Left ventricular assist devices (LVAD) provide cardiac support for patients with end-stage heart disease as either bridge or destination therapy, and have significantly improved the survival of these patients. Whereas earlier models were designed to mimic the human heart by producing a pulsatile flow in parallel with the patient’s heart, newer devices, which are smaller and more durable, provide continuous blood flow along an axial path using an internal rotor in the blood. However, device-related hemostatic complications remain common and have negatively affected patients’ recovery and quality of life. In most patients, the von Willebrand factor (VWF) rapidly loses large multimers and binds poorly to platelets and subendothelial collagen upon LVAD implantation, leading to the term acquired von Willebrand syndrome (AVWS). These changes in VWF structure and adhesive activity recover quickly upon LVAD explantation and are not observed in patients with heart transplant. The VWF defects are believed to be caused by excessive cleavage of large VWF multimers by the metalloprotease ADAMTS-13 in an LVAD-driven circulation. However, evidence that this mechanism could be the primary cause for the loss of large VWF multimers and LVAD-associated bleeding remains circumstantial. This review discusses changes in VWF reactivity found in patients on LVAD support. It specifically focuses on impacts of LVAD-related mechanical stress on VWF structural stability and adhesive reactivity in exploring multiple causes of AVWS and LVAD-associated hemostatic complications.

The first ventricular assist device (VAD) was implanted by Michael E. Debakey in 1966 at the Methodist Hospital in Houston, Texas. The interest in developing a device for artificially supporting the circulation was further increased by the first total artificial heart implantation, performed by Denton A. Cooley in Houston on April 4, 1969.1  In 1986, O. H. Frazier and his colleagues began implanting the first generation of pneumatic pulsatile LVADs (the Heartmate 1000 IP) to improve end-organ perfusion in candidates for heart transplant.2  The patients with LVAD implants experienced significant improvements in long-term survival. The first continuous-flow LVAD—the Hemopump—was subsequently implanted in 1988.3  It was followed by successful implantations of the HeartMate XVE in 1991,4  the Jarvick 2000 in 2000,5  and the HeartMate2 (Figure 1) in 2003 by O. H. Frazier and his colleagues at St. Luke’s Hospital in Houston, Texas.6  Continuous-flow LVADs are designed to provide blood flow of up to 10 L/min by generating a continuous blood flow from the inlet through the revolving impeller to the outlet. They energize blood flow throughout the cardiac cycle, thereby activating the normally passive flow of diastole. This continuous and unidirectional flow allows for a simplified device design and a smaller size, because it obviates the need for unidirectional valves.

Figure 1

Schematic illustration of an implanted continuous-flow HeartMate2 LVAD (with permission from Thoratec Corporation).

Figure 1

Schematic illustration of an implanted continuous-flow HeartMate2 LVAD (with permission from Thoratec Corporation).

Close modal

The axial continuous-flow HeartMate2 is currently the device approved by the U.S. Food and Drug Administration (FDA) for both bridge-to-transplant (BTT)7  and destination therapy (DT).8  It has been implanted in >15 000 patients worldwide.9  The centrifugal continuous-flow Heartware HVAD is an intrapericardial pump with an impeller suspended by a passive magnetic and hydrodynamic bearing that produces contact-free rotation (FDA approved for BTT10 ).

Heart failure is a severe medical condition affecting more than 5 million individuals and it is a leading cause of death in the US.11  Cardiac transplantation remains a valid option in the treatment of these patients. However, only 2000 to 2500 transplants are performed each year because of limited organ availability. More than 4000 patients are currently on the waiting list (www.unos.org/data/transplant-trends/#waitlists_by_organ), and this number is expected to grow substantially because of our increasingly aging population. Patients on the waiting list have a significantly higher rate of mortality than those who receive the implants. LVADs were introduced as a rescue strategy for patients dying while on the heart transplant waiting list. They were used as a temporary solution until a donor heart could be transplanted (BTT).2  The early positive results of BTT led the FDA to approve the use of LVADs as an alternative to heart transplants, serving as a destination therapy.12  In some cases, LVADs have been shown to restore sufficient cardiac function and end-organ perfusion that patients who were previously ineligible for heart transplants eventually met transplantation criteria (bridge to decision). In rare cases, patients have had their LVADs successfully explanted (bridge to recovery).13,14  All surgical interventions alter internal anatomy to a condition that is not normal, but is better than the diseased condition it corrects. In the case of LVAD, the alteration is to the blood flow. Although patients have survived with normal activity for >10 years without a pulse, a LVAD can result in complications in some patients related to the unique levels and patterns of blood flow created by the device.

Major LVAD-associated complications include bleeding, neurologic dysfunction (transient ischemic attack and stroke), pump thrombosis, infection, right ventricular failure, and aortic regurgitation as codified by the Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS).15  The registry collects clinical data on mechanical circulatory support devices from index hospitalization through follow-up evaluation on 16 268 patients from 163 centers (as of August 2015).

Neurologic dysfunction, pump thrombosis, and bleeding are common hemostatic complications. A stroke rate of 0.064 event per patient per year (EPPY) was reported in a 7.5-year study of 230 patients implanted with the HeartMate2 as either BTT (80.4%) or DT (19.6%),16  significantly higher than the 0.013 to 0.035 EPPY reported for patients with advanced heart failure.8,17  The stroke rates for the Heartware were 0.11 and 0.09 EPPY for ischemic and hemorrhagic stroke, respectively.10  LVAD-supported patients who experienced stroke had twice the risk of death as stroke-free patients.16 

Pump thrombosis is usually located at the bearing of the device and presents at inspection as a chronic fibrous pannus formation that responds poorly to thrombolytic therapy. Pump exchange is the treatment of choice.18  The incidence of pump thrombosis with the HeartMate2 was 2% to 4%, with 0.03 EPPY in the first year in a BTT trial7 ; and 0.024 EPPY over a 2-year period in a DT trial.8 

LVAD-associated bleeding is the most frequent complication, occurring in 19% to 40% of patients on HeartMate2 support.19,20  INTERMACS defines LVAD-related bleeding as episodes of internal or external bleeding that result in death, reoperation, hospitalization, or transfusion of packed red blood cells (>4 U in the first 7 days, or any units >7 days after the implant). Bleeding within 7 days is primarily caused by surgery and the use of anticoagulants after the LVAD implantation. However, nonsurgical bleeding that occurs later has emerged as a major challenge to the recovery of patients on LVAD support. The mean time for nonsurgical bleeding and the rate of recurrent bleeding after LVAD implant varies significantly among published reports, ranging from 10 to 154 days,21-23  and 9.3% to 60%, respectively.22,24,25  Gastrointestinal (GI) bleeding and epistaxis are more common than mediastinum/thorax and intracranial bleeding.26-30  The incidence of GI bleeding increased significantly, from 0.068 EPPY for pulsatile LVADs to 0.63 EPPY for continuous-flow LVADs.31 

Patients with clinical GI bleeding are treated in consultation with gastroenterologists, undergoing colonoscopy, upper endoscopic evaluation, and/or small bowel capsule endoscopy. In the presence of persistent GI bleeding with negative endoscopic findings, a tagged red blood scan or angiography is recommended. Optimal anticoagulation levels and an antiplatelet regimen for patients on LVAD support is an active area of research. In the absence of evident pump dysfunction, current guidelines32  recommend holding anticoagulation and antiplatelet therapy in the presence of clinically significant bleeding. During this holding period, strict clinical follow-ups are imperative because of the increased risk for device malfunction, pump thrombosis, and ischemic stroke.32,33  The US-TRACE study found that recurrent bleeding was reported in as much as 52% of cases despite the reduction in antithrombotic therapy, suggesting that anticoagulation might be only partially responsible for the hemorrhagic complications in these patients.33  Anticoagulation and antiplatelet therapy can be reintroduced with careful monitoring when the bleeding episode has resolved. In the case of refractory bleeding, the indication for warfarin and target international normalized ratio should be re-evaluated depending on the bleeding severity and pump types.32  Octreotide and thalidomide have been reported as off-label adjuvants for controlling recurrent GI bleeding,34-36  but neither has been systematically studied for the treatment of LVAD-related bleeding. Thalidomide is also associated with an increased risk of thrombosis.25,37 

Attempts have been made to operate a continuous-flow device at a lower speed to reduce shear stress and to reintroduce pulsatility. At a lower LVAD speed, the native heart may increase (transiently and in systole only) the total cardiac output with minimal stroke volume. This approach has not been fully validated and may limit the amount of LVAD support to the native heart. New-generation devices have incorporated operational algorithms to introduce cardiac output variations over several cardiac cycles. These algorithms aim at simulating an arterial pulse pressure and, if effective, may reduce LVAD-induced bleeding. Additional information related to the clinical presentation and managements of LVAD-associated hemostatic defects is discussed by Susen et al in a recent review.38  In this review, we focus on the underlying mechanisms of LVAD-associated VWF abnormalities and their contributions to clinical bleeding.

