Von Willebrand factor in patients on mechanical circulatory support - a double-edged sword between bleeding and thrombosis

Development and validation of a mathematical model for evaluating shear-induced damage of von Willebrand factor

PURPOSE

To develop a mathematical model for predicting shear-induced von Willebrand factor (vWF) function modification which can be used to guide ventricular assist devices (VADs) design, and evaluate the damage of high molecular weight multimers (HMWM)-vWF in VAD patients for reducing clinical complications.

METHODS

Mathematical models were constructed based on three morphological variations (globular vWF, unfolded vWF and degraded vWF) of vWF under shear stress conditions, in which parameters were obtained from previous studies or fitted by experimental data. Different clinical support modes (pediatric vs. adult mode), different VAD operating states (pulsation vs. constant mode) and different clinical VADs (HeartMate II, HeartWare and CentriMag) were utilized to analyze shear-induced damage of HMWM-vWF based on our vWF model. The accuracy and feasibility of the models were evaluated using various experimental and clinical cases, and the biomechanical mechanisms of HMWM-vWF degradation induced by VADs were further explained.

RESULTS

The mathematical model developed in this study predicted VAD-induced HMWM-vWF degradation with high accuracy (correlation with experimental data r2 > 0.99). The numerical results showed that VAD in the pediatric mode resulted in more HMWM-vWF degradation per unit time and per unit flow rate than in the adult mode. However, the total degradation of HMWM-vWF is less in the pediatric mode than in the adult mode because the pediatric mode has fewer times of blood circulation than the adult mode in the same amount of time. The ratio of HMWM-vWF degradation was lower in the pulsation mode than in the constant mode. This is due to the increased flushing of VADs in the pulsation mode, which avoids prolonged stagnation of blood in high shear regions. This study also found that the design feature, rotor size and volume of the VADs, and the superimposed regions of high shear stress and long residence time inside VADs affect the degradation of HMWM-vWF. The axial flow VADs (HeartMate II) showed higher degradation of HMWM-vWF compared to centrifugal VADs (HeartWare and CentriMag). Compared to fully magnetically suspended VADs (CentriMag), hydrodynamic suspended VADs (HeartWare) produced extremely high degradation of HWMW-vWF in its narrow hydrodynamic clearance. Finally, the study used a mathematical model of HMWM-vWF degradation to interpret the clinical statistics from a biomechanical perspective and found that minimizing the rotating speed of VADs within reasonable limits helps to reduce HWMW-vWF degradation. All predicted conclusions are supported by the experimental and clinical data.

CONCLUSION

This study provides a validated mathematical model to assess the shear-induced degradation of HMWM-vWF, which can help to evaluate the damage of HMWM-vWF in patients implanted with VADs for reducing clinical complications, and to guide the optimization of VADs for improving hemocompatibility.

Platelet Secretion Defects and Acquired von Willebrand Syndrome in Patients With Ventricular Assist Devices

Background

The number of implanted ventricular assist devices (VADs) has increased significantly recently. Bleeding, the most frequent complication, cannot be solely attributed to anticoagulation therapy. Acquired von Willebrand syndrome (AVWS) caused by increased shear stress is frequent in VAD patients and can increase the bleeding risk. The HeartMate III (HM III) is a novel left VAD featuring potential improvements over the HeartMate II.

Methods and Results

In this study, we investigated the prevalence and onset of AVWS in 198 VAD patients. To our knowledge, this is the largest cohort of VAD patients whose longitudinal data on AVWS have been collected. We also analyzed whether AVWS is less severe in HM III patients than in HeartMate II patients. Because platelet dysfunction can raise the bleeding risk, we investigated platelet function in a subset of patients. In total, 198 VAD patients and 60 patients with heart transplants as controls were included in this study. The ratio of von Willebrand factor collagen binding capacity to von Willebrand factor:antigen, multimer analyses, and platelet function (especially secretion of α‐ and δ‐granules) were investigated. All 198 VAD patients developed AVWS. As soon as the VAD was explanted, the AVWS disappeared within hours. AVWS was less severe in the HM III patients than in the HeartMate II patients. The HM III patients had fewer bleeding symptoms. In addition, VAD patients exhibited a platelet α‐ and δ‐granule secretion defect.

Conclusions

AVWS develops in VAD patients and may increase the bleeding risk. The HM III device causes less severe AVWS. Platelet secretion defects should be investigated in VAD patients because they also raise the bleeding risk.

Clinical Trial Registration

https://www.drks.de/drks_web. Unique identifier: DRKS00000649.

Monitoring hemostasis parameters in left ventricular assist device recipients - a preliminary report

Introduction

Mechanical circulatory support (MCS) therapy is associated with the improvement of long-term prognosis in patients with end-stage heart failure. For years it has been used as a bridge to transplant. However, more recently it is even being used as a destination therapy. Recently, clinicians have identified common MCS therapy-associated complications: pump thrombosis, bleeding, and hemolysis. These complications are very challenging with regard to both diagnosis and management.

Aim

To determine time-dependant changes of selected hemostasis/coagulation parameters in patients with end-stage heart failure treated with MCS and antithrombotic therapy.

Material and methods

Sixteen patients with end-stage heart failure on left ventricular assist device (LVAD) were followed for 6 weeks (six blood samples for each patient). Every week an extended hemostasis panel was assessed, including activated partial thromboplastin time, prothrombin time, international normalized ratio, von Willebrand factor (vWF) activity, factor VIII activity, fibrinogen level, D-dimer, platelet response to arachidonic acid (ASPI test) and adenosine diphosphate (ADP test), thrombin receptor activating peptide-6 (TRAP test) and collagen (COL test).

Results

The study population comprised 16 men. The median time from LVAD implantation was 120 days (100–150 days). During the study period the D-dimer and fibrinogen concentrations were elevated but remained similar throughout all six measurements. Meanwhile factor VIII and vWF activities were elevated in the first two measurements and then subsequently declined. Inhibition of platelet aggregation was greater early after LVAD implantation. During subsequent weeks the inhibition of platelet aggregation was less pronounced. No patient developed any bleeding or thrombo-embolic event during the study period.

Conclusions

Patients on MCS therapy demonstrate significant time-dependant changes in hemostasis parameters (both in the coagulation system and platelet aggregation).