High-speed visualization of disturbed pathlines in axial flow ventricular assist device under pulsatile conditions

Bioprosthetic Total Artificial Heart in Autoregulated Mode Is Biologically Hemocompatible: Insights for Multimers of von Willebrand Factor

BACKGROUND

Carmat bioprosthetic total artificial heart (Aeson; A-TAH) is a pulsatile and autoregulated device. The aim of this study is to evaluate level of hemolysis potential acquired von Willebrand syndrome after A-TAH implantation.

METHODS

We examined the presence of hemolysis and acquired von Willebrand syndrome in adult patients receiving A-TAH support (n=10) during their whole clinical follow-up in comparison with control subjects and adult patients receiving Heartmate II or Heartmate III support. We also performed a fluid structure interaction model coupled with computational fluid dynamics simulation to evaluate the A-TAH resulting shear stress and its distribution in the blood volume.

RESULTS

The cumulative duration of A-TAH support was 2087 days. A-TAH implantation did not affect plasma free hemoglobin over time, and there was no association between plasma free hemoglobin and cardiac output or beat rate. For VWF (von Willebrand factor) evaluation, A-TAH implantation did not modify multimers profile of VWF in contrast to Heartmate II and Heartmate III. Furthermore, fluid structure interaction coupled with computational fluid dynamics showed a gradually increase of blood damage according to increase of cardiac output (P<0.01), however, the blood volume fraction that endured significant shear stresses was always inferior to 0.03% of the volume for both ventricles in all regimens tested. An inverse association between cardiac output, beat rate, and high-molecular weight multimers ratio was found.

CONCLUSIONS

We demonstrated that A-TAH does not cause hemolysis or AWVS. However, relationship between HMWM and cardiac output depending flow confirms relevance of VWF as a biological sensor of blood flow, even in normal range.

Hemodynamic investigation and in vitro evaluation of a novel mixed flow blood pump

INTRODUCTION

Ventricular assist devices (VADs) are considered as an effective treatment for patients with advanced heart failure, while complications associated with blood damage remain a burden. Structure design innovation has potential to reduce hemolysis and improve hemocompatibility.

METHODS

In this research, a novel mixed flow blood pump that integrates structural features of the axial and centrifugal VADs was proposed. The pump consists of an inducer, a mixed impeller supported by two ceramic pivot bearings and a volute. The flow field and laminar viscous shear stress were analyzed by the in-silico simulation. The hydraulic and hemolytic performance were evaluated in vitro by using a 3D printed pump.

RESULTS

The flow field distribution showed that streamlines in the connection area were smoothly transitioned through structural integration and no irregular flow occurred in the entire flow channel. The axial blades work as a fluid accelerator (generating 18.56% of the energy), and the centrifugal blades provide the main pressure head. The proportion of fluid inside the pump exposed to low laminar viscous shear stress (<50 Pa) and high laminar viscous shear stress (>150 Pa) was 99.02% and 0.03%, respectively. The in vitro hemolysis test results showed that the NIH (Normalized Index of Hemolysis) value of the mixed pump is 0.0079 ± 0.0039 g/100 L (n=6).

CONCLUSION

It can be concluded that the mixed flow structure is effective to improve hydraulic performance, eliminate flow disturbance and minimize shear stresses. This novel pump design is expected to provide a new direction for the development of next-generation VADs.

Hemolytic Footprint of Rotodynamic Blood Pumps

OBJECTIVE

In preclinical examinations, rotodynamic blood pumps (RBPs) are predominantly evaluated at design-point conditions. In clinical practice, however, they run at diversified modes of operation. This study aimed at extending current preclinical evaluation of hemolytic profiles in RBPs toward broader, clinically relevant ranges of operation.

METHODS

Two implantable RBPs the HeartMate 3 (HM3) and the HeartWare Ventricular Assist Device (HVAD) were analyzed at three pump speeds (HM3: 4300, 5600, 7000rpm; HVAD: 1800, 2760, 3600rpm) with three flow rates (1-9L/min) per speed setting. Hemolysis measurements were performed in heparinized bovine blood. The delta free hemoglobin (dfHb) and the normalized index of hemolysis (NIH) served as hemolytic measures. Statistical analysis was performed by multiple comparison of the 9 operating conditions. Moreover, computational fluid dynamics (CFD) was applied to provide mechanistic insights into the interrelation between hydraulics and hemolysis by correlating numerically computed hydraulic losses with in-vitro hemolytic measures.

