Biological effects of dynamic shear stress in cardiovascular pathologies and devices

Platelet Biorheology and Mechanobiology in Thrombosis and Hemostasis: Perspectives from Multiscale Computation

Thrombosis is the pathological clot formation under abnormal hemodynamic conditions, which can result in vascular obstruction, causing ischemic strokes and myocardial infarction. Thrombus growth under moderate to low shear (<1000 s-1) relies on platelet activation and coagulation. Thrombosis at elevated high shear rates (>10,000 s-1) is predominantly driven by unactivated platelet binding and aggregating mediated by von Willebrand factor (VWF), while platelet activation and coagulation are secondary in supporting and reinforcing the thrombus. Given the molecular and cellular level information it can access, multiscale computational modeling informed by biology can provide new pathophysiological mechanisms that are otherwise not accessible experimentally, holding promise for novel first-principle-based therapeutics. In this review, we summarize the key aspects of platelet biorheology and mechanobiology, focusing on the molecular and cellular scale events and how they build up to thrombosis through platelet adhesion and aggregation in the presence or absence of platelet activation. In particular, we highlight recent advancements in multiscale modeling of platelet biorheology and mechanobiology and how they can lead to the better prediction and quantification of thrombus formation, exemplifying the exciting paradigm of digital medicine.

In vitro thrombogenicity evaluation of rotary blood pumps by thromboelastometry

In vitro thrombogenicity tests for rotary blood pumps (RBPs) could benefit from assessing coagulation kinematics, as RBP design improves. In this feasibility study, we investigated if the method of thromboelastometry (TEM) is able to assess coagulation kinematics under the in vitro conditions of RBP tests. We conducted in vitro thrombogenicity tests (n=4) by placing Deltastream^® DP3 pumps into test loops that were filled with 150 mL of slightly anti-coagulated porcine blood, adjusted to an activated clotting time (ACT) well below clinically recommended levels. Blood samples were taken at certain time points during the experiment until a continuous decrease in pump flow indicated major thrombus formation. Blood samples were analyzed for ACT, platelet count (PLT), and several TEM parameters. While visible thrombus formation was observed in three pumps, ACT indicated an ongoing activation of coagulation, PLT might have indicated platelet consumption. Unexpectedly, most TEM results gave no clear indications. Nonetheless, TEM clotting time obtained by non-anticoagulated and chemically non-activated whole blood (HEPNATEM-CT) appeared to be more sensitive for the activation of coagulation in vitro than ACT, which might be of interest for future pump tests. However, more research regarding standardization of thrombogenicity pump tests is urgently required.

Characterization of the competing role of surface-contact and shear stress on platelet activation in the setting of blood contacting devices

Supraphysiological shear stress and surface-contact are recognized as driving mechanisms of platelet activation (PA) in blood contacting devices (BCDs). However, the competing role of these mechanisms in triggering thrombogenic events is poorly understood. Here, we characterized the dynamics of PA in response to the combined effect of shear stress and material exposure. Human platelets were stimulated with different levels of shear stress (500, 750, 1000 dynes/cm²) over a range of exposure times (10, 20, and 30 min) within capillary tubes made of various polymeric materials. Polyethylene (PE), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), and polyether ether ketone (PEEK), used for BCDs fabrication, were investigated as compared to glass and thromboresistant Sigma™-coated glass. PA was quantified using the Platelet Activity State assay. Our results indicate that mechanical stimulation and polymer surface-contact both significantly contribute to PA. Notably, the contribution of the mechanical stimulus ranges between +36% and +43%, while that associated with polymer surface-contact ranges from +48% to +59%, depending on the exposure time. In more detail, our results indicate that: (i) PA increases with increasing shear stress magnitude; (ii) PA has a non-linear, time-dependent relationship to exposure time; (iii) PA is largely influenced by biomaterials, with PE and PEEK having respectively the lowest and highest prothrombotic potential; (iv) the effects of polymer surface-contact and shear stress are not correlated and can be studied separately. Our results suggest the importance of incorporating the evaluation of platelet activation driven by the combined effect of shear stress and polymer surface-contact for the comprehensive assessment, and eventually minimization, of BCDs thrombogenic potential.

