Experimental Study on Shear Stress Distributions in a Centrifugal Blood Pump

Hemocompatibility and biophysical interface of left ventricular assist devices and total artificial hearts

ABSTRACT

Over the past 2 decades, there has been a significant increase in the utilization of long-term mechanical circulatory support (MCS) for the treatment of cardiac failure. Left ventricular assist devices (LVADs) and total artificial hearts (TAHs) have been developed in parallel to serve as bridge-to-transplant and destination therapy solutions. Despite the distinct hemodynamic characteristics introduced by LVADs and TAHs, a comparative evaluation of these devices regarding potential complications in supported patients, has not been undertaken. Such a study could provide valuable insights into the complications associated with these devices. Although MCS has shown substantial clinical benefits, significant complications related to hemocompatibility persist, including thrombosis, recurrent bleeding, and cerebrovascular accidents. This review focuses on the current understanding of hemostasis, specifically thrombotic and bleeding complications, and explores the influence of different shear stress regimens in long-term MCS. Furthermore, the role of endothelial cells in protecting against hemocompatibility-related complications of MCS is discussed. We also compared the diverse mechanisms contributing to the occurrence of hemocompatibility-related complications in currently used LVADs and TAHs. By applying the existing knowledge, we present, for the first time, a comprehensive comparison between long-term MCS options.

A novel device for elimination of cancer cells from blood specimens

Circulating tumor cells (CTCs) are derivatives of solid cancerous lesions that detach from the tumor mass and enter the blood circulation. CTCs are considered to be the precursors of metastasis in several cancer types. They are present in the blood of cancer patients as single cells or clusters, with the latter being associated with a higher metastatic potential. Methods to eliminate CTCs from the bloodstream are currently lacking. Here, we took advantage of the lower shear stress-resistance of cancer cells compared to blood cells, and developed a device that can eliminate cancer cells without blood damage. The device consists of an axial pump and a coupled rotating throttle, controllable to prevent local blood flow impairment, yet maintaining a constant shear performance. When processing cancer cells through our device, we observe cancer cell-cluster disruption and viability reduction of single cancer cells, without noticeable effects on human blood cells. When injecting cancer cell-containing samples into tumor-free recipient mice, processed samples fail to generate metastasis. Together, our data show that a selective disruption of cancer cells is possible while preserving blood cells, paving the way towards the development of novel, implantable tools for CTC disruption and metastasis prevention.

Development of the channel type centrifugal pump

A channel centrifugal pump has been developed which have calculated parameters during the nominal operating mode based on 3-dimensional computer simulation  (flow rate 5 l/min, pressure drop 100 mm). In addition, pump’s operating conditions in ECMO mode are considered at high pressure drops of 200–300 mm Hg with a  speed of rotor up to 3500 rpm. Simulation result was a creation of a new channel- type centrifugal pump with shear stress that do not exceed the allowable threshold  of 150 Pa, and also minimizing stagnation and flow recirculation zones. The  obtained data were also the result of use design of rotor with constant cross-section channels formed along a logarithmic curve and ensuring minimum turbulence due to the minimum outlet angle of the flow.

Durable, flexible, superhydrophobic and blood-repelling surfaces for use in medical blood pumps

A new sand-casting method for fabricating superhydrophobic materials gives highly durable, flexible, and blood-repelling surfaces useful for cardiovascular medical devices.

von Willebrand factor disruption and continuous-flow circulatory devices

Bleeding events remain a significant and frequent complication of continuous-flow left ventricular assist devices (VADs). von Willebrand factor (VWF) is critical to hemostasis by acting as a bridging molecule at sites of vascular injury for normal platelet adhesion as well as promoting platelet aggregation under conditions of high shear. Clinical and experimental data support a role for acquired von Willebrand disease in VAD bleeding episodes caused by shear-induced qualitative defects in VWF. Pathologic shear induces VWF unfolding and proteolysis of large multimers into smaller less hemostatic multimers via ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). This review outlines the pathobiology of VWF disruption in the context of VADs as well as current diagnostic and management strategies of the associated acquired von Willebrand disease.

Effects of HeartWare ventricular assist device on the von Willebrand factor: results of an academic Belgian center

Background

Left Ventricular Assist Device (LVAD) is a promising therapy for patients with advanced heart failure (HF), but bleeding complications remain an important issue. Previous series show that acquired von Willebrand syndrome was present in up to 100 % of first generation LVAD recipients. We report the effects of new generation LVADs on vW factor (vWF) metabolism and activity in our center.

Methods

Fifteen LVAD recipients (HeartWare®, Framingham, MA, USA) were compared to 12 HF patients, matched for age and body mass index. vWF antigen and activity, as well as D-dimers, were measured on hemostasis analyzers. A vWF LVAD-induced alteration was evocated when the [vWF activity]/[vWF antigen] ratio was <0.6. ADAMTS13 and high molecular weight multimers of vWF were also assessed.

Results

LVAD recipients had similar levels of endothelial vWF production than the HF subjects (137 ± 14.5 vs. 147 ± 11.7 %; respectively, p = 0.611) but a decreased vWF activity (90 ± 11 vs. 132.6 ± 13 %; respectively, p = 0.017). [vWF activity]/[vWF antigen] ratio was 0.65 ± 0.02 in the LVAD recipients and 0.92 ± 0.06 in the subjects with HF (p = 0.001). ADAMTS13 activity was 80.3 ± 4.7 % in LVAD recipients and 96.2 ± 3.5 % in the HF patients (p = 0.016). LVAD patients disclosed markedly elevated D-dimers (3217.7 ± 735 vs. 680.6 ± 223.2 ng/mL FEU in the HF patients, p = 0.006). The LVAD patients experienced one major hemorrhagic event and one systemic thrombotic event during the median follow-up of 345 days.

