can ensue with the development of IgG antibodies against heparin/PF4 complexes. The IgG/heparin/PF4 immune complexes activate platelets with intravascular platelet consumption resulting in thrombocytopenia and increased thrombin production with paradoxical, potentially life-threatening, thrombotic complications instead of bleeding. HIT diagnosis is based on the combination of pre-test clinical probability and the detection of platelet activating antibodies against heparin/PF4 complexes with immunological and functional assays. When HIT is confirmed, any form of heparin should be stopped and non-heparin alternative anticoagulants should be started. Only argatroban and danaparoid are currently licensed for HIT. Cardiac surgery should be delayed in patients with acute or sub-acute HIT until anti-heparin/PF4 antibodies are detectable. In case of urgent cardiac surgery or procedures, bivalirudin is an option, although no reversal agents are available. Other options are pre-operative therapeutic plasmapheresis or use of anti-platelet agents such as tirofiban, epoprostenol with UFH, albeit with increased risk of bleeding.