A high gas transfer efficiency microfluidic oxygenator for extracorporeal respiratory assist applications in critical care medicine

Advances in microfluidics technologies have spurred the development of a new generation of microfluidic respiratory assist devices, constructed using microfabrication techniques capable of producing microchannel dimensions similar to those found in human capillaries and gas transfer films in the same thickness range as the alveolar membrane. These devices have been tested in laboratory settings and in some cases in extracorporeal animal experiments, yet none have been advanced to human clinical studies. A major challenge in the development of microfluidic oxygenators is the difficulty in scaling the technology toward high blood flows necessary to support adult humans; such scaling efforts are often limited by the complexity of the fabrication process and the manner in which blood is distributed in a three-dimensional network of microchannels. Conceptually, a central advantage of microfluidic oxygenators over existing hollow-fiber membrane-based configurations is the potential for shallower channels and thinner gas transfer membranes, features that reduce oxygen diffusion distances, to result in a higher gas transfer efficiency defined as the ratio of the volume of oxygen transferred to the blood per unit time to the active surface area of the gas transfer membrane. If this ratio is not significantly higher than values reported for hollow fiber membrane oxygenators (HFMO), then the expected advantage of the microfluidic approach would not be realized in practice, potentially due to challenges encountered in blood distribution strategies when scaling microfluidic designs to higher flow rates. Here, we report on scaling of a microfluidic oxygenator design from 4 to 92 mL/min blood flow, within an order of magnitude of the flow rate required for neonatal applications. This scaled device is shown to have a gas transfer efficiency higher than any other reported system in the literature, including other microfluidic prototypes and commercial HFMO cartridges. While the high oxygen transfer efficiency is a promising advance toward clinical scaling of a microfluidic architecture, it is accompanied by an excessive blood pressure drop in the circuit, arising from a combination of shallow gas transfer channels and equally shallow distribution manifolds. Therefore, next-generation microfluidic oxygenators will require novel design and fabrication strategies to minimize pressure drops while maintaining very high oxygen transfer efficiencies.

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.

Platelet activation via dynamic conformational changes of von Willebrand factor under shear

Shear-induced conformational changes of von Willebrand factor (VWF) play an important role in platelet activation. A novel approach describing VWF unfolding on the platelet surface under dynamic shear stress is proposed. Cumulative effect of dynamic shear on platelet activation via conformational changes of VWF is analysed. The critical condition of shear-induced platelet activation is formulated. The explicit expression for the threshold value of cumulative shear stress as a function of VWF multimer size is derived. The results open novel prospects for pharmacological regulation of shear-induced platelet activation through control of VWF multimers size distribution.

Evaluating medical device and material thrombosis under flow: current and emerging technologies

This review highlights the importance of flow in medical device thrombosis and explores current and emerging technologies to evaluate dynamic biomaterial Thrombosis in vitro.

Oscillatory shear potentiates latent TGF-β1 activation more than steady shear as demonstrated by a novel force generator

Cardiovascular mechanical stresses trigger physiological and pathological cellular reactions including secretion of Transforming Growth Factor β1 ubiquitously in a latent form (LTGF-β1). While complex shear stresses can activate LTGF-β1, the mechanisms underlying LTGF-β1 activation remain unclear. We hypothesized that different types of shear stress differentially activate LTGF-β1. We designed a custom-built cone-and-plate device to generate steady shear (SS) forces, which are physiologic, or oscillatory shear (OSS) forces characteristic of pathologic states, by abruptly changing rotation directions. We then measured LTGF-β1 activation in platelet releasates. We modeled and measured flow profile changes between SS and OSS by computational fluid dynamics (CFD) simulations. We found a spike in shear rate during abrupt changes in rotation direction. OSS activated TGF-β1 levels significantly more than SS at all shear rates. OSS altered oxidation of free thiols to form more high molecular weight protein complex(es) than SS, a potential mechanism of shear-dependent LTGF-β1 activation. Increasing viscosity in platelet releasates produced higher shear stress and higher LTGF-β1 activation. OSS-generated active TGF-β1 stimulated higher pSmad2 signaling and endothelial to mesenchymal transition (EndoMT)-related genes PAI-1, collagen, and periostin expression in endothelial cells. Overall, our data suggest variable TGF-β1 activation and signaling occurs with competing blood flow patterns in the vasculature to generate complex shear stress, which activates higher levels of TGF-β1 to drive vascular remodeling.