GI bleeding is often found at the site of an arteriovenous malformation (AVM).39,40  There are at least 2 theories regarding how an AVM is formed. One is that high intraluminal pressure causes reflective and localized smooth muscle relaxation and arteriovenular dilation.39  The other is that intestinal hypoperfusion resulting from reduced pulse pressure leads to regional hypoxia, vascular dilation, and subsequent angiodysplasia.26  However, neither theory explains why the AVM is primarily formed in the GI track. Nevertheless, it was the association of AVM with GI bleeding that led to the hypothesis that LVAD-associated bleeding is similar to aortic valve stenosis (Heyde syndrome)41  in that it is caused by the deficiency of the adhesive ligand VWF and thus is often called acquired von Willebrand syndrome (AVWS).31,42,43 

VWF mediates platelet tethering to the subendothelial matrix at sites of vascular injury to initiate hemostasis. It also protects the coagulation factor VIII (FVIII) against proteolytic degradation by forming a VWF–FVIII complex in the circulation.44  As an acute-phase reactant, VWF synthesis and secretion is rapidly and significantly increased in response to inflammatory and ischemic insults. Elevated levels of VWF antigen and/or adhesion to platelets are, therefore, widely used as markers for endothelial perturbation and propensity for thrombosis and thromboembolism.

VWF is synthesized in megakaryocytes and endothelial cells as a single-chain propolypeptide of 2813 amino acids.44  A mature VWF monomer contains 4 types of repeated domains in the order of D1-D2-D′-D3-A1-A2-A3-D4-B1-B2-B3-C1-C2-CK (Figure 2A). The D′ and D3 domains mediate VWF multimerization and FVIII binding.45-48  The A1 and A3 domains contain the binding sites for the platelet glycoprotein (GP) Ib-IX-V complex and the subendothelial collagen, respectively. The A2 domain contains the Tyr1605-Met1606 peptide bond that is cleaved by a disintegrin and metalloprotease with thrombospondin type 1 repeat 13 (ADAMTS-13).49-51  The C domain contains an RGD sequence that interacts with integrins. Free thiols in the C domain are also involved in the shear-induced lateral association of VWF multimers to form highly adhesive fibrils.52,53 

Figure 2

VWF domain structure and potential lateral association. (A) VWF domain structure. ProVWF forms disulfide-linked dimers (B) that further multimerize covalently to multimers in the Golgi (C). These multimers are packaged in the storage granule Weibel-Palade body of endothelial cells (D), where the multimerization process is likely to continue, but factors that regulate its rate and termination remain largely unknown. The stored VWF multimers are released from endothelial cells that are activated by inflammatory stimuli. These newly released VWF multimers form fibrillary meshes that capture platelets (E) and are proteolytically released from the surface of endothelial cells by ADAMTS-13 (F).

Figure 2

VWF domain structure and potential lateral association. (A) VWF domain structure. ProVWF forms disulfide-linked dimers (B) that further multimerize covalently to multimers in the Golgi (C). These multimers are packaged in the storage granule Weibel-Palade body of endothelial cells (D), where the multimerization process is likely to continue, but factors that regulate its rate and termination remain largely unknown. The stored VWF multimers are released from endothelial cells that are activated by inflammatory stimuli. These newly released VWF multimers form fibrillary meshes that capture platelets (E) and are proteolytically released from the surface of endothelial cells by ADAMTS-13 (F).

Close modal

Once synthesized, pro-VWF monomers first dimerize through C-terminal disulfide bonds.54  A variable number of dimers then multimerize through the N-terminal disulfide bonds55-58  after the proteolytic removal of a 741-amino-acid propeptide (Figure 2B-C).54,59-62  The newly synthesized VWF multimers are either constitutively released or stored in the Weibel-Palade bodies (Figure 2D) of endothelial cells and the α-granules of megakaryocytes and platelets.63,64  VWF multimers stored in these granules are enriched in large and ultralarge (UL) forms that are released from activated endothelial cells.62,63,65-67  Newly released VWF multimers can be anchored to the surface of endothelial cells to form elongated and membrane-anchored VWF fibrils that are cleaved by ADAMTS-13 and released from the cells (Figure 2E).68,69  It is because of this unique structure that VWF adhesive activity is determined not only by antigen level, but also by the size of multimers, with large and UL multimers being most active in hemostasis and thrombosis/thromboembolism, respectively.56,70  This large and flexible structure also renders VWF prone to undergoing changes in conformation and adhesive activity in response to fluidic mechanical forces that are closely associated with LVAD implantation.

The most drastic change brought by a continuous LVAD is the conversion of a pulsatile flow of blood to a continuous flow with a significantly elevated shear rate and stress, not only in the device, but also systemically. An increase in fluid shear stress above certain thresholds is known to alter the structure of VWF multimers to induce VWF cleavage by ADAMTS-13 and also VWF binding to platelets (Figure 3).71-73 

Figure 3

Shear-induced platelet activation. Platelets in diluted PRP or whole blood subjected to fluid shear in a cone-plate viscometer abruptly undergo activation above a critical wall shear stress of 8-10 Pa (arrow). Inset shows that this activation is a strong function of the mechanical force applied on the platelet GP Ibα by the VWF that is bound to it. Here, doubling media viscosity more than doubles the extent of cell activation (adapted from Shankaran et al71 ).

Figure 3

Shear-induced platelet activation. Platelets in diluted PRP or whole blood subjected to fluid shear in a cone-plate viscometer abruptly undergo activation above a critical wall shear stress of 8-10 Pa (arrow). Inset shows that this activation is a strong function of the mechanical force applied on the platelet GP Ibα by the VWF that is bound to it. Here, doubling media viscosity more than doubles the extent of cell activation (adapted from Shankaran et al71 ).

Close modal

Among the continuous-flow devices, axial-flow LVADs have a high impeller speed that results in a high local shear rate and stress. Platelets in this device experience stresses of >10 Pa (1 Pa = 1 N/m2 = 10 dynes/cm2) during their passage, and some are exposed to stress as high as 600 Pa (Table 1).74-77  A centrifugal pump has a lower impeller velocity than an axial pump and lower wall shear stress throughout the device.76,77  Regions of high, nonphysiologic flow are formed in axial pumps with an estimated wall shear stress of >10 Pa, as is found in most of the devices.76,78  However, a recent study that compared the performance of centrifugal and axial-flow LVADs in 102 patients showed an equivalent loss of large VWF multimers and similar incidences of bleeding and thromboembolic events.79 

Table 1

Typical range of wall shear rates and stresses in blood vessels

Blood vesselShear rate (/s)Shear stress (Pa)
Large arteries* 300-800 1.4-3.6 
Arterioles* 450-1600 2-7.2 
Veins* 15-200 0.07-0.9 
Stenotic vessels* 800-10 000 3.6-45 
Axial flow LVAD — 600 
Centrifugal flow LVAD — 150-230 
Blood vesselShear rate (/s)Shear stress (Pa)
Large arteries* 300-800 1.4-3.6 
Arterioles* 450-1600 2-7.2 
Veins* 15-200 0.07-0.9 
Stenotic vessels* 800-10 000 3.6-45 
Axial flow LVAD — 600 
Centrifugal flow LVAD — 150-230 
*

Values for different vessels are adapted from Kroll et al74 ; mean wall shear rate/stress values are presented. Peak values could be higher.

Present peak value in typical devices based on experimental measurements and computational fluid dynamics calculations (blood flow rate was 4-7 L/min). These values were summarized from Girdhar et al,75  Selgrade and Truskey,76  and Fraser et al.77 

A growing body of evidence suggests that a local shear stress in excess of 10 Pa can result in the application of forces >∼10-15 pN (piconewton = 10−12 N), which induces VWF to undergo conformational transition from a “resting” to an “active” state80  and to self-associate.71,81  Similar magnitudes of fluidic stress also promote the proteolysis by ADAMTS-13 of the cryptic Tyr1605-Met1606 peptide bond in the VWF A2-domain.72,73  In this regard, whereas VWF coexists with ADAMTS-13 in plasma, it remains mostly uncleaved unless the Tyr1605-Met1606 scissile bond is exposed by mechanical forces in solution82  or upon binding to a platelet,83  or collagen,84  or being anchored to endothelial cells.68  The force required for inducing the cleavage lies in the range of 7 to 14 pN that unfolds the VWF A2 domain.72  A slightly larger force is needed to unfold and cleave a recombinant VWF A1-A2-A3 tri-domain.73  More importantly, this magnitude of force also induces and strengthens VWF binding to platelet GP Ibα.85,86  These opposing effects of high shear stress on VWF raise the important question of which of the 2 processes (excessive cleavage by ADAMTS-13 or shear-induced binding to platelets) occurs predominantly in an LVAD-driven circulation and how the 2 processes are differentially regulated by confounding factors.