Conclusions

LVAD recipients achieved a new hemostatic equilibrium characterized by infrequent major hemorrhagic and thrombotic events, despite a mildly impaired vWF function and a markedly enhanced thrombin formation.

Trial registration

ISRCTN39517567

Electronic supplementary material

The online version of this article (doi:10.1186/s12872-016-0334-z) contains supplementary material, which is available to authorized users.

Acquired von Willebrand syndrome in patients with a centrifugal or axial continuous flow left ventricular assist device

OBJECTIVES

The aim of this study was to determine whether differences in continuous flow left ventricular assist devices (LVADs) may lead to differences in the von Willebrand profile and the occurrence of bleeding and thromboembolic events.

BACKGROUND

The HeartMate II (Thoratec Corp., Pleasanton, California) and HeartWare Ventricular Assist Device (HVAD) (HeartWare, Inc., Framingham, Massachusetts) systems are the most frequently implanted LVADs worldwide. In all patients with an axial-flow HeartMate II, acquired von Willebrand syndrome (AvWS) due to the loss of large molecular weight multimers was found. The large molecular weight multimers of the von Willebrand factor (vWF) play a key role in primary hemostasis through interactions with platelets.

METHODS

This was a retrospective study of the vWF profile and incidence of bleeding and thromboembolic events in 102 patients receiving the HeartMate II (n = 51) and HVAD (n = 51). Between January 2003 and December 2010, vWF testing was performed in 102 of 175 consecutive patients after LVAD implantation.

RESULTS

AvWS was found in all patients, demonstrated by a decrease in the high molecular weight multimers of vWF to 30 ± 14% in HeartMate II patients and 34 ± 13% in patients with an HVAD. Significant predictors of vWF antigen included age (p = 0.011), number of days on the device (p = 0.035), C-reactive protein (p < 0.001), and blood group (p = 0.007). Bleeding and thromboembolic event rates were similar. However, lower fractions of vWF antigen and high molecular weight multimers did not correlate with the rate of bleeding complications or thromboembolic events.

CONCLUSIONS

AvWS developed in all patients after centrifugal or axial flow pump implantation. Different patterns of AvWS were seen between the devices as well as individually. However, the complication rates after implantation were similar.

Acquired von Willebrand syndrome in patients with extracorporeal life support (ECLS)

PURPOSE

Extracorporeal life support (ECLS) is used for patients with refractory heart failure with or without respiratory failure. This temporary support is provided by blood pumps which are connected to large vessels. Bleeding episodes are a typical complication in patients with ECLS. Recently, several studies illustrated that acquired von Willebrand syndrome (AVWS) can contribute to bleeding tendencies in patients with long-term ventricular assist devices (VAD). AVWS is characterized by loss of the high molecular weight (HMW) multimers of von Willebrand factor (VWF) as a result of high shear stress and leads to impaired binding of VWF to platelets and to subendothelial matrix. Since ECLS and VAD share several features, we investigated patients with ECLS for AVWS.

METHODS

We analyzed 32 patients with ECLS and 19 of them without support. To diagnose AVWS, ratios of ristocetin cofactor activity (VWF:RCo) and collagen binding capacity (VWF:CB) to VWF antigen (VWF:Ag) were employed in conjunction with multimeric analysis.

RESULTS

Reduced VWF:RCo/VWF:Ag ratios were identified in 28 ECLS patients. Furthermore, VWF:CB/VWF:Ag ratios were decreased in 31 patients. HMW multimers of VWF were missing in the same 31 patients. Thus, 31 of 32 ECLS patients presented with AVWS. Twenty-two of the 32 patients suffered from bleeding complications. Without support, AVWS was not detectable in any analyzed patient.

CONCLUSION

Our data indicate that AVWS is a typical disorder in patients with ECLS. We hypothesize that AVWS could contribute to aggravation of bleeding tendencies in ECLS patients.

The use of computational fluid dynamics in the development of ventricular assist devices

Progress in the field of prosthetic cardiovascular devices has significantly contributed to the rapid advancements in cardiac therapy during the last four decades. The concept of mechanical circulatory assistance was established with the first successful clinical use of heart-lung machines for cardiopulmonary bypass. Since then a variety of devices have been developed to replace or assist diseased components of the cardiovascular system. Ventricular assist devices (VADs) are basically mechanical pumps designed to augment or replace the function of one or more chambers of the failing heart. Computational Fluid Dynamics (CFD) is an attractive tool in the development process of VADs, allowing numerous different designs to be characterized for their functional performance virtually, for a wide range of operating conditions, without the physical device being fabricated. However, VADs operate in a flow regime which is traditionally difficult to simulate; the transitional region at the boundary of laminar and turbulent flow. Hence different methods have been used and the best approach is debatable. In addition to these fundamental fluid dynamic issues, blood consists of biological cells. Device-induced biological complications are a serious consequence of VAD use. The complications include blood damage (haemolysis, blood cell activation), thrombosis and emboli. Patients are required to take anticoagulation medication constantly which may cause bleeding. Despite many efforts blood damage models have still not been implemented satisfactorily into numerical analysis of VADs, which severely undermines the full potential of CFD. This paper reviews the current state of the art CFD for analysis of blood pumps, including a practical critical review of the studies to date, which should help device designers choose the most appropriate methods; a summary of blood damage models and the difficulties in implementing them into CFD; and current gaps in knowledge and areas for future work.