Platelet factor 4-positive thrombi adhering to the ventricles of a ventricular assist device in patients with heparin-induced thrombocytopenia type II

Background: Thromboembolism is a major complication in patients with ventricular assist devices (VADs). Drug anticoagulation and the use of biocompatible surfaces, such as coating with heparin, aim to reduce thromboembolism in these patients. Administration of heparin can lead to heparin-induced thrombocytopenia (HIT) type II, mainly through heparin/platelet factor 4 (PF4) antibodies. We assessed the presence of PF4 antibodies in VAD thrombi of patients with heparin-coated VADs and HIT II.

Methods: Thrombi (n = 6) were obtained from the replaced Excor ventricles of patients with HIT II after biventricular VAD implantation (Excor Adult; Berlin Heart, Germany). Excor ventricles were changed after clinical examination and suspicion of thrombi in the polyurethane valves. Expression of PF4- antibodies was assessed with the use of a polyclonal rabbit antibody (anti-PF4 antibody; Abcam, USA). Expression was assessed by 2 independent observers.

Results: Biopsies of all thrombi showed an extreme positive immunoreaction for PF4. No differences between the different thrombi and localization (left/right Excor ventricle) were observed. The thrombi were organized, without lamination of fibrin and cellular layers.

Conclusions: Platelet surface expression of PF4 in the thrombi reflects HIT antigen presentation. The physical relationship between the PF4-positive thrombi and the heparin-coated surface suggests that onset of HIT II could be influenced by the immobilized heparin coating.