The high shear rates are induced by artificial heart valves or left ventricular assist devices cause acquired von Willebrand factor syndrome (aVWS). We hypothesised that an ex vivo model could be established to study whether mechanical shear alone causes aVWS or whether this process depends also on the VWF cleavage protein ADAMTS-13.
Healthy volunteers and two patients with congenital ADAMTS-13 deficiency donated blood. In vitro extracorporeal circuits were established using medically approved left ventricular assist devices (Heartware®) and silicone tubings with blood inlets and outlets. The pump speed and circulating blood volume were adjusted to correspond to in vivo use in humans. Anticoagulated blood circulated for 1-4 hours in the extracorporeal circuits. VWF multimers were quantified by sodium dodecyl sulphate-agarose discontinuous gel (1.2%) electrophoresis followed by Western-Blotting and consequent quantification with a luminescence image analyzer and a commercially available software, VWF antigen was measured by an STA assay and ristocetin co-factor activity (VWF:RCo) was quantified by turbidometry using a commercial kit (BC von Willebrand reagent; Dade Behring/Siemens, Marburg, Germany). Platelet function was measured by multiple electrode aggregometry in whole blood.
The high shear stress in the extracorporeal circulation rapidly decreased VWF:RCo and thereby the VWF:RCo/VWF:Ag ratio by 47% (p<0.01) to pathologically low values. Concomitantly, high molecular weight multimers (HMWM) decreased, resembling the pathophysiological events that occur when an LVAD is implanted into humans. Up to 14-15 mers were visible on the gels at baseline, which were reduced by a maximum of 6-7 mers, corresponding to an average 68% lower densitometry signal of HMWM (p<0.01). This was accompanied by a 3-fold reduction in ristocetin induced aggregation (p<0.01). In contrast, the two patients with congenital thrombocytopenic purpura and virtually complete deficiency of ADAMTS-13 activity had only a minimal or no decrease in multimers (p<0.05 vs. healthy controls).
A model for LVAD associated aVWS was established, which demonstrated that ADAMTS-13 activity is essential for the depletion of HMWM of VWF that occurs in VAD-associated aVWS.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.