Two scenarios should be considered in elucidating the effects of mechanical forces on adhesive reactivity and cleavage when VWF is (1) subjected to shear stress in solution, because this can occur during its passage through an LVAD; and (2) attached to the biomaterials that compose the prosthetic device or to the surface of platelets or endothelial cells.

In the first scenario, VWF in flowing blood experiences periodic variations in shear forces as it tumbles through the circulation. Shear forces are at their maximal level when the protein is oriented either parallel or perpendicular to the direction of flow, and stretching forces that pull the molecule apart are at their maximum level when the protein is oriented at 45° to the flow direction.87  These oscillatory force cycles repeat every ∼10 msec when the applied shear stress is 10 Pa, with increasing shear stress proportionally decreasing the duration of force oscillation.87  Biophysical calculations based on protein geometry show that the maximal force applied to VWF during these periodic cycles is tensile in nature (rather than shear). This tensile force tends to stretch or unfold VWF multimers87  and is maximal at the geometric center of VWF when it is oriented at 45° to the direction of flow. For a VWF protomer composed of two C-terminal dimerized VWF monomers, this peak tensile force is estimated to be Fp ∼0.01 · τ, where Fp is the applied force in pN and τ is the applied shear stress in Pa.87,88  Extending this estimate to multimeric VWF with Np dimeric repeats stretched into a maximally extended linear structure, the force applied to multimeric VWF is approximately Fmulti= Fp · Np2/2.72  This derivation assumes that the force applied at the center of a fully extended VWF multimer can be estimated by summing up the hydrodynamic forces on VWF doublets that were located more outward, toward the ends of the VWF multimer. Thus, at a shear stress of 10 Pa, the peak force on a 20-mer VWF multimer would be 20 pN (0.01 × 10 × (20)2/2) applied at the midpoint of the molecule.

Studies using single-molecule fluorescence microscopy,80  small-angle neutron scattering (SANS),89  and fluorescence spectroscopy90  have suggested that VWF can undergo structural transitions under shear stresses of ∼8-10 Pa. More specifically, the fluorescence microscopy reveals a large-scale extension of VWF, with end-to-end lengths of up to 10 μm at a shear stress of >5 Pa.80  At shear stresses of >8 Pa, VWF multimers also bind to a fluorescent probe bis-ANS that recognizes nonpolar cavities exposed on the sheared protein.90  These hydrophobic pockets gradually become buried over a period of time ranging from minutes to hours, suggesting that once unfolded, proteins may not recover their native conformation immediately upon the removal of shear stress. This delayed recovery is also demonstrated in the long relaxation time of VWF fibrils returning back to their original conformation after a mechanical force applied to induce domain-unfolding was removed.52,81  A shear stress of ∼8 Pa is also necessary for VWF self-association in solution as measured using light-scattering.71  Similar magnitudes of forces enable the formation of VWF bundles or fibers in synthetic microvessels that are lined with endothelial cells.91  Thus, ∼8-10 Pa represents a critical shear threshold at which a number of investigators have noted protein structural changes measured using different techniques.

Stretching forces are also applied when VWF multimers form a bridge between 2 platelets that are tumbling together in a shear field.83,87  In this situation, the force applied to a VWF at a given shear rate would be 2 to 3 orders of magnitude greater than the force applied to free VWF in solution. Given the size of a typical human platelet, this force is estimated to be Fplat-plat(pN) ∼45 · τ.87,88  Thus, the bridging of 2 platelets by VWF at 10 Pa would result in the application of a ∼450 pN force on both the VWF multimers and the platelet GP Ibα. This force estimate may explain why VWF cleavage by ADAMTS-13 is enhanced when the VWF multimers are platelet-bound.83  The formation of VWF-crosslinked platelets through supraphysiologic shear in an LVAD would, therefore, increase force application, leading to VWF-mediated mechanotransduction/platelet activation and VWF proteolysis.71,92,93 

In the second scenario, shear-driven VWF immobilization on such substrates as collagen and the biomaterial surfaces of LVADs could result in changes to VWF conformation. This was originally demonstrated using atomic force microscopy on hydrophobic substrates in which the application of wall shear stress >3.5 Pa triggered VWF unfolding.94  Extended VWF fiber meshes have also been reported on collagen substrates at shear stresses >10 Pa.84,95  These extended VWF bundles may themselves be susceptible to cleavage by ADAMTS-13, because the attachment of platelets to the immobilized VWF further enhances the force applied to both VWF and GP Ibα. The force applied to platelet-bound VWF strings can be estimated at ∼76 τ pN.88  At 10 Pa, a 760-pN force would, therefore, be applied to both VWF and the attached platelet GP Ibα, triggering both platelet mechanotransduction that activates platelets and ADAMTS-13–mediated proteolysis. Overall, when the applied shear stress exceeds the physiologic range, as is found in an LVAD-driven circulation, biophysical processes are simultaneously triggered that together promote both VWF binding to GP Ibα (and subsequent platelet activation) and its proteolysis by ADAMTS-13.

Finally, Da et al report that free hemoglobin augments platelet adhesion and microthrombus formation on an extracellular matrix at high shear stresses in a GP Ib and VWF-dependent manner.96  Hemoglobin is also reported to sterically hinder ADAMTS-13–VWF interaction.97  These findings are relevant because hemolysis often occurs in patients on LVAD support and is associated with thrombotic events.98-101  They also raise an important question regarding the potential redox-regulation of VWF reactivity and cleavage by hemoglobin, which is often called a “biological Fenton’s reagent” for its strong oxidative activity (from the heme iron).102  Oxidative stress has indeed been shown to make VWF hyper-reactive, render them resistant to ADAMTS-13, and inhibit ADAMTS-13 activity.103-106  Some of redox-sensitive residues in VWF and ADAMTS-13 are exposed by the high shear stress104  found in the LVAD-driven circulation.

LVAD-associated AVWS has so far been defined by laboratory findings that plasma VWF from patients on LVAD support lacks large multimers (Figure 4)31,42,79,107-110  and has reduced activity in agglutinating lyophilized platelets (ristocetin cofactor activity, VWF:Rco) and in binding the collagen matrix (VWF:CB).31,79,107,108,111-115  These VWF defects are observed soon after LVAD implantation,42,43  rapidly resolve after LVAD explantation,109,111-113  and were not observed in heart transplant recipients,31,43  indicating their close association with the level and pattern of the LVAD-driven blood flow. The rapid loss of large VWF multimers and their quick recovery was also observed in a rabbit model of reversible arterial stenosis to alter blood flow.112 

Figure 4

VWF multimer found in patients on LVAD. Examples of VWF multimer patterns found in normal plasma pooled from 32 healthy subjects and in patients on LVAD support who developed myocardial infarction (Patient A), no complication (Patient B), and severe GI bleeding (Patient C). Samples were collected before LVAD implant (BL), 3 months after implant (3), and at the time of readmission for the clinical complications (E).

Figure 4

VWF multimer found in patients on LVAD. Examples of VWF multimer patterns found in normal plasma pooled from 32 healthy subjects and in patients on LVAD support who developed myocardial infarction (Patient A), no complication (Patient B), and severe GI bleeding (Patient C). Samples were collected before LVAD implant (BL), 3 months after implant (3), and at the time of readmission for the clinical complications (E).

