The role of von Willebrand factor in primary haemostasis under conditions of haemodilution.

INTRODUCTION: Severe blood loss and related haemodilution during cardiac surgery result in a reduced platelet count which may lead to impaired primary haemostasis. Additionally, the reduced haematocrit lowers rheological forces in circulation and may account for lowered platelet adhesiveness and potentially reduced von Willebrand factor (VWF) activity. These mechanisms may lead to postoperative bleeding. Aim of this study was the examination of VWF activity and VWF-mediated platelet adhesion to collagen under conditions of haemodilution.

MATERIALS AND METHODS: An in vitro flow chamber was utilized to examine the primary haemostasis under a high arterial shear rate of 1500s-1 at variable VWF concentrations, platelet counts and haematocrit levels.

RESULTS: Under a high arterial shear rate, VWF activity is highly dependent on blood viscosity. Both VWF-collagen binding and VWF-mediated platelet adhesion to collagen were significantly increased with increasing haematocrit. Interestingly, we found slight differences in the VWF multimer sizes able to bind collagen under different shear stress conditions. Under conditions of haemodilution, platelet adhesion was strongly dependent on VWF concentration. Increasing VWF concentration improved platelet adhesiveness under low haematocrit conditions (30%) and variable platelet counts (80, 150 and 250×109 /L). This effect was nearly abolished at very low platelet count levels of 50×109 /L.

CONCLUSIONS: VWF improves platelet function under conditions of haemodilution. Therefore, increasing VWF concentration may represent a complementary strategy to administration of platelet concentrates for the management of bleeding in thrombocytopenia.