Microfluidic flow-based platforms for induction and analysis of dynamic shear-mediated platelet activation-Initial validation versus the standardized hemodynamic shearing device

A microfluidic flow-based platform (μFP), able to stimulate platelets via exposure of shear stress patterns pertinent to cardiovascular devices and prostheses, was compared to the Hemodynamic Shearing Device (HSD)-a state-of-the-art bench-top system for exposure of platelets to defined levels and patterns of shear. Platelets were exposed to time-varying shear stress patterns in the two systems; in detail, platelets were recirculated in the μFP or stimulated in the HSD to replicate comparable exposure time. Shear-mediated platelet activation was evaluated via (i) the platelet activity state assay, allowing the measurement of platelet-mediated thrombin generation and associated prothrombotic tendencies, (ii) scanning electron microscopy to evaluate morphological changes of sheared platelets, and (iii) flow cytometry for the determination of platelet phosphatidylserine exposure as a marker of shear activation. The results revealed good matching and comparability between the two systems, with similar trends of platelet activation, formation of microaggregates, and analogous trends of activation marker exposure for both the HSD and microfluidic-stimulated samples. These findings support future translation of the microfluidic platform as a Point-of-Care facsimile system for the diagnosis of thrombotic risk in patients implanted with cardiovascular devices.

Is There a Relationship between Pressure Gradients through Contemporary Oxygenators and Immune Cell Proliferation during Cardiopulmonary Bypass? A Pilot Study

There have been many advances in the perfusion equipment used for cardiopulmonary bypass (CPB) surgery. A key component, the membrane oxygenator, has had a number of modifications in recent years and a recent clinical evaluation demonstrated disparity in various aspects of device performance. One difference among oxygenators, which to-date has received little attention, was the impact on the patient's immune cells, with some oxygenators producing a significantly greater increase in immune cell numbers after cross clamp. Such increases in immune cell proliferation may contribute to the development of a systemic inflammatory response (SIR), which has been demonstrated to have a negative impact on patient outcomes. Although factors contributing to immune cell proliferation during CPB are recognized to be multi-factorial, the goal of the current communication was to perform an ad hoc analysis of these raw data for evidence that pressure gradients through an oxygenator contributes to this outcome. Despite the observation that higher-pressure gradient oxygenators appeared to associate with increased immune cell proliferation, no correlation was detected in this analysis. This finding, however, provides further evidence for the complex nature of inflammation during CPB, which deserves ongoing discussion and investigation.

High Frequency Components of Hemodynamic Shear Stress Profiles are a Major Determinant of Shear-Mediated Platelet Activation in Therapeutic Blood Recirculating Devices

We systematically analyzed the relative contributions of frequency component elements of hemodynamic shear stress waveforms encountered in cardiovascular blood recirculating devices as to overall platelet activation over time. We demonstrated that high frequency oscillations are the major determinants for priming, triggering and yielding activated “prothrombotic behavior” for stimulated platelets, even if the imparted shear stress has low magnitude and brief exposure time. Conversely, the low frequency components of the stress signal, with limited oscillations over time, did not induce significant activation, despite being of high magnitude and/or exposure time. In vitro data were compared with numerical predictions computed according to a recently proposed numerical model of shear-mediated platelet activation. The numerical model effectively resolved the correlation between platelet activation and the various frequency components examined. However, numerical predictions exhibited a different activation trend compared to experimental results for different time points of a stress activation sequence. With this study we provide a more fundamental understanding for the mechanobiological responsiveness of circulating platelets to the hemodynamic environment of cardiovascular devices, and the importance of these environments in mediating life-threatening thromboembolic complications associated with shear-mediated platelet activation. Experimental data will guide further optimization of the thromboresistance of cardiovascular implantable therapeutic devices.