Close modal

One widely held view is that the loss of VWF large multimers and reduction in VWF adhesive activity are caused by excessive cleavage of VWF by ADAMTS-13 under a persistently elevated level of shear stress in the LVAD-supported circulation. This shear-induced cleavage reduces VWF size and adhesive activity in a pattern similar to patients with type 2A VWD.31,79,107,108,116  However, this tentative mechanism has not been experimentally validated, and significant gaps remain. First, there is no direct evidence that VWF multimers are excessively cleaved by ADAMTS-13 in patients on LVAD support. Alternative VWF cleavage by other proteases such as the serine protease granzyme,117,118  plasmin,119  and Staphylococcus aureus V-8 protease,58,120  and its effect on VWF reactivity in a LVAD setting, have not been examined. For example, it has been shown that granzyme secreted from immune and cytotoxic lymphocytes in conditions of inflammation proteolytically reduces VWF adhesive activity117-119  and disrupts VWF–FVIII interaction.118  The serine protease plasmin cleaves VWF fibrils that are resistant to ADAMTS-13.119  Second, VWF antigen was significantly increased in all patients,79,108,109  a condition that is not common in patients with type 2 VWD.121,122  The question is whether the loss of large VWF multimers can be compensated for by an increase in VWF antigen. Third, the loss of large VWF multimers has been observed in nearly all patients, but only a small portion of them have clinically relevant bleeding. More importantly, neither the loss of large VWF multimers nor a reduced VWF:Rco activity is associated with bleeding, thrombosis, or the need for transfusion in patients on LVAD support.79,87,109,114,123  Although this lack of association may be attributed to clinical variations among patients, it is also possible that AVWS constitutes a baseline condition that has minimal clinical consequences on its own, and that requires a trigger or additive event to propagate bleeding and/or thrombosis in patients on LVAD support. Fourth, while it facilitates VWF cleavage by ADAMTS-13,72,73,82  a high fluid shear stress or a tensile force also unfolds VWF to bind and activate platelets,74,81,124-126  potentially leading to a consumptive deficiency in large VWF multimers, similar to patients with type 2B VWD. This shear-induced platelet aggregation depends on the attachment of large VWF multimers to GP Ibα and the activation of integrin αIIbβ3.125,127,128  This gain-of-function VWF phenotype can not only remove large VWF multimers from the circulation, but also occlude cerebral and GI microvessels, producing punctate injury, bleeding, and AVM. In this case, a patient with deficient large VWF multimers may be prone to thrombosis, not bleeding. If the theory of shear-induced platelet aggregation is to be credence, one may consider that shear- and/or oxidative stress-mediated VWF binding to platelets is effectively prevented in most patients by therapeutic agents that block platelet ADP receptors (clopidogrel) and/or integrin αIIbβ3 (tirofiban, eptifibatide).129,130  The patients who develop bleeding or thromboembolism may have an inadequate therapeutic blockade of platelets in terms of the dosage, type, or protocol used during their care. Furthermore, aspirin is widely used to inhibit platelet activation in patients on LVAD support, but has been found to be ineffective in vitro to prevent shear-induced and VWF-mediated aggregation of platelets.125  A distinction between excessive VWF cleavage (loss-of-function) and enhanced VWF binding to platelets (gain-of-function) may, therefore, provide further insights into the causes of LVAD-induced loss of large VWF multimers and its association with bleeding and thrombosis.

The evidence in support of the excessive cleavage of large VWF multimers by ADAMTS-13 as a primary cause of LVAD-associated bleeding remains circumstantial. A large body of clinical and laboratory evidence suggests that the LVAD-associated loss of large VWF multimers can also be caused by shear- and oxidative stress–induced VWF binding to platelets. This conjecture is consistent with a recent report that thromboembolic events were 7.4 times as likely to occur in LVAD patients who had experienced prior GI bleeding.121  Defining this phenotypic diversity may hold the key to understanding the contribution of AVWS to the bleeding and thrombotic complications associated with LVAD implantation. It may also be essential for devising effective preventative and therapeutic strategies for the debilitating complications.

This work is supported by National Institute of Health, National Heart, Lung, and Blood Institute grants HL77258 (S.N.), and HL71895 and HL125957 (J.-f.D.).

A.N., S.N., and J.-f.D. conducted background research, and all authors wrote the manuscript.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Jing-fei Dong, BloodWorks Research Institute, 1551 Eastlake Ave East, Seattle, WA 98102; e-mail: jfdong@bloodworksnw.org.

1
Cooley
 
DA
Liotta
 
D
Hallman
 
GL
Bloodwell
 
RD
Leachman
 
RD
Milam
 
JD
Orthotopic cardiac prosthesis for two-staged cardiac replacement.
Am J Cardiol
1969
, vol. 
24
 
5
(pg. 
723
-
730
)
2
Frazier
 
OH
Rose
 
EA
McCarthy
 
P
et al. 
Improved mortality and rehabilitation of transplant candidates treated with a long-term implantable left ventricular assist system.
Ann Surg
1995
, vol. 
222
 
3
(pg. 
327
-
336, discussion 336-338
)
3
Frazier
 
OH
Wampler
 
RK
Duncan
 
JM
et al. 
First human use of the Hemopump, a catheter-mounted ventricular assist device.
Ann Thorac Surg
1990
, vol. 
49
 
2
(pg. 
299
-
304
)
4
Frazier
 
OH
First use of an untethered, vented electric left ventricular assist device for long-term support.
Circulation
1994
, vol. 
89
 
6
(pg. 
2908
-
2914
)
5
Frazier
 
OH
Gregoric
 
ID
Delgado
 
RM
et al. 
Initial experience with the Jarvik 2000 left ventricular assist system as a bridge to transplantation: report of 4 cases.
J Heart Lung Transplant
2001
, vol. 
20
 
2
pg. 
201
 
6
Frazier
 
OH
Delgado
 
RM
Kar
 
B
Patel
 
V
Gregoric
 
ID
Myers
 
TJ
First clinical use of the redesigned HeartMate II left ventricular assist system in the United States: a case report.
Tex Heart Inst J
2004
, vol. 
31
 
2
(pg. 
157
-
159
)
7
Miller
 
LW
Pagani
 
FD
Russell
 
SD
et al. 
HeartMate II Clinical Investigators
Use of a continuous-flow device in patients awaiting heart transplantation.
N Engl J Med
2007
, vol. 
357
 
9
(pg. 
885
-
896
)
8
Slaughter
 
MS
Rogers
 
JG
Milano
 
CA
et al. 
HeartMate II Investigators
Advanced heart failure treated with continuous-flow left ventricular assist device.
N Engl J Med
2009
, vol. 
361
 
23
(pg. 
2241
-
2251
)
9
Kirklin
 
JK
Naftel
 
DC
Pagani
 
FD
et al. 
Seventh INTERMACS annual report: 15,000 patients and counting.
J Heart Lung Transplant
2015
, vol. 
34
 
12
(pg. 
1495
-
1504
)
10
Aaronson
 
KD
Slaughter
 
MS
Miller
 
LW
et al. 
HeartWare Ventricular Assist Device (HVAD) Bridge to Transplant ADVANCE Trial Investigators
Use of an intrapericardial, continuous-flow, centrifugal pump in patients awaiting heart transplantation.
Circulation
2012
, vol. 
125
 
25
(pg. 
3191
-
3200
)
11
Lloyd-Jones
 
D
Adams
 
RJ
Brown
 
TM
et al. 
WRITING GROUP MEMBERS; American Heart Association Statistics Committee and Stroke Statistics Subcommittee
Heart disease and stroke statistics--2010 update: a report from the American Heart Association.
Circulation
2010
, vol. 
121
 
7
(pg. 
e46
-
e215
)
12
Rose
 
EA
Gelijns
 
AC
Moskowitz
 
AJ
et al. 
Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure (REMATCH) Study Group
Long-term use of a left ventricular assist device for end-stage heart failure.
N Engl J Med
2001
, vol. 
345
 
20
(pg. 
1435
-
1443
)
13
Birks
 
EJ
Tansley
 
PD
Hardy
 
J
et al. 
Left ventricular assist device and drug therapy for the reversal of heart failure.
N Engl J Med
2006
, vol. 
355
 
18
(pg. 
1873
-
1884
)
14
Mancini
 
DM
Beniaminovitz
 
A
Levin
 
H
et al. 
Low incidence of myocardial recovery after left ventricular assist device implantation in patients with chronic heart failure.
Circulation
1998
, vol. 
98
 
22
(pg. 
2383
-
2389
)
16
Harvey
 
L
Holley
 
C
Roy
 
SS
et al. 
Stroke After Left Ventricular Assist Device Implantation: Outcomes in the Continuous-Flow Era.
Ann Thorac Surg
2015
, vol. 
100
 
2
(pg. 
535
-
541
)
17
Pullicino
 
PM
Halperin
 
JL
Thompson
 
JL
Stroke in patients with heart failure and reduced left ventricular ejection fraction.
Neurology
2000
, vol. 
54
 