RESULTS

In both devices, dfHb increased toward increasing speeds, particularly during low but also during high flow condition. By contrast, in both RBPs magnitudes of NIH were significantly elevated during low flow operation compared to high flow conditions (p<0.0036). Maps of hemolytic metrics revealed morphologically similar trends to in-silico hydraulic losses (r>0.793).

CONCLUSIONS

While off-design operation is associated with increased hemolytic profiles, the setting of different operating conditions render a preclinical prediction of clinical impact with current hemolysis metrics difficult.

SIGNIFICANCE

The identified increase in hemolytic measures during episodes of off-design operation is highlighting the need to consider worst-case operation during preclinical examinations.

High-speed visualization of ingested, ejected, adherent, and disintegrated thrombus in contemporary ventricular assist devices

Biocompatibility of ventricular assist devices (VADs) has been steadily improving, yet the rate of neurological events remains unacceptably high. Recent speculation for elevated stroke rates centers on ingestion of thrombi originating upstream of the pump, such as in the ventricle or left atrial appendage. These thrombi may be ejected by the VAD or become deposited within the blood flow pathway, presenting serious complications to the patient. This study was performed to visualize and quantify the degree of disruption, adherence, and disintegration of thrombi that are ingested by the three most implanted VADs: the HeartMate II, HeartMate 3, and HVAD. Clot analogs of varying microstructure compositions (red, white) and sizes (0.5, 1, 2 cm³ ) were synthesized in vitro based on clinical explant data. These were introduced individually into an in vitro flow loop with a transparent replica of the HMII, HM3, and HVAD operated at nominal steady flow (2.3-4.0 L/min). High-speed videography (up to 10 000 fps) revealed the ingestion, disruption, ejection, and adherence of thrombus fragments. Thromboemboli of varying compositions and sizes were observed mechanically attaching to components in all 3 VAD models. In some instances, ingested thrombi physically obstructed portions of the blood flow path; 18% (3 of 17 total) of red thrombi adhered to the inflow straightener of the transparent HMII. In the HVAD model, fewer than 4% of clots were adherent or trapped within the pump, irrespective of microstructure or initial volume. In comparison, 100% (4 of 4 total) of 1-cm³ white (fibrin) clots became lodged within the transparent HM3 while, in contrast, less than 5% of macerated red clots (3 of 63 total) of the same volume were adherent inside the pump. A significant proportion of ingested thrombi were macerated into infinitesimal fragments; 84% and 74% of 2-cm³ red thrombi in the HVAD and HM3 models, respectively, were found to have disintegrated upon ingestion. However, large emboli were also discharged from both centrifugal VADs; these fragments, ranging from 0.01 to 0.29 cm³ regardless of microstructure and original volume, may be capable of occluding an intracranial vessel. Therefore, ingested thrombus may explain, in part, elevated stroke rates in contemporary blood pumps in the absence of adherent pump thrombosis.

Food and Drug Administration Malfunction Recalls of Left Ventricular Assist Devices

Left ventricular assist devices (LVADs) are being increasingly implanted given the increasing prevalence of patients with advanced heart failure stages. However, they are not exempt from device malfunctions. A PubMed search for the key words (left ventricular assist device malfunction) (ventricular assist system malfunction) was performed. We identified 28 publications in the US Food and Drug Administration (FDA) website database that addressed LVAD malfunction. Twenty-nine FDA recalls were identified regarding LVAD malfunctions: 17 regarding HeartWare ventricular assist device, six for HeartMate II, three for HeartMate 3, and three for total artificial heart. Mechanisms involved in LVAD malfunction include battery malfunction, loose driveline connector, malfunction of the system controller, loose power supply connector ports, malfunction of the driveline splice kit, problems with the percutaneous lead connection, disconnection of the bend relief and outflow graft and outflow graft occlusion among others. Multiple mechanisms could be linked to LVAD malfunction. However, multiple device modifications have been developed over the past decade to avoid recurrent malfunctions. Constant improvements and research in biotechnology are needed to prevent these complications. It remains to be seen if newer generation devices will lead to improved patient outcomes over the long term.

