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Comparative Analysis of von Willebrand Factor Profiles in Pulsatile and Continuous Left Ventricular Assist Device Recipients

Crow, Sheri*; Milano, Carmelo†; Joyce, Lyle‡; Chen, Dong§; Arepally, Gowthami¶; Bowles, Dawn†; Thomas, William∥; Ortiz, Nestor Villamizar†

doi: 10.1097/MAT.0b013e3181e5de0a
Clinical Cardiovascular

A higher rate of nonsurgical bleeding has been observed in nonpulsatile left ventricular assist device (LVAD) recipients. von Willebrand factor (vWF) profiles were compared for nonpulsatile and pulsatile LVAD recipients to explore mechanisms that may contribute to the development of postimplant nonsurgical bleeding. The nonpulsatile mechanism may impair vWF function by creating a deficiency in vWF high molecular weight multimers (HMWMs), essential for hemostasis. High molecular weight multimer deficiency should result in low ristocetin cofactor (RCo) to vWF antigen ratios (vWF:RCo/vWF:Ag) because of impaired platelet (plt)-binding ability. von Willebrand factor profiles and HMWM were measured pre- and post-LVAD placement in 11 nonpulsatile (HeartMate II [HM II[) and 3 pulsatile (HeartMate XVE [HM XVE]) recipients. All the nonpulsatile LVAD recipients exhibited loss of HMWM 30 days postimplant. The vWF:RCo/vWF:Ag ratio was significantly lower after LVAD placement in the nonpulsatile group when compared with the pulsatile group. In addition, the vWF:RCo/vWF:Ag ratio decreased significantly from baseline 30 days postimplant within the nonpulsatile recipients. All nonpulsatile LVAD recipients had low vWF:RCo/vWF:Ag ratios 30 days post-LVAD even if the values were normal at baseline. These data suggest that nonpulsatile HM II recipients develop HMWM loss and impaired vWF platelet (plt)-binding ability after LVAD placement. Similar results were not observed in our small series of pulsatile HM XVE recipients. This finding could suggest a contributing factor to the increase in nonsurgical bleeding observed in nonpulsatile LVAD patients. Further investigation is ongoing to identify specific causes of vWF impairment.

From the *Division of Pediatric Critical Care Medicine, Mayo Clinic, Rochester, Minnesota; †Division of Cardiothoracic Surgery, Duke University, Durham, North Carolina; ‡Division of Cardiothoracic Surgery, §Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota; ¶Division of Hematology, Duke University, Durham, North Carolina; and ∥Division of Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota.

Submitted for consideration January 2010; accepted for publication in revised form April 2010.

Presented, in part, at the 55th Annual ASAIO Conference, Dallas, TX, May 27–29, 2009.

Lyle Joyce was a principal investigator for the HeartMate II bridge to transplantation and destination therapy trials. He has served as an instructor for the University of Minnesota's training site for the implantation of Thoratec devices.

Reprint Requests: Sheri Crow, MD, Pediatric Critical Care Medicine, Mayo Clinic, 200 First Street SW, Rochester, MN 55905. Email:

During the past decade, left ventricular assist devices (LVADs) have become an effective strategy for prolonging life in patients with New York Heart Association Class IV congestive heart failure. A recent clinical trial comparing pulsatile vs. nonpulsatile (continuous flow) LVAD support demonstrated comparable survival outcomes without significant differences in complication rates.1 However, in the past 2 years, two retrospective analyses have demonstrated an increased rate of nonsurgical bleeding in nonpulsatile LVAD recipients during follow-up. Crow et al. compared gastrointestinal (GI) bleeding rates among nonpulsatile and pulsatile LVAD recipients. The nonpulsatile recipients had a bleeding event rate of 63 per 100 patient years compared with a rate of only 6.8 per 100 patient years in the pulsatile recipients.2 A follow-up analysis of LVAD recipients at Duke University by Pedrotty et al. demonstrated similar differences in bleeding rates between the device types. In their series of 99 patients, 65% of HeartMate II (HM II) recipients experienced nonsurgical bleeding compared to only 44% in HeartMate XVE (HM XVE) patients.3

We sought to perform a detailed comparison of von Willebrand factor (vWF) profiles in nonpulsatile and pulsatile LVAD recipients with the goal of identifying a potential cause for the increased bleeding propensity observed in nonpulsatile LVAD recipients.

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Patients and Methods


This research protocol was approved by the University of Minnesota and Duke University institutional review boards. Informed consent was obtained from patients included in the study. The enrollment period extended from July through December 2008. Eleven HM II and three HM XVE recipients were enrolled in the study, and blood samples were obtained before and 30 days after LVAD placement.