2
(pg. 
288
-
294
)
18
Stevenson
 
LW
Pagani
 
FD
Young
 
JB
et al. 
INTERMACS profiles of advanced heart failure: the current picture.
J Heart Lung Transplant
2009
, vol. 
28
 
6
(pg. 
535
-
541
)
19
Starling
 
RC
Naka
 
Y
Boyle
 
AJ
et al. 
Results of the post-U.S. Food and Drug Administration-approval study with a continuous flow left ventricular assist device as a bridge to heart transplantation: a prospective study using the INTERMACS (Interagency Registry for Mechanically Assisted Circulatory Support).
J Am Coll Cardiol
2011
, vol. 
57
 
19
(pg. 
1890
-
1898
)
20
Lahpor
 
J
Khaghani
 
A
Hetzer
 
R
et al. 
European results with a continuous-flow ventricular assist device for advanced heart-failure patients.
Eur J Cardiothorac Surg
2010
, vol. 
37
 
2
(pg. 
357
-
361
)
21
Tsiouris
 
A
Paone
 
G
Nemeh
 
HW
Brewer
 
RJ
Morgan
 
JA
Factors determining post-operative readmissions after left ventricular assist device implantation.
J Heart Lung Transplant
2014
, vol. 
33
 
10
(pg. 
1041
-
1047
)
22
Marsano
 
J
Desai
 
J
Chang
 
S
Chau
 
M
Pochapin
 
M
Gurvits
 
GE
Characteristics of gastrointestinal bleeding after placement of continuous-flow left ventricular assist device: a case series.
Dig Dis Sci
2015
, vol. 
60
 
6
(pg. 
1859
-
1867
)
23
Akhter
 
SA
Badami
 
A
Murray
 
M
et al. 
Hospital Readmissions After Continuous-Flow Left Ventricular Assist Device Implantation: Incidence, Causes, and Cost Analysis.
Ann Thorac Surg
2015
, vol. 
100
 
3
(pg. 
884
-
889
)
24
Morgan
 
JA
Paone
 
G
Nemeh
 
HW
et al. 
Gastrointestinal bleeding with the HeartMate II left ventricular assist device.
J Heart Lung Transplant
2012
, vol. 
31
 
7
(pg. 
715
-
718
)
25
Draper
 
KV
Huang
 
RJ
Gerson
 
LB
GI bleeding in patients with continuous-flow left ventricular assist devices: a systematic review and meta-analysis.
Gastrointest Endosc
2014
, vol. 
80
 
3
(pg. 
435
-
446.e1
)
26
Crow
 
S
John
 
R
Boyle
 
A
et al. 
Gastrointestinal bleeding rates in recipients of nonpulsatile and pulsatile left ventricular assist devices.
J Thorac Cardiovasc Surg
2009
, vol. 
137
 
1
(pg. 
208
-
215
)
27
Genovese
 
EA
Dew
 
MA
Teuteberg
 
JJ
et al. 
Incidence and patterns of adverse event onset during the first 60 days after ventricular assist device implantation.
Ann Thorac Surg
2009
, vol. 
88
 
4
(pg. 
1162
-
1170
)
28
Pagani
 
FD
Miller
 
LW
Russell
 
SD
et al. 
HeartMate II Investigators
Extended mechanical circulatory support with a continuous-flow rotary left ventricular assist device.
J Am Coll Cardiol
2009
, vol. 
54
 
4
(pg. 
312
-
321
)
29
Kirklin
 
JK
Naftel
 
DC
Kormos
 
RL
et al. 
Third INTERMACS Annual Report: the evolution of destination therapy in the United States.
J Heart Lung Transplant
2011
, vol. 
30
 
2
(pg. 
115
-
123
)
30
Suarez
 
J
Patel
 
CB
Felker
 
GM
Becker
 
R
Hernandez
 
AF
Rogers
 
JG
Mechanisms of bleeding and approach to patients with axial-flow left ventricular assist devices.
Circ Heart Fail
2011
, vol. 
4
 
6
(pg. 
779
-
784
)
31
Crow
 
S
Milano
 
C
Joyce
 
L
et al. 
Comparative analysis of von Willebrand factor profiles in pulsatile and continuous left ventricular assist device recipients.
ASAIO J
2010
, vol. 
56
 
5
(pg. 
441
-
445
)
32
Feldman
 
D
Pamboukian
 
SV
Teuteberg
 
JJ
et al. 
International Society for Heart and Lung Transplantation
The 2013 International Society for Heart and Lung Transplantation Guidelines for mechanical circulatory support: executive summary.
J Heart Lung Transplant
2013
, vol. 
32
 
2
(pg. 
157
-
187
)
33
Katz
 
JN
Adamson
 
RM
John
 
R
et al. 
TRACE study
Safety of reduced anti-thrombotic strategies in HeartMate II patients: A one-year analysis of the US-TRACE Study.
J Heart Lung Transplant
2015
, vol. 
34
 
12
(pg. 
1542
-
1548
)
34
Loyaga-Rendon
 
RY
Hashim
 
T
Tallaj
 
JA
et al. 
Octreotide in the management of recurrent gastrointestinal bleed in patients supported by continuous flow left ventricular assist devices.
ASAIO J
2015
, vol. 
61
 
1
(pg. 
107
-
109
)
35
Draper
 
K
Kale
 
P
Martin
 
B
Cordero
 
K
Ha
 
R
Banerjee
 
D
Thalidomide for treatment of gastrointestinal angiodysplasia in patients with left ventricular assist devices: case series and treatment protocol.
J Heart Lung Transplant
2015
, vol. 
34
 
1
(pg. 
132
-
134
)
36
Ray
 
R
Kale
 
PP
Ha
 
R
Banerjee
 
D
Treatment of left ventricular assist device-associated arteriovenous malformations with thalidomide.
ASAIO J
2014
, vol. 
60
 
4
(pg. 
482
-
483
)
37
Aggarwal
 
A
Pant
 
R
Kumar
 
S
et al. 
Incidence and management of gastrointestinal bleeding with continuous flow assist devices.
Ann Thorac Surg
2012
, vol. 
93
 
5
(pg. 
1534
-
1540
)
38
Susen
 
S
Rauch
 
A
Van Belle
 
E
Vincentelli
 
A
Lenting
 
PJ
Circulatory support devices: fundamental aspects and clinical management of bleeding and thrombosis.
J Thromb Haemost
2015
, vol. 
13
 
10
(pg. 
1757
-
1767
)
39
Letsou
 
GV
Shah
 
N
Gregoric
 
ID
Myers
 
TJ
Delgado
 
R
Frazier
 
OH
Gastrointestinal bleeding from arteriovenous malformations in patients supported by the Jarvik 2000 axial-flow left ventricular assist device.
J Heart Lung Transplant
2005
, vol. 
24
 
1
(pg. 
105
-
109
)
40
Demirozu
 
ZT
Radovancevic
 
R
Hochman
 
LF
et al. 
Arteriovenous malformation and gastrointestinal bleeding in patients with the HeartMate II left ventricular assist device.
J Heart Lung Transplant
2011
, vol. 
30
 
8
(pg. 
849
-
853
)
41
Vincentelli
 
A
Susen
 
S
Le Tourneau
 
T
et al. 
Acquired von Willebrand syndrome in aortic stenosis.
N Engl J Med
2003
, vol. 
349
 
4
(pg. 
343
-
349
)
42
Heilmann
 
C
Geisen
 
U
Beyersdorf
 
F
et al. 
Acquired Von Willebrand syndrome is an early-onset problem in ventricular assist device patients.
Eur J Cardiothorac Surg
2011
, vol. 
40
 
6
(pg. 
1328
-
1333, discussion 1233
)
43
Geisen
 
U
Heilmann
 
C
Beyersdorf
 
F
et al. 
Non-surgical bleeding in patients with ventricular assist devices could be explained by acquired von Willebrand disease.
Eur J Cardiothorac Surg
2008
, vol. 
33
 
4
(pg. 
679
-
684
)
44
Sadler
 
JE
Biochemistry and genetics of von Willebrand factor.
Annu Rev Biochem
1998
, vol. 
67
 (pg. 
395
-
424
)
45
Foster
 
PA
Fulcher
 
CA
Marti
 
T
Titani
 
K
Zimmerman
 
TS
A major factor VIII binding domain resides within the amino-terminal 272 amino acid residues of von Willebrand factor.
J Biol Chem
1987
, vol. 
262
 