The platelet hammer: In vitro platelet activation under repetitive hypershear

Mechanical circulatory support (MCS) devices, such as ventricular assist devices and the total artificial heart, have emerged as a vital therapy for advanced and end-stage heart failure. However, MCS patients face life-long antiplatelet and anticoagulant therapy to minimize thrombotic complications resulting from the dynamic and supraphysiologic device-associated shear stress conditions, whose effect on platelet activation is poorly understood. We repeatedly exposed platelets to average shear stresses up to 1000 dyne/cm(2) for as short as 25 ms in the "platelet hammer," a syringe-capillary viscometer. Platelet activation state was measured using a modified prothrombinase assay and morphological changes analyzed using scanning electron microscopy. An increase in stress accumulation (SA), the product of shear stress and exposure time, led to an increase in the platelet activation state and post-high shear platelet activation rate, or sensitization. A significant increase in platelet activation state was observed beyond an SA of 1500 dyne-s/cm(2), with a marked increase in pseudopod length visible beyond an SA of 1000 dyne-s/cm(2). The platelet hammer may be used to study other shear-dependent pathologies and may ultimately enhance the safety and effectiveness of MCS devices and objective antithrombotic pharmacotherapy management.

Disturbed flow mediated modulation of shear forces on endothelial plane: A proposed model for studying endothelium around atherosclerotic plaques

Disturbed fluid flow or modulated shear stress is associated with vascular conditions such as atherosclerosis, thrombosis, and aneurysm. In vitro simulation of the fluid flow around the plaque micro-environment remains a challenging approach. Currently available models have limitations such as complications in protocols, high cost, incompetence of co-culture and not being suitable for massive expression studies. Hence, the present study aimed to develop a simple, versatile model based on Computational Fluid Dynamics (CFD) simulation. Current observations of CFD have shown the regions of modulated shear stress by the disturbed fluid flow. To execute and validate the model in real sense, cell morphology, cytoskeletal arrangement, cell death, reactive oxygen species (ROS) profile, nitric oxide production and disturbed flow markers under the above condition were assessed. Endothelium at disturbed flow region which had been exposed to low shear stress and swirling flow pattern showed morphological and expression similarities with the pathological disturbed flow environment reported previously. Altogether, the proposed model can serve as a platform to simulate the real time micro-environment of disturbed flow associated with eccentric plaque shapes and the possibilities of studying its downstream events.

Microfluidic approaches for the assessment of blood cell trauma: a focus on thrombotic risk in mechanical circulatory support devices

INTRODUCTION

Mechanical circulatory support devices (MCSDs) are emerging as a valuable therapeutic option for the management of end-stage heart failure. However, although recipients are routinely administered with anti-thrombotic (AT) drugs, thrombosis persists as a severe post-implant complication. Conventional clinical assays and coagulation markers demonstrate partial ability in preventing the onset of thrombosis. Through years, different laboratory techniques have been proposed as potential tools for the evaluation of platelets' hemostatic response in MCSD recipients. Most rely on platelet aggregation tests; they are performed in static or low shear conditions, neglecting the prominent contribution of MCSD shear-induced mechanical load in enhancing platelet activation (PA). On the other hand, those tests able to account for shear-induced PA have limited possibility of effective clinical translation.

AIMS AND METHODS

Advances on this side have been addressed by microfluidic technology. Microfluidic devices have been developed for AT drug monitoring under flow, able to replicate physiological and/or constant shear flow conditions in vitro. In this paper, we present a newly developed microfluidic platform able to expose platelets to MCSD-specific dynamic shear stress patterns. We performed in vitro tests circulating human platelets in the microfluidic platform and quantifying the dynamics of PA by means of the Platelet Activity State (PAS) assay.

RESULTS

Our results prove the feasibility of using microfluidics for the diagnosis of MCSD-related thrombotic risk. This study paves the way for the development of a miniaturized point-of-care device for monitoring AT drug regimen. Such a system may have significant impact on limiting the incidence of thrombosis in MCSD recipients.

Aspirin has limited ability to modulate shear-mediated platelet activation associated with elevated shear stress of ventricular assist devices