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Device Placement/Surgical Procedure

All eligible patients had New York Heart Association functional class IV congestive heart failure. In all cases, LVAD implantation was performed through a median sternotomy, using cardiopulmonary bypass. The inflow cannula was inserted into the left ventricular apex, and the outflow cannula was anastomosed to the ascending aorta. Pump flow rates were adjusted to achieve optimal cardiac output and resolution of signs and symptoms of cardiac failure.

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Nonpulsatile device recipients (HM II) were placed on a regimen of warfarin sodium and aspirin, with a goal international normalized ratio (INR) of 1.5. Pulsatile device recipients (HM XVE) took an aspirin once a day.

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Laboratory Analysis

Blood was collected preimplantation and 30 days postimplantation in tubes containing 3.2% sodium citrate. The blood was analyzed for INR, activated partial thromboplastin time (aPTT), platelet (PLT) count, vWF antigen (vWF:Ag), ristocetin cofactor (RCo) activity (vWF:RCo), factor VIII (FVIII), ADAMTS13 levels, and presence of vWF high molecular weight multimers (HMWM).

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Data Collection

Bleeding was defined as a guaiac-positive stool, melena, hematochezia, or epistaxis, which resulted in some type of intervention to achieve bleeding cessation. Those interventions included the following: ear, nose, and throat cauterization or nasal packing for epistaxis, colonoscopy or upper endoscopy for melena, and hematochezia or blood transfusion. International normalized ratio, aPTT, PLT count, and von Willebrand profiles were measured at the time of presentation when patients experienced a bleeding event.

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Statistical Analysis

Data for all continuous variables (age, laboratory values, and implant duration) are presented as mean ± standard deviation. Pulsatile and nonpulsatile groups were compared using t tests for continuous variables and χ2 tests for categorical variables. Within-group changes were assessed with paired t tests. The p values <0.05 were considered statistically significant.

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A total of 11 HM II (nonpulsatile) and 3 HM XVE (pulsatile) LVADs were implanted during the study period from July through December 2008. The baseline characteristics of the nonpulsatile and pulsatile LVAD recipients are outlined in Table 1. There were no significant differences in age, gender, or device purpose (bridge to transplantation vs. destination therapy) between the two groups.

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von Willebrand Factor Profile Comparison of Nonpulsatile and Pulsatile LVAD Recipients

Laboratory analyses for vWF:Ag, vWF:RCo, FVIII, ADAMTS13, and presence of vWF high molecular weight multimers (HMWM) were performed pre-LVAD and 30 days post-LVAD implantation. The results are shown in Table 2. There was no significant change in FVIII, vWF:Ag, vWF:RCo, or ADAMTS13 levels pre- and post-LVAD placement for either group. The pulsatile group had equivalent vWF:RCo to vWF:Ag ratios (vWF:RCo/vWF:Ag) pre-LVAD and 30 days post-LVAD implantation. The vWF:RCo/vWF:Ag ratio was significantly lower after LVAD placement in the nonpulsatile group when compared with nonpulsatile pre-LVAD levels and pulsatile 30 days post-LVAD levels. All nonpulsatile LVAD recipients had low vWF:RCo/vWF:Ag ratios 30 days post-LVAD even if the values were normal at baseline.

All patients except one HM II recipient had normal HMWM levels pre-LVAD implantation. High molecular weight multimers were absent in 100% of the HM II recipients 30 days postimplantation. High molecular weight multimer levels were measured in a single HM XVE recipient 30 days postimplant, and these levels were normal.

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Nonsurgical Bleeding Events

There were no bleeding complications experienced by the pulsatile HM XVE LVAD recipients. However, 33% or 4 of 11 patients implanted with the nonpulsatile HM II developed clinically significant nonsurgical bleeding after device implantation. The average time from device implantation to bleeding event was 29 days. Three of the patients experienced GI bleeding with sufficient hemoglobin drops to require transfusion with packed red blood cells. One patient had epistaxis that required nasal tamponade and cauterization to achieve bleeding cessation. Baseline characteristic and coagulation profile comparisons between the nonpulsatile bleeders and nonbleeders are presented in Table 3.

Age, PLT count, INR, and aPTT measured before and 30 days post-LVAD placement were not significantly different between the bleeders and nonbleeders. For patients who experienced bleeding, there was no difference between the INR, aPTT, and PLT count measured pre-LVAD, 30 days postimplant, and that measured at presentation with a bleeding event. The results for von Willebrand profile comparison between nonpulsatile bleeders and nonbleeders are summarized in Table 4.

