Stress and Exposure Time on von Willebrand Factor Degradation

Abstract

Despite the prevailing use of the continuous flow left ventricular assist devices (cf-LVADs), acquired von Willebrand syndrome (AvWS) associated with cf-LVADs still remains a major complication. As AvWS is known to be dependent on shear stress ( ) and exposure time (t exp ), this study examined the degradation of high molecular weight multimers (HMWM) of von Willebrand factor (vWF) in terms of τ and t exp . Two custom apparatus, i.e., capillary-tubing-type degrader (CTD) and Taylor-Couettetype degrader (TCD) were developed for short-term (0.033 sec ≤ t exp ≤ 1.05 s) and long-term (10 s ≤ t exp ≤ 10 min) shear exposures of vWF, respectively. Flow conditions indexed by Reynolds number (Re) for CTD were 14 ≤ Re ≤ 288 with corresponding laminar stress level of 52 ≤ CTD ≤ 1042 dyne/ cm 2 . Flow conditions for TCD were 100 ≤ Re ≤ 2500 with corresponding rotor speed of 180 ≤ ω o ≤ 4000 RPM and laminar stress level of 50 ≤ TCD ≤ 1114 dyne/cm 2 . Due to transitional and turbulent flows in TCD at Re > 1117, total stress (i.e., total = laminar + turbulent) was also calculated using a computational fluid dynamics (CFD) solver, Converge CFD. Inhibition of ADAMTS13 with different concentrations of EDTA (5 mM and 10 mM) was also performed to investigate the mechanism of cleavage in terms of mechanical and enzymatic aspects. Degradation of HMWM with CTD was negligible at all given testing conditions. Although no degradation of HMWM was observed with TCD at Re < 1117 ( total = 1012 dyne/cm 2 ), an increase in degradation of HMWM was observed beyond Re of 1117 for all given exposure times. At Re ~ 2500 ( total = 3070 dyne/ cm 2 ) with t exp = 60 s, a severe degradation of HMWM (90.7 ± 3.8%, abnormal) was observed, and almost complete degradation of HMWM (96.1 ± 1.9%, abnormal) was observed with t exp = 600 s. The inhibition studies with 5 mM EDTA at Re ~ 2500 showed that loss of HMWM was negligible (<10%, normal) for all given exposure times except for t exp = 10 min (39.5 ± 22.3%, borderline-abnormal).

Pathologic Shear and Elongation Rates Do Not Cause Cleavage of Von Willebrand Factor by ADAMTS13 in a Purified System

“…For example, turbulent flow conditions are present in severe AS 10,26 and these have been shown to cause excessive cleavage in the presence of ADAMTS13 by our group and others. 12,28 Prior studies conducted by Schneider et al reported VWF extension at SR above 1000 s À1 . 44 At such rates the platelet binding capacity of VWF, mediated by the A1 domain, might be substantial.…”

“…Platelets contain the VWF's binding receptor platelet glycoprotein Ib alpha (GP1ba), and VWF/platelet binding could contribute to cleavage by increasing the force across the VWF molecule. 20,28 Since the goal of our biophysical study was to evaluate the role of elongation and shear rates on VWF cleavage by ADAMTS13, we utilized recombinant VWF (VonvendiÒ) as all molecules in the sample are of similar size. This allowed us to limit the potential variability associated with the heterogeneous size distribution of VWF in plasma that results from the presence of ADAMTS13.…”

“…As such, we could not simulate the effects of secondary and turbulent flows that had been reported to play a role in VWF cleavage. 12,28 Second, the CFD model assumes an axisymmetric idealized aortic valve using physiological velocity waveforms as input data. These simulations act like an axisymmetric constriction, as opposed to a more complex AS that involves potentially calcified leaflets and an aortic sinus.…”

Artificial Organs 2019: A year in review

“…Choon‐Sik Jhun et al of the Penn State College of Medicine, Hershey, PA, USA investigated the dependence of shear stress (τ) and exposure time (t exp) of acquired von Willebrand syndrome (AvWS) with continuous flow left ventricular assist devices (cf‐LVADs). They investigated these variables on the degradation of high molecular weight multimers (HMWM) of vWF.…”

Interplay of pump design elements and bleeding predilection—Mechanisms for a forward momentum

“…6 With the advent of rotary blood pumps, this figurative "blender" phenomenon appeared as a common denominator for all such devices and became accepted as a necessary shortcoming of this technology. Curiously, while it was clear that high shear stress in the continuous flow LVADs reduces the levels of HMvWF, 7 it was less obvious how or if different pump designs may influence the integrity of these multimers.…”