Detection of circulating platelet microaggregates and surface microthrombi during continuous flow LVAD: evidence for shear-induced platelet activation

Continuous flow left ventricular assist devices (LVADs) are used for circulatory support. Despite heparin anticoagulation, thromboembolic complications occur frequently. It is believed that the thrombus formation is a result of activation of the intrinsic coagulation pathway by the pump surface. However, elevated fluid shear forces can activate platelets and induce platelet aggregates and thrombi. Shear-induced platelet activation requires von Willebrand factor (vWF) binding sequentially to platelet glycoprotein (GP) Ib-IX-V complex and GPIIb-IIIa. In this study, LVAD perfusion was studied, using an artificial circulatory system with heparinized whole blood. After perfusion, circulating platelet microaggregates were detected and quantified with flow cytometry. Surface microthrombi were detected with scanning electron microscopy and quantified with ELISA assay. Blocking vWF-GPIb interaction with aurin tricarboxylic acid (ATA) suppressed platelet aggregation almost completely and blocking vWF-GPIIb-IIIa with c7E3 reduced aggregation extensively. ATA or c7E3 also reduced surface microthrombi significantly, in a dose-dependent manner. Moreover, perfused platelets had a significant increase of surface-bound vWF, which was suppressed by ATA, but was not significantly reduced by the GPIIb-IIIa blocker, TAK-029. The results suggest that shear-induced platelet activation occurs during LVAD perfusion and the inhibition of shear-induced platelet functions may be useful in preventing thromboembolic complications during mechanical circulatory support.