The 30 days post-LVAD vWF:RCo/vWF:Ag ratios were significantly lower than pre-LVAD levels in three of four patients who experienced bleeding. However, there was no significant difference in the 30 days postimplant vWF:RCo/vWF:Ag ratio values between bleeders and nonbleeders. Pre-LVAD FVIII levels were significantly lower in the nonbleeders than the bleeders. Pre-LVAD vWF:Ag and vWF:RCo levels were lower in the nonbleeders, but this finding did not reach statistical significance.

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Patients implanted with LVADs for treatment of end-stage congestive heart failure experience the benefits of increased life expectancy and improved quality of life.4 Although continuous flow devices such as the HM II offer the advantage of smaller size and enhanced durability, nonsurgical bleeding remains a significant problem for LVAD recipients. Determining potential causes and risk factors for the development of complications is essential to improve the quality of life that these patients can anticipate after LVAD placement.

The primary source of bleeding experienced by nonpulsatile LVAD recipients continues to involve the GI tract and nasal mucosa. Previous analyses have demonstrated that there is no increase in surgical bleeding for these patients, suggesting normal ability to maintain postsurgical hemostasis.1,2 Instead, the bleeding pattern demonstrated is more reminiscent of that observed in patients with severe aortic stenosis (AS), first described as Heyde's5 syndrome in 1958. The bleeding dyscrasia observed in these patients went on to be described as acquired von Willebrand syndrome (AVWS).6–9

von Willebrand factor is a 250-kDa protein synthesized as monomers by endothelial cells and megakaryocytes. These monomers assemble into dimers and subsequently multimers. von Willebrand factor multimers are stored in Weible-Palade bodies within endothelial cells and released after endothelial cell activation. Large vWF multimers that are 10 dimers or larger are referred to as HMWM.10 These HMWM are essential for promoting PLT adhesion in high shear stress areas such as GI arteriovenous malformations.8,11 Aortic stenosis is believed to create an environment of high shear stress as the blood flows across the calcific aortic valve.9 This stress is believed to produce a conformational change in vWF that increases its susceptibility to proteolysis and cleavage by ADAMTS13 forming smaller multimers. The result is an AVWS caused by a deficiency of the largest vWF multimers reducing vWF's ability to bind collagen and PLTs. The absence of HMWM in patients with calcific AS and GI bleeding is well documented. The overall quantity of vWF in HMWM deficiency remains unchanged. This may explain the absence of bleeding from other areas that are less dependent on HMWM for hemostasis. After aortic valve replacement and correction of AS patients reestablish normal levels of circulating HMWMs and bleeding resolves.9,12

The finding of an isolated increase in the rate of GI and nasal mucosal bleeding in nonpulsatile continuous flow LVAD recipients is similar to that seen in AS. We theorized that the bleeding complications observed in nonpulsatile device recipients may similarly be due to the development of an AVWS in this patient population. The continuous impeller mechanism of nonpulsatile LVADs such as the HM II may result in vWF deformation, proteolysis, and cleavage leading to a loss or reduction in HMWM. In this setting, patients with previously asymptomatic GI and nasal angiodysplasias would have impaired PLT-mediated hemostasis and exhibit increased tendency for bleeding after LVAD placement.

The cause for nonsurgical bleeding in nonpulsatile recipients is undoubtedly multifactorial in nature. Traditional laboratory analysis for von Willebrand disease includes measuring vWF Ag, RCo activity (vWF:RCo), and FVIII levels. von Willebrand factor:Ag levels reflect the quantity of vWF but give no information about its functional capability. In contrast, vWF:RCo measurement demonstrates the ability of a patient's vWF to agglutinate fresh or formalin-fixed PLTs in the presence of ristocetin. The ratio of vWF:RCo to vWF:Ag (vWF:RCo/vWF:Ag) can be calculated to evaluate functional vWF PLT-binding activity. If a HMWM deficiency, i.e., AVWS exists, the vWF:RCo/vWF:Ag ratio should be abnormally low after LVAD placement reflecting impaired PLT-binding capacity. However, currently, there is no established normal cutoff ratio for defining AVWS.