COMPARISON AND EXPERIMENTAL VALIDATION OF TURBULENCE MODELS FOR AN AXIAL FLOW BLOOD PUMP

Computational fluid dynamics (CFD) has become an essential tool for designing and optimizing the structure of blood pumps. However, it is still questionable which turbulence model can better obtain the flow information for axial flow blood pump. In this study, the axial flow blood pump was used as the object, and the influence of the common turbulence models on simulation was compared. Six turbulence models (standard [Formula: see text]–[Formula: see text] model, RNG [Formula: see text]–[Formula: see text] model, standard [Formula: see text]–[Formula: see text] model, SST [Formula: see text]–[Formula: see text] model, Spalart–Allmaras model, SSG Reynolds stress model) were used to simulate the pressure difference and velocity field of the pump. In parallel, we designed a novel drive system of the axial flow blood pump, which allowed the camera to capture the internal flow field. Then we measured the flow field in the impeller region based on particle image velocimetry (PIV). Through the comparison of experiments and simulation results, the average errors of velocity field obtained by the above models are 30.97%, 19.40%, 24.25%, 15.28%, 28.51%, 23.00%, respectively. Since the SST [Formula: see text]–[Formula: see text] model has the smallest error, and the streamline is consistent with the experimental results, it is recommended to use SST [Formula: see text]–[Formula: see text] model for numerical analysis of the axial flow blood pump.

Cavitation in left ventricular assist device patients: a potential early sign of pump thrombosis

Mechanical ventricular support with left ventricular assist device (LVAD) has emerged as a durable and safe therapy, both as bridge-to-transplant (BTT) or destination therapy (DT), in patients with advanced heart failure (HF). However, the occurrence of pump thrombosis (PT) still represents a serious complication, especially when LVADs of first or second generation are implanted. During the latest years, some investigations have recognized the occurrence of cavitation, evidenced through transthoracic echocardiography (TTE), as a potential early and indirect sign of PT. In the present manuscript, we reviewed the available data on the occurrence of cavitation in LVAD patients as an early potential marker of PT, also presenting the hemodynamic mechanisms involved.

Clinical Relevance of Histopathologic Analysis of HeartMate II Thrombi

Left ventricular assist device (LVAD) thrombosis is a devastating complication that occurs in about 10% of patients despite anticoagulation and antiplatelet treatment. How the thrombus initiates and propagates is unknown. We pathologically and immunohistochemically examined 28 thrombi removed from 17 HeartMate II LVADs. Two groups of thrombi were found: those formed in the inlet/outlet and those on the rotor. The four thrombi found at the inlet (three inlet conduit and one inlet tube) and outlet (three at outlet elbow and one outlet graft) appeared similar and were composed of a loose meshwork of fibrin(ogen), von Willebrand factor, leukocytes, and aggregated platelets. The majority of the thrombi (20/28), however, were located on the rotor: nine at the inlet bearing, five on the rotor vanes, and six at the outlet bearing. Laminated thrombi formed around the inlet bearing in rings, an area of blood recirculation. The inner rings of the thrombus had fibrin and von Willebrand factor. Aggregated platelets were found in the outer thrombi rings, but limited evidence of platelets within the laminated thrombi was noted. The presence of distinct rings suggests development of the clot over time. The increased platelets in the outer rings of the inlet bearing thrombi would support further investigation into their role in thrombus growth. Initiating events require further investigation, but the fibrin-rich structure of HeartMate II thrombi suggests that alternative anticoagulation strategies are needed to prevent thrombosis in our LVAD patients.

Restoring pulsatility and peakVO2 in the era of continuous flow, fixed pump speed, left ventricular assist devices: 'A hypothesis of pump's or patient's speed?'

Despite significant improvement in survival and functional capacity after continuous flow left ventricular assist device implantation, the patient's quality of life may remain limited by complications such as aortic valve insufficiency, thromboembolic episodes and gastrointestinal bleeding attributed to high shear stress continuous flow with attenuated or absence of pulsatile flow and by a reduced peak oxygen consumption (peakVO₂) primarily associated with a fixed pump speed operation. Revision of current evidence suggests that high technology pump speed algorithms, a 'hypothesis of decreasing pump's speed' to promote pulsatile flow and a 'hypothesis of increasing pump's speed' to increase peakVO₂, may only partially reverse these barriers. A 'hypothesis of increasing patient's speed' is introduced, suggesting that exercise training may further contribute to the patient's recovery, enhancing peakVO₂ and pulsatile flow by improving skeletal muscle oxidative capacity and strength, peripheral vasodilatory and ventilatory responses, favour changes in preload/afterload and facilitate native flow, formulating the rationale for further studies in the field.

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.

Debate: creating adequate pulse with a continuous flow ventricular assist device: can it be done and should it be done? Probably not, it may cause more problems than benefits!