Continuous flow ventricular assist devices (cfVADs) while effective in advanced heart failure, remain plagued by thrombosis related to abnormal flows and elevated shear stress. To limit cfVAD thrombosis, patients utilize complex anti-thrombotic regimens built upon a foundation of aspirin (ASA). While much data exists on ASA as a modulator of biochemically-mediated platelet activation, limited data exists as to the efficacy of ASA as a means of limiting shear-mediated platelet activation, particularly under elevated shear stress common within cfVADs. We investigated the ability of ASA (20, 25 and 125 μM) to limit shear-mediated platelet activation under conditions of: 1) constant shear stress (30 dynes/cm(2) and 70 dynes/cm(2)); 2) dynamic shear stress, and 3) initial high shear exposure (70 dynes/cm(2)) followed by low shear exposure - i.e. a platelet sensitization protocol, utilizing a hemodynamic shearing device providing uniform shear stress in vitro. The efficacy of ASA to limit platelet activation mediated via passage through a clinical cfVAD system (DeBakey Micromed) in vitro was also studied. ASA reduced platelet activation only under conditions of low shear stress (38% reduction compared to control, n=10, p<0.004), with minimal protection at higher shear stress and under dynamic conditions (n=10, p>0.5) with no limitation of platelet sensitization. ASA had limited ability (25.6% reduction in platelet activation rate) to modulate shear-mediated platelet activation induced via cfVAD passage. These findings, while performed under "deconstructed" non-clinical conditions by utilizing purified platelets alone in vitro, provide a potential contributory mechanistic explanation for the persistent thrombosis rates experienced clinically in cfVAD patients despite ASA therapy. An opportunity exists to develop enhanced pharmacologic strategies to limit shear-mediated platelet activation at elevated shear levels associated with mechanical circulatory support devices.

Microfluidic emulation of mechanical circulatory support device shear-mediated platelet activation

Thrombosis of ventricular assist devices (VADs) compromises their performance, with associated risks of systemic embolization, stroke, pump stop and possible death. Anti-thrombotic (AT) drugs, utilized to limit thrombosis, are largely dosed empirically, with limited testing of their efficacy. Further, such testing, if performed, typically examines efficacy under static conditions, which is not reflective of actual shear-mediated flow. Here we adopted our previously developed Device Thrombogenicity Emulation methodology to design microfluidic platforms able to emulate representative shear stress profiles of mechanical circulatory support (MCS) devices. Our long-term goal is to utilize these systems for point-of-care (POC) personalized testing of AT efficacy under specific, individual shear profiles. First, we designed different types of microfluidic channels able to replicate sample shear stress patterns observed in MCS devices. Second, we explored the flexibility of microfluidic technology in generating dynamic shear stress profiles by modulating the geometrical features of the channels. Finally, we designed microfluidic channel systems able to emulate the shear stress profiles of two commercial VADs. From CFD analyses, the VAD-emulating microfluidic systems were able to replicate the main characteristics of the shear stress waveforms of the macroscale VADs (i.e., shear stress peaks and duration). Our results establish the basis for development of a lab-on-chip POC system able to perform device-specific and patient-specific platelet activation state assays.

Repetitive Hypershear Activates and Sensitizes Platelets in a Dose-Dependent Manner

Implantation of mechanical circulatory support (MCS) devices-ventricular assist devices and the total artificial heart-has emerged as a vital therapy for advanced and end-stage heart failure. Unfortunately, MCS patients face the requirement of life-long antiplatelet and anticoagulant therapy to combat thrombotic complications resulting from the dynamic and supraphysiologic shear stress conditions associated with such devices, whose effect on platelet activation is poorly understood. We developed a syringe-capillary viscometer-the "platelet hammer"-that repeatedly exposed platelets to average shear stresses up to 1000 dyne/cm(2) for as short as 25 ms. Platelet activation state was measured using a modified prothrombinase assay, with morphological changes analyzed using scanning electron microscopy. We observed an increase in platelet activation state and post-high shear platelet activation rate, or sensitization, with an increase in stress accumulation (SA), the product of shear stress and exposure time. A significant increase in platelet activation state was observed beyond an SA of 1500 dyne-s/cm(2) , with a marked increase in pseudopod length visible beyond an SA of 1000 dyne-s/cm(2) . Utility of the platelet hammer extends to studies of other shear-dependent pathologies, and may assist development of approaches to enhance the safety and effectiveness of MCS devices and objective antithrombotic pharmacotherapy management.

Platelet GpIba binding to von Willebrand Factor under fluid shear:contributions of the D′D3-domain, A1-domain flanking peptide and O-linked glycans

BACKGROUND

Von Willebrand Factor (VWF) A1-domain binding to platelet receptor GpIbα is an important fluid-shear dependent interaction that regulates both soluble VWF binding to platelets, and platelet tethering onto immobilized VWF. We evaluated the roles of different structural elements at the N-terminus of the A1-domain in regulating shear dependent platelet binding. Specifically, the focus was on the VWF D'D3-domain, A1-domain N-terminal flanking peptide (NFP), and O-glycans on this peptide.