18
(pg. 
8443
-
8446
)
46
Takahashi
 
Y
Kalafatis
 
M
Girma
 
JP
Sewerin
 
K
Andersson
 
LO
Meyer
 
D
Localization of a factor VIII binding domain on a 34 kilodalton fragment of the N-terminal portion of von Willebrand factor.
Blood
1987
, vol. 
70
 
5
(pg. 
1679
-
1682
)
47
Bahou
 
WF
Ginsburg
 
D
Sikkink
 
R
Litwiller
 
R
Fass
 
DN
A monoclonal antibody to von Willebrand factor (vWF) inhibits factor VIII binding. Localization of its antigenic determinant to a nonadecapeptide at the amino terminus of the mature vWF polypeptide.
J Clin Invest
1989
, vol. 
84
 
1
(pg. 
56
-
61
)
48
Shiltagh
 
N
Kirkpatrick
 
J
Cabrita
 
LD
et al. 
Solution structure of the major factor VIII binding region on von Willebrand factor.
Blood
2014
, vol. 
123
 
26
(pg. 
4143
-
4151
)
49
Furlan
 
M
Robles
 
R
Lämmle
 
B
Partial purification and characterization of a protease from human plasma cleaving von Willebrand factor to fragments produced by in vivo proteolysis.
Blood
1996
, vol. 
87
 
10
(pg. 
4223
-
4234
)
50
Tsai
 
HM
Physiologic cleavage of von Willebrand factor by a plasma protease is dependent on its conformation and requires calcium ion.
Blood
1996
, vol. 
87
 
10
(pg. 
4235
-
4244
)
51
Levy
 
GG
Nichols
 
WC
Lian
 
EC
et al. 
Mutations in a member of the ADAMTS gene family cause thrombotic thrombocytopenic purpura.
Nature
2001
, vol. 
413
 
6855
(pg. 
488
-
494
)
52
Choi
 
H
Aboulfatova
 
K
Pownall
 
HJ
Cook
 
R
Dong
 
JF
Shear-induced disulfide bond formation regulates adhesion activity of von Willebrand factor.
J Biol Chem
2007
, vol. 
282
 
49
(pg. 
35604
-
35611
)
53
Li
 
Y
Choi
 
H
Zhou
 
Z
et al. 
Covalent regulation of ULVWF string formation and elongation on endothelial cells under flow conditions.
J Thromb Haemost
2008
, vol. 
6
 
7
(pg. 
1135
-
1143
)
54
Wagner
 
DD
Lawrence
 
SO
Ohlsson-Wilhelm
 
BM
Fay
 
PJ
Marder
 
VJ
Topology and order of formation of interchain disulfide bonds in von Willebrand factor.
Blood
1987
, vol. 
69
 
1
(pg. 
27
-
32
)
55
Vischer
 
UM
Wagner
 
DD
von Willebrand factor proteolytic processing and multimerization precede the formation of Weibel-Palade bodies.
Blood
1994
, vol. 
83
 
12
(pg. 
3536
-
3544
)
56
Mayadas
 
TN
Wagner
 
DD
In vitro multimerization of von Willebrand factor is triggered by low pH. Importance of the propolypeptide and free sulfhydryls.
J Biol Chem
1989
, vol. 
264
 
23
(pg. 
13497
-
13503
)
57
Furlan
 
M
Von Willebrand factor: molecular size and functional activity.
Ann Hematol
1996
, vol. 
72
 
6
(pg. 
341
-
348
)
58
Girma
 
JP
Chopek
 
MW
Titani
 
K
Davie
 
EW
Limited proteolysis of human von Willebrand factor by Staphylococcus aureus V-8 protease: isolation and partial characterization of a platelet-binding domain.
Biochemistry
1986
, vol. 
25
 
11
(pg. 
3156
-
3163
)
59
Voorberg
 
J
Fontijn
 
R
Calafat
 
J
Janssen
 
H
van Mourik
 
JA
Pannekoek
 
H
Assembly and routing of von Willebrand factor variants: the requirements for disulfide-linked dimerization reside within the carboxy-terminal 151 amino acids.
J Cell Biol
1991
, vol. 
113
 
1
(pg. 
195
-
205
)
60
Journet
 
AM
Saffaripour
 
S
Wagner
 
DD
Requirement for both D domains of the propolypeptide in von Willebrand factor multimerization and storage.
Thromb Haemost
1993
, vol. 
70
 
6
(pg. 
1053
-
1057
)
61
Bonthron
 
DT
Handin
 
RI
Kaufman
 
RJ
et al. 
Structure of pre-pro-von Willebrand factor and its expression in heterologous cells.
Nature
1986
, vol. 
324
 
6094
(pg. 
270
-
273
)
62
Mayadas
 
TN
Wagner
 
DD
Vicinal cysteines in the prosequence play a role in von Willebrand factor multimer assembly.
Proc Natl Acad Sci USA
1992
, vol. 
89
 
8
(pg. 
3531
-
3535
)
63
Sporn
 
LA
Marder
 
VJ
Wagner
 
DD
Inducible secretion of large, biologically potent von Willebrand factor multimers.
Cell
1986
, vol. 
46
 
2
(pg. 
185
-
190
)
64
Brouland
 
JP
Egan
 
T
Roussi
 
J
et al. 
In vivo regulation of von willebrand factor synthesis: von Willebrand factor production in endothelial cells after lung transplantation between normal pigs and von Willebrand factor-deficient pigs.
Arterioscler Thromb Vasc Biol
1999
, vol. 
19
 
12
(pg. 
3055
-
3062
)
65
Tsai
 
HM
Nagel
 
RL
Hatcher
 
VB
Seaton
 
AC
Sussman
 
II
The high molecular weight form of endothelial cell von Willebrand factor is released by the regulated pathway.
Br J Haematol
1991
, vol. 
79
 
2
(pg. 
239
-
245
)
66
Counts
 
RB
Paskell
 
SL
Elgee
 
SK
Disulfide bonds and the quaternary structure of factor VIII/von Willebrand factor.
J Clin Invest
1978
, vol. 
62
 
3
(pg. 
702
-
709
)
67
Fowler
 
WE
Fretto
 
LJ
Hamilton
 
KK
Erickson
 
HP
McKee
 
PA
Substructure of human von Willebrand factor.
J Clin Invest
1985
, vol. 
76
 
4
(pg. 
1491
-
1500
)
68
Dong
 
JF
Moake
 
JL
Nolasco
 
L
et al. 
ADAMTS-13 rapidly cleaves newly secreted ultralarge von Willebrand factor multimers on the endothelial surface under flowing conditions.
Blood
2002
, vol. 
100
 
12
(pg. 
4033
-
4039
)
69
Chauhan
 
AK
Goerge
 
T
Schneider
 
SW
Wagner
 
DD
Formation of platelet strings and microthrombi in the presence of ADAMTS-13 inhibitor does not require P-selectin or beta3 integrin.
J Thromb Haemost
2007
, vol. 
5
 
3
(pg. 
583
-
589
)
70
Sporn
 
LA
Marder
 
VJ
Wagner
 
DD
von Willebrand factor released from Weibel-Palade bodies binds more avidly to extracellular matrix than that secreted constitutively.
Blood
1987
, vol. 
69
 
5
(pg. 
1531
-
1534
)
71
Shankaran
 
H
Alexandridis
 
P
Neelamegham
 
S
Aspects of hydrodynamic shear regulating shear-induced platelet activation and self-association of von Willebrand factor in suspension.
Blood
2003
, vol. 
101
 
7
(pg. 
2637
-
2645
)
72
Zhang
 
X
Halvorsen
 
K
Zhang
 
CZ
Wong
 
WP
Springer
 
TA
Mechanoenzymatic cleavage of the ultralarge vascular protein von Willebrand factor.
Science
2009
, vol. 
324
 
5932
(pg. 
1330
-
1334
)
73
Wu
 
T
Lin
 
J
Cruz
 
MA
Dong
 
JF
Zhu
 
C
Force-induced cleavage of single VWFA1A2A3 tridomains by ADAMTS-13.
Blood
2010
, vol. 
115
 
2
(pg. 
370
-
378
)
74
Kroll
 
MH
Hellums
 
JD
McIntire
 
LV
Schafer
 
AI
Moake
 
JL
Platelets and shear stress.
Blood
1996
, vol. 
88
 
5
(pg. 
1525
-
1541
)
75
Girdhar
 
G
Xenos
 
M
Alemu
 
Y
et al. 
Device thrombogenicity emulation: a novel method for optimizing mechanical circulatory support device thromboresistance.
PLoS One
2012
, vol. 
7
 