The pre-LVAD vWF:Ag levels in our study cohort were actually increased above the normal range in both the pulsatile and nonpulsatile groups. Previous studies have also demonstrated higher than normal vWF:Ag levels in LVAD recipients compared with controls.13 Levels of vWF:Ag can increase during illness as a part of an acute phase reaction. Klovaite et al.14 measured increased levels of c-reactive protein in the blood of patients with congestive heart failure or HM II LVADs. Patients with AS and AVWS demonstrate normal vWF:Ag and vWF:RCo test results despite loss of HMWM.8,9 Other cardiovascular disorders that are associated with HMWM loss but normal vWF:Ag and vWF:RCo activity include the following: hypertrophic cardiomyopathy, certain congenital cardiac defects, and severe peripheral artery disease.6

Laboratory comparisons of vWF profiles and HMWM levels in different LVAD designs have potential to highlight mechanistic features that minimize impact on hemostatic forces. The differences in vWF profile measurements and HMWM levels for nonpulsatile devices that use axial vs. centrifugal flow have not been evaluated. There is some data to suggest that pulsatile HM XVE recipients experience less hematologic derangements after LVAD placement. Malehsa et al. demonstrated normal levels of HMWM in a HM XVE patient. Repeat analysis after replacement of the HM XVE with a HM II showed complete loss of the HMWMs.15 In our cohort, we observed a reduction in vWF:RCo/vWF:Ag ratios after HM II implantation that was not observed after HM XVE placement. This data suggest that HM II patients in our series developed impaired PLT-mediated hemostasis after LVAD implantation. However, these results must be interpreted cautiously because they were measured in only a small number of patients.

Geisen et al.16 recently examined HMWM levels in five pulsatile (Thoratec BIVAD; Thoratec Corporation, Pleasanton, CA) and seven nonpulsatile (Thoratec HM II; Thoratec Corporation, Pleasanton, CA) ventricular assist device (VAD) recipients.Although the exact timing of laboratory analysis is not reported, all measurements were made within 30 days of having the VAD implanted. In the analysis by Geisen et al., all VAD recipients regardless of device mechanism demonstrated impaired coagulation as measured by vWF:RCo/vWF:Ag ratios postimplantation. Eight heart transplant patients were used as controls for this study. The vWF:RCo/vWF:Ag ratios were significantly lower in VAD recipients compared with heart transplant patients (VAD, 0.6 ± 0.1 vs. heart transplant [HTX], 0.9 ± 0.2, p = 0.002). Ten of the VAD patients in this group also underwent HMWM analysis within the first 30 days after VAD implantation. All 10 VAD patients tested demonstrated the absence of HMW vWF multimers. A second measurement of multimer levels in three BIVAD and four HM II recipients at >10 weeks after implantation demonstrated persistence in these patient's HMW multimer deficiencies.16 These findings would suggest that HMWM deficiency alone does not explain the increased incidence of nonsurgical bleeding in nonpulsatile LVAD recipients.

In our series, post-LVAD vWF:RCo/vWF:Ag ratio reductions were observed only in the nonpulsatile device recipients. The difference in our results from the results of Geisen et al. may be due to the differences in the timings of laboratory analysis or variability in RCo activity testing between institutions. The patients in our series had blood sampled at identical time points post-LVAD placement. In addition, we provide the first measurements of vWF profiles pre-LVAD placement, enabling patients to serve as their own controls.

Interpretation of our results is limited by the small number of patients evaluated, particularly in the HM XVE group. Further analysis of vWF profiles in a larger cohort of nonpulsatile and pulsatile LVAD recipients may suggest additional predictive indices for those patients who are at an increased risk for bleeding complications. Our preliminary data (Table 4) demonstrate a statistically significant difference in FVIII levels pre-LVAD between the bleeders and the nonbleeders (p = 0.02). In addition, although the finding only trended toward significance, vWF:Ag and vWF:RCo levels were lower pre-LVAD in patients who did not bleed. These values can be increased during inflammatory states and potentially suggests that patients who bleed may be sicker than their nonbleeding counterparts. Based on our small subset of data, pre-LVAD FVIII levels that fall outside the normal range (74%–212%) have 50% sensitivity and 100% specificity for predicting development of a post-LVAD bleeding complication. However, analysis of vWF profiles in a larger series of patients is required to determine whether these findings persist.

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Our findings demonstrate that the nonpulsatile (continuous flow) LVAD mechanism may impair vWF function and PLT-binding ability, as measured by a decreased ristocetin to Ag ratio and loss of HMWM after LVAD implantation. The small number of HM XVE pulsatile recipients evaluated in our series did not exhibit markers for PLT dysfunction after LVAD placement. These results suggest that impaired PLT-binding ability may contribute to the increased risk for nonsurgical bleeding observed in nonpulsatile LVAD recipients. Analysis of vWF profiles and HMWM levels in a larger group of patients is required to identify laboratory markers that may be predictive of bleeding risk in LVAD recipients.

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Supported by a research and an educational grant provided by Thoratec Corporation (to C.M.). A portion of this study is supported by a Thoratec grant.

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