METHODS AND RESULTS

Full-length dimeric VWF (ΔPro-VWF), dimeric VWF lacking the D'D3 domain (ΔD'D3-VWF), and ΔD'D3-VWF variants lacking either the NFP (ΔD'D3NFP(─)-VWF) or just O-glycans on this peptide (ΔD'D3OG(─)-VWF) were expressed. Monomeric VWF-A1 and D'D3-A1 were also produced. In ELISA, the apparent dissociation constant (KD) of soluble ΔPro-VWF binding to immobilized GpIbα (KD≈100 nmol/L) was 50- to 100-fold higher than other proteins lacking the D'D3 domain (KD~0.7 to 2.5 nmol/L). Additionally, in surface plasmon resonance studies, the on-rate of D'D3-A1 binding to immobilized GpIbα (kon=1.8±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.7 μmol/L) was reduced compared with the single VWF-A1 domain (kon=5.1±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.2 μmol/L). Thus, VWF-D'D3 primarily controls soluble VWF binding to GpIbα. In contrast, upon VWF immobilization, all molecular features regulated A1-GpIbα binding. Here, in ELISA, the number of apparent A1-domain sites available for binding GpIbα on ΔPro-VWF was ≈50% that of the ΔD'D3-VWF variants. In microfluidics based platelet adhesion measurements on immobilized VWF and thrombus formation assays on collagen, human platelet recruitment varied as ΔPro-VWF<ΔD'D3-VWF<ΔD'D3NFP(─)-VWF<ΔD'D3OG(─)-VWF.

CONCLUSIONS

Whereas VWF-D'D3 is the major regulator of soluble VWF binding to platelet GpIbα, both the D'D3-domain and N-terminal peptide regulate platelet translocation and thrombus formation.

The role of hemostasis in infective endocarditis

Infective endocarditis (IE) is a thromboinflammatory disease of the endocardium, with pathophysiology mostly the result of the interplay between microorganisms and modifiers of the hemostasis system. In this setting, the evidence gathered so far warrants a more systematic appraisal. In this review article, experimental and clinical data on the role of hemostasis in IE are summarized. Starting from the current pathogenetic model of IE, we discuss the dual role of platelets in this condition, the microbial interaction with the hemostasis system, also describing nonspecific hemostasis changes during sepsis. We finally propose our hypothesis of thrombophilia as a possible trigger of IE, highlighting the challenges that the study of hemostasis in IE presents. The role of hemostasis in IE appears to be an exciting field of research. The activity of the hemostasis system is highly relevant in terms of susceptibility, progression, and treatment of IE. Pharmacologic modulation of hemostasis before and after IE onset is possible and represents still a largely unexplored area of study.

Prosthetic heart valve obstruction: thrombolysis or surgical treatment?

Prosthetic valve thrombosis is a potentially life-threatening complication associated with high morbidity and mortality. Transthorasic and transoesophageal echocardiography play an important role to the diagnosis and provides incremental information about the optimal treatment strategy, while fluoroscopy and cardiac computed tomography may be of added value. Guidelines differ on whether surgical treatment or fibrinolysis should be the treatment of choice for the management of left-sided prosthetic valve thrombosis and these uncertainties underline the need for further prospective randomized controlled trials. Thrombus size, New York Heart Association functional class of the patient, the possible contraindications, the availability of each therapeutic option and the clinician's experience are important determinants for the management of prosthetic valve thrombosis.

Polymeric trileaflet prosthetic heart valves: evolution and path to clinical reality

Present prosthetic heart valves, while hemodynamically effective, remain limited by progressive structural deterioration of tissue valves or the burden of chronic anticoagulation for mechanical valves. An idealized valve prosthesis would eliminate these limitations. Polymeric heart valves (PHVs), fabricated from advanced polymeric materials, offer the potential of durability and hemocompatibility. Unfortunately, the clinical realization of PHVs to date has been hampered by findings of in vivo calcification, degradation and thrombosis. Here, the authors review the evolution of PHVs, evaluate the state of the art of this technology and propose a pathway towards clinical reality. In particular, the authors discuss the development of a novel aortic PHV that may be deployed via transcatheter implantation, as well as its optimization via device thrombogenicity emulation.