3
pg. 
e32463
 
76
Selgrade
 
BP
Truskey
 
GA
Computational fluid dynamics analysis to determine shear stresses and rates in a centrifugal left ventricular assist device.
Artif Organs
2012
, vol. 
36
 
4
(pg. 
E89
-
E96
)
77
Fraser
 
KH
Zhang
 
T
Taskin
 
ME
Griffith
 
BP
Wu
 
ZJ
A quantitative comparison of mechanical blood damage parameters in rotary ventricular assist devices: shear stress, exposure time and hemolysis index.
J Biomech Eng
2012
, vol. 
134
 
8
pg. 
081002
 
78
Curtas
 
AR
Wood
 
HG
Allaire
 
PE
McDaniel
 
JC
Day
 
SW
Olsen
 
DB
Computational fluid dynamics modeling of impeller designs for the HeartQuest left ventricular assist device.
ASAIO J
2002
, vol. 
48
 
5
(pg. 
552
-
561
)
79
Meyer
 
AL
Malehsa
 
D
Budde
 
U
Bara
 
C
Haverich
 
A
Strueber
 
M
Acquired von Willebrand syndrome in patients with a centrifugal or axial continuous flow left ventricular assist device.
JACC Heart Fail
2014
, vol. 
2
 
2
(pg. 
141
-
145
)
80
Schneider
 
SW
Nuschele
 
S
Wixforth
 
A
et al. 
Shear-induced unfolding triggers adhesion of von Willebrand factor fibers.
Proc Natl Acad Sci USA
2007
, vol. 
104
 
19
(pg. 
7899
-
7903
)
81
Wijeratne
 
SS
Botello
 
E
Yeh
 
HC
et al. 
Mechanical activation of a multimeric adhesive protein through domain conformational change.
Phys Rev Lett
2013
, vol. 
110
 
10
pg. 
108102
 
82
Tsai
 
HM
Sussman
 
II
Nagel
 
RL
Shear stress enhances the proteolysis of von Willebrand factor in normal plasma.
Blood
1994
, vol. 
83
 
8
(pg. 
2171
-
2179
)
83
Shim
 
K
Anderson
 
PJ
Tuley
 
EA
Wiswall
 
E
Sadler
 
JE
Platelet-VWF complexes are preferred substrates of ADAMTS13 under fluid shear stress.
Blood
2008
, vol. 
111
 
2
(pg. 
651
-
657
)
84
Barg
 
A
Ossig
 
R
Goerge
 
T
et al. 
Soluble plasma-derived von Willebrand factor assembles to a haemostatically active filamentous network.
Thromb Haemost
2007
, vol. 
97
 
4
(pg. 
514
-
526
)
85
Kim
 
J
Zhang
 
CZ
Zhang
 
X
Springer
 
TA
A mechanically stabilized receptor-ligand flex-bond important in the vasculature.
Nature
2010
, vol. 
466
 
7309
(pg. 
992
-
995
)
86
Yago
 
T
Lou
 
J
Wu
 
T
et al. 
Platelet glycoprotein Ibalpha forms catch bonds with human WT vWF but not with type 2B von Willebrand disease vWF.
J Clin Invest
2008
, vol. 
118
 
9
(pg. 
3195
-
3207
)
87
Shankaran
 
H
Neelamegham
 
S
Hydrodynamic forces applied on intercellular bonds, soluble molecules, and cell-surface receptors.
Biophys J
2004
, vol. 
86
 
1 Pt 1
(pg. 
576
-
588
)
88
Gogia
 
S
Neelamegham
 
S
Role of fluid shear stress in regulating VWF structure, function and related blood disorders.
Biorheology
2015
, vol. 
52
 
5-6
(pg. 
319
-
335
)
89
Singh
 
I
Themistou
 
E
Porcar
 
L
Neelamegham
 
S
Fluid shear induces conformation change in human blood protein von Willebrand factor in solution.
Biophys J
2009
, vol. 
96
 
6
(pg. 
2313
-
2320
)
90
Themistou
 
E
Singh
 
I
Shang
 
C
Balu-Iyer
 
SV
Alexandridis
 
P
Neelamegham
 
S
Application of fluorescence spectroscopy to quantify shear-induced protein conformation change.
Biophys J
2009
, vol. 
97
 
9
(pg. 
2567
-
2576
)
91
Zheng
 
Y
Chen
 
J
López
 
JA
Flow-driven assembly of VWF fibres and webs in in vitro microvessels.
Nat Commun
2015
, vol. 
6
 pg. 
7858
 
92
Dayananda
 
KM
Singh
 
I
Mondal
 
N
Neelamegham
 
S
von Willebrand factor self-association on platelet GpIbalpha under hydrodynamic shear: effect on shear-induced platelet activation.
Blood
2010
, vol. 
116
 
19
(pg. 
3990
-
3998
)
93
Zhang
 
W
Deng
 
W
Zhou
 
L
et al. 
Identification of a juxtamembrane mechanosensitive domain in the platelet mechanosensor glycoprotein Ib-IX complex.
Blood
2015
, vol. 
125
 
3
(pg. 
562
-
569
)
94
Siedlecki
 
CA
Lestini
 
BJ
Kottke-Marchant
 
KK
Eppell
 
SJ
Wilson
 
DL
Marchant
 
RE
Shear-dependent changes in the three-dimensional structure of human von Willebrand factor.
Blood
1996
, vol. 
88
 
8
(pg. 
2939
-
2950
)
95
Colace
 
TV
Diamond
 
SL
Direct observation of von Willebrand factor elongation and fiber formation on collagen during acute whole blood exposure to pathological flow.
Arterioscler Thromb Vasc Biol
2013
, vol. 
33
 
1
(pg. 
105
-
113
)
96
Da
 
Q
Teruya
 
M
Guchhait
 
P
Teruya
 
J
Olson
 
JS
Cruz
 
MA
Free hemoglobin increases von Willebrand factor-mediated platelet adhesion in vitro: implications for circulatory devices.
Blood
2015
, vol. 
126
 
20
(pg. 
2338
-
2341
)
97
Zhou
 
Z
Han
 
H
Cruz
 
MA
López
 
JA
Dong
 
JF
Guchhait
 
P
Haemoglobin blocks von Willebrand factor proteolysis by ADAMTS-13: a mechanism associated with sickle cell disease.
Thromb Haemost
2009
, vol. 
101
 
6
(pg. 
1070
-
1077
)
98
Katz
 
JN
Jensen
 
BC
Chang
 
PP
Myers
 
SL
Pagani
 
FD
Kirklin
 
JK
A multicenter analysis of clinical hemolysis in patients supported with durable, long-term left ventricular assist device therapy.
J Heart Lung Transplant
2015
, vol. 
34
 
5
(pg. 
701
-
709
)
99
Whitson
 
BA
Eckman
 
P
Kamdar
 
F
et al. 
Hemolysis, pump thrombus, and neurologic events in continuous-flow left ventricular assist device recipients.
Ann Thorac Surg
2014
, vol. 
97
 
6
(pg. 
2097
-
2103
)
100
Ravichandran
 
AK
Parker
 
J
Novak
 
E
et al. 
Hemolysis in left ventricular assist device: a retrospective analysis of outcomes.
J Heart Lung Transplant
2014
, vol. 
33
 
1
(pg. 
44
-
50
)
101
Bartoli
 
CR
Ghotra
 
AS
Pachika
 
AR
Birks
 
EJ
McCants
 
KC
Hematologic markers better predict left ventricular assist device thrombosis than echocardiographic or pump parameters.
Thorac Cardiovasc Surg
2014
, vol. 
62
 
5
(pg. 
414
-
418
)
102
Sadrzadeh
 
SM
Graf
 
E
Panter
 
SS
Hallaway
 
PE
Eaton
 
JW
Hemoglobin. A biologic fenton reagent.
J Biol Chem
1984
, vol. 
259
 
23
(pg. 
14354
-
14356
)
103
Chen
 
J
Fu
 
X
Wang
 
Y
et al. 
Oxidative modification of von Willebrand factor by neutrophil oxidants inhibits its cleavage by ADAMTS13.
Blood
2010
, vol. 
115
 
3
(pg. 
706
-
712
)
104
Fu
 
X
Chen
 
J
Gallagher
 
R
Zheng
 
Y
Chung
 
DW
López
 
JA
Shear stress-induced unfolding of VWF accelerates oxidation of key methionine residues in the A1A2A3 region.
Blood
2011
, vol. 
118
 