PURPOSE OF REVIEW

The feasibility and benefits of creating adequate pulsatility with continuous flow left ventricular assist devices (LVADs) have long been debated. This review discusses recent technical and clinical findings to answer whether such intervention should be implemented in the standard patient management.

RECENT FINDINGS

Only a limited amount of pulsatility can be generated by periodic speed steps, both considerably smaller in flow increase and in pace rate than the natural circulation. Organ systems are not impeded in their normal function and even not in recovery by a continuous flow. Known problems such as gastrointestinal bleeding are not necessarily due to pulsatility per se, or not important for therapeutic progress, such as minor modifications of the arterial walls.

SUMMARY

The speculative benefits of augmented pulsatility with continuous flow LVADs could be overrated and are still incompletely evaluated. Potential risks that might arise from this strategy should be carefully weighed before implementing extensive pulsatility as standard patient management.

High fidelity computational simulation of thrombus formation in Thoratec HeartMate II continuous flow ventricular assist device

Continuous flow ventricular assist devices (cfVADs) provide a life-saving therapy for severe heart failure. However, in recent years, the incidence of device-related thrombosis (resulting in stroke, device-exchange surgery or premature death) has been increasing dramatically, which has alarmed both the medical community and the FDA. The objective of this study was to gain improved understanding of the initiation and progression of thrombosis in one of the most commonly used cfVADs, the Thoratec HeartMate II. A computational fluid dynamics simulation (CFD) was performed using our recently updated mathematical model of thrombosis. The patterns of deposition predicted by simulation agreed well with clinical observations. Furthermore, thrombus accumulation was found to increase with decreased flow rate, and can be completely suppressed by the application of anticoagulants and/or improvement of surface chemistry. To our knowledge, this is the first simulation to explicitly model the processes of platelet deposition and thrombus growth in a continuous flow blood pump and thereby replicate patterns of deposition observed clinically. The use of this simulation tool over a range of hemodynamic, hematological, and anticoagulation conditions could assist physicians to personalize clinical management to mitigate the risk of thrombosis. It may also contribute to the design of future VADs that are less thrombogenic.

New Management Strategies in Heart Failure

Despite >100 clinical trials, only 2 new drugs had been approved by the US Food and Drug Administration for the treatment of chronic heart failure in more than a decade: the aldosterone antagonist eplerenone in 2003 and a fixed dose combination of hydralazine-isosorbide dinitrate in 2005. In contrast, 2015 has witnessed the Food and Drug Administration approval of 2 new drugs, both for the treatment of chronic heart failure with reduced ejection fraction: ivabradine and another combination drug, sacubitril/valsartan or LCZ696. Seemingly overnight, a range of therapeutic possibilities, evoking new physiological mechanisms, promise great hope for a disease that often carries a prognosis worse than many forms of cancer. Importantly, the newly available therapies represent a culmination of basic and translational research that actually spans many decades. This review will summarize newer drugs currently being used in the treatment of heart failure, as well as newer strategies increasingly explored for their utility during the stages of the heart failure syndrome.

Visualization and analysis of biomaterial-centered thrombus formation within a defined crevice under flow

The blood flow pathway within a device, together with the biomaterial surfaces and status of the patient's blood, are well-recognized factors in the development of thrombotic deposition and subsequent embolization. Blood flow patterns are of particular concern for devices such as blood pumps (i.e. ventricular assist devices, VADs) where shearing forces can be high, volumes are relatively large, and the flow fields can be complex. However, few studies have examined the effect of geometric irregularities on thrombus formation on clinically relevant opaque materials under flow. The objective of this study was to quantify human platelet deposition onto Ti6Al4V alloys, as well as positive and negative control surfaces, in the region of defined crevices (∼50-150 μm in width) that might be encountered in many VADs or other cardiovascular devices. To achieve this, reconstituted fresh human blood with hemoglobin-depleted red blood cells (to achieve optical clarity while maintaining relevant rheology), long working optics, and a custom designed parallel plate flow chamber were employed. The results showed that the least amount of platelet deposition occurred in the largest crevice size examined, which was counterintuitive. The greatest levels of deposition occurred in the 90 μm and 53 μm crevices at the lower wall shear rate. The results suggest that while crevices may be unavoidable in device manufacturing, the crevice size might be tailored, depending on the flow conditions, to reduce the risk of thromboembolic events. Further, these data might be used to improve the accuracy of predictive models of thrombotic deposition in cardiovascular devices to help optimize the blood flow path and reduce device thrombogenicity.