19
(pg. 
5283
-
5291
)
105
Lancellotti
 
S
De Filippis
 
V
Pozzi
 
N
et al. 
Formation of methionine sulfoxide by peroxynitrite at position 1606 of von Willebrand factor inhibits its cleavage by ADAMTS-13: A new prothrombotic mechanism in diseases associated with oxidative stress.
Free Radic Biol Med
2010
, vol. 
48
 
3
(pg. 
446
-
456
)
106
Wang
 
Y
Chen
 
J
Ling
 
M
López
 
JA
Chung
 
DW
Fu
 
X
Hypochlorous acid generated by neutrophils inactivates ADAMTS13: an oxidative mechanism for regulating ADAMTS13 proteolytic activity during inflammation.
J Biol Chem
2015
, vol. 
290
 
3
(pg. 
1422
-
1431
)
107
Crow
 
S
Chen
 
D
Milano
 
C
et al. 
Acquired von Willebrand syndrome in continuous-flow ventricular assist device recipients.
Ann Thorac Surg
2010
, vol. 
90
 
4
(pg. 
1263
-
1269, discussion 1269
)
108
Uriel
 
N
Pak
 
SW
Jorde
 
UP
et al. 
Acquired von Willebrand syndrome after continuous-flow mechanical device support contributes to a high prevalence of bleeding during long-term support and at the time of transplantation.
J Am Coll Cardiol
2010
, vol. 
56
 
15
(pg. 
1207
-
1213
)
109
Goda
 
M
Jacobs
 
S
Rega
 
F
et al. 
Time course of acquired von Willebrand disease associated with two types of continuous-flow left ventricular assist devices: HeartMate II and CircuLite Synergy Pocket Micro-pump.
J Heart Lung Transplant
2013
, vol. 
32
 
5
(pg. 
539
-
545
)
110
Restle
 
DJ
Zhang
 
DM
Hung
 
G
et al. 
Preclinical Models for Translational Investigations of Left Ventricular Assist Device-Associated von Willebrand Factor Degradation.
Artif Organs
2015
, vol. 
39
 
7
(pg. 
569
-
575
)
111
Meyer
 
AL
Malehsa
 
D
Bara
 
C
et al. 
Acquired von Willebrand syndrome in patients with an axial flow left ventricular assist device.
Circ Heart Fail
2010
, vol. 
3
 
6
(pg. 
675
-
681
)
112
Van Belle
 
E
Rauch
 
A
Vincentelli
 
A
et al. 
Von Willebrand factor as a biological sensor of blood flow to monitor percutaneous aortic valve interventions.
Circ Res
2015
, vol. 
116
 
7
(pg. 
1193
-
1201
)
113
Davis
 
ME
Haglund
 
NA
Tricarico
 
NM
Matafonov
 
A
Gailani
 
D
Maltais
 
S
Immediate recovery of acquired von Willebrand syndrome after left ventricular assist device explantation: implications for heart transplantation.
ASAIO J
2015
, vol. 
61
 
1
(pg. 
e1
-
e4
)
114
Klovaite
 
J
Gustafsson
 
F
Mortensen
 
SA
Sander
 
K
Nielsen
 
LB
Severely impaired von Willebrand factor-dependent platelet aggregation in patients with a continuous-flow left ventricular assist device (HeartMate II).
J Am Coll Cardiol
2009
, vol. 
53
 
23
(pg. 
2162
-
2167
)
115
Baghai
 
M
Heilmann
 
C
Beyersdorf
 
F
et al. 
Platelet dysfunction and acquired von Willebrand syndrome in patients with left ventricular assist devices.
Eur J Cardiothorac Surg
2015
, vol. 
48
 
3
(pg. 
421
-
427
)
116
Michiels
 
JJ
Berneman
 
Z
Gadisseur
 
A
et al. 
Classification and characterization of hereditary types 2A, 2B, 2C, 2D, 2E, 2M, 2N, and 2U (unclassifiable) von Willebrand disease.
Clin Appl Thromb Hemost
2006
, vol. 
12
 
4
(pg. 
397
-
420
)
117
Buzza
 
MS
Dyson
 
JM
Choi
 
H
et al. 
Antihemostatic activity of human granzyme B mediated by cleavage of von Willebrand factor.
J Biol Chem
2008
, vol. 
283
 
33
(pg. 
22498
-
22504
)
118
Hollestelle
 
MJ
Lai
 
KW
van Deuren
 
M
et al. 
Cleavage of von Willebrand factor by granzyme M destroys its factor VIII binding capacity.
PLoS One
2011
, vol. 
6
 
9
pg. 
e24216
 
119
Tersteeg
 
C
de Maat
 
S
De Meyer
 
SF
et al. 
Plasmin cleavage of von Willebrand factor as an emergency bypass for ADAMTS13 deficiency in thrombotic microangiopathy.
Circulation
2014
, vol. 
129
 
12
(pg. 
1320
-
1331
)
120
Fretto
 
LJ
Fowler
 
WE
McCaslin
 
DR
Erickson
 
HP
McKee
 
PA
Substructure of human von Willebrand factor. Proteolysis by V8 and characterization of two functional domains.
J Biol Chem
1986
, vol. 
261
 
33
(pg. 
15679
-
15689
)
121
Federici
 
AB
Rand
 
JH
Bucciarelli
 
P
et al. 
Subcommittee on von Willebrand Factor
Acquired von Willebrand syndrome: data from an international registry.
Thromb Haemost
2000
, vol. 
84
 
2
(pg. 
345
-
349
)
122
Sadler
 
JE
Budde
 
U
Eikenboom
 
JC
et al. 
Working Party on von Willebrand Disease Classification
Update on the pathophysiology and classification of von Willebrand disease: a report of the Subcommittee on von Willebrand Factor.
J Thromb Haemost
2006
, vol. 
4
 
10
(pg. 
2103
-
2114
)
123
Stulak
 
JM
Lee
 
D
Haft
 
JW
et al. 
Gastrointestinal bleeding and subsequent risk of thromboembolic events during support with a left ventricular assist device.
J Heart Lung Transplant
2014
, vol. 
33
 
1
(pg. 
60
-
64
)
124
Ikeda
 
Y
Handa
 
M
Kawano
 
K
et al. 
The role of von Willebrand factor and fibrinogen in platelet aggregation under varying shear stress.
J Clin Invest
1991
, vol. 
87
 
4
(pg. 
1234
-
1240
)
125
Moake
 
JL
Turner
 
NA
Stathopoulos
 
NA
Nolasco
 
L
Hellums
 
JD
Shear-induced platelet aggregation can be mediated by vWF released from platelets, as well as by exogenous large or unusually large vWF multimers, requires adenosine diphosphate, and is resistant to aspirin.
Blood
1988
, vol. 
71
 
5
(pg. 
1366
-
1374
)
126
Auton
 
M
Sowa
 
KE
Smith
 
SM
Sedlák
 
E
Vijayan
 
KV
Cruz
 
MA
Destabilization of the A1 domain in von Willebrand factor dissociates the A1A2A3 tri-domain and provokes spontaneous binding to glycoprotein Ibalpha and platelet activation under shear stress.
J Biol Chem
2010
, vol. 
285
 
30
(pg. 
22831
-
22839
)
127
Moake
 
JL
Turner
 
NA
Stathopoulos
 
NA
Nolasco
 
LH
Hellums
 
JD
Involvement of large plasma von Willebrand factor (vWF) multimers and unusually large vWF forms derived from endothelial cells in shear stress-induced platelet aggregation.
J Clin Invest
1986
, vol. 
78
 
6
(pg. 
1456
-
1461
)
128
Peterson
 
DM
Stathopoulos
 
NA
Giorgio
 
TD
Hellums
 
JD
Moake
 
JL
Shear-induced platelet aggregation requires von Willebrand factor and platelet membrane glycoproteins Ib and IIb-IIIa.
Blood
1987
, vol. 
69
 
2
(pg. 
625
-
628
)
129
Turner
 
NA
Moake
 
JL
Kamat
 
SG
et al. 
Comparative real-time effects on platelet adhesion and aggregation under flowing conditions of in vivo aspirin, heparin, and monoclonal antibody fragment against glycoprotein IIb-IIIa.
Circulation
1995
, vol. 
91
 
5
(pg. 
1354
-
1362
)
130
Turner
 
NA
Moake
 
JL
McIntire
 
LV
Blockade of adenosine diphosphate receptors P2Y(12) and P2Y(1) is required to inhibit platelet aggregation in whole blood under flow.
Blood
2001
, vol. 
98
 
12
(pg. 
3340
-
3345
)
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