A novel mathematical model of activation and sensitization of platelets subjected to dynamic stress histories

Blood recirculating devices, such as ventricular assist devices and prosthetic heart valves, are burdened by thromboembolic complications requiring complex and lifelong anticoagulant therapy with its inherent hemorrhagic risks. Pathologic flow patterns occurring in such devices chronically activate platelets, and the optimization of their thrombogenic performance requires the development of flow-induced platelet activation models. However, existing models are based on empirical correlations using the well-established power law paradigm of constant levels of shear stress during certain exposure times as factors for mechanical platelet activation. These models are limited by their range of application and do not account for other relevant phenomena, such as loading rate dependence and platelet sensitization to high stress conditions, which characterize the dynamic flow conditions in devices. These limitations were addressed by developing a new class of phenomenological stress-induced platelet activation models that specifies the rate of platelet activation as a function of the entire stress history and results in a differential equation that can be directly integrated to calculate the cumulative levels of activation. The proposed model reverts to the power law under constant shear stress conditions and is able to describe experimental results in response to a diverse range of highly dynamic stress conditions found in blood recirculating devices. The model was tested in vitro under emulated device flow conditions and correlates well with experimental results. This new model provides a reliable and robust mathematical tool that can be incorporated into computational fluid dynamic studies in order to optimize design, with the goal of improving the thrombogenic performance of blood recirculating devices.

Device thrombogenicity emulation: a novel methodology for optimizing the thromboresistance of cardiovascular devices

Thrombotic complications with mechanical circulatory support (MCS) devices remain a critical limitation to their long-term use. Device-induced shear forces may enhance the thrombotic potential of MCS devices through chronic activation of platelets, with a known dose-time response of the platelets to the accumulated stress experienced while flowing through the device-mandating complex, lifelong anticoagulation therapy. To enhance the thromboresistance of these devices for facilitating their long-term use, a universal predictive methodology entitled device thrombogenicity emulation (DTE) was developed. DTE is aimed at optimizing the thromboresistance of any MCS device. It is designed to test device-mediated thrombogenicity, coupled with virtual design modifications, in an iterative approach. This disruptive technology combines in silico numerical simulations with in vitro measurements, by correlating device hemodynamics with platelet activity coagulation markers-before and after iterative design modifications aimed at achieving optimized thrombogenic performance. The design changes are first tested in the numerical domain, and the resultant device conditions are then emulated in a hemodynamic shearing device (HSD) in which platelet activity is measured under device emulated conditions. As such, DTE can be easily incorporated during the device research and development phase-achieving minimization of the device thrombogenicity before prototypes are built and tested thereby reducing the ultimate cost of preclinical and clinical trials. The robust capability of this predictive technology is demonstrated here in various MCS devices. The presented examples indicate the potential of DTE for reducing device thrombogenicity to a level that may obviate or significantly reduce the extent of anticoagulation currently mandated for patients implanted with MCS devices for safe long-term clinical use.

Design optimization of a mechanical heart valve for reducing valve thrombogenicity-A case study with ATS valve

Patients implanted with mechanical heart valves (MHV) or with ventricular assist devices that use MHV require mandatory lifelong anticoagulation for secondary stroke prevention. We recently developed a novel Device Thrombogenicity Emulator (DTE) methodology that interfaces numerical and experimental approaches to optimize the thrombogenic performance of the device and reduce the bleeding risk associated with anticoagulation therapy. Device Thrombogenicity Emulator uses stress-loading waveforms in pertinent platelet flow trajectories that are extracted from highly resolved numerical simulations and emulates these flow conditions in a programmable hemodynamic shearing device (HSD) by which platelet activity is measured. We have previously compared two MHV, ATS and the St. Jude Medical, and demonstrated that owing to its nonrecessed hinge design, the ATS valve offers improved thrombogenic performance. In this study, we further optimize the ATS valve thrombogenic performance, by modifying various design features of the valve, intended to achieve reduced thrombogenicity: 1) optimizing the leaflet-housing gap clearance; 2) increasing the effective maximum opening angle of the valve; and 3) introducing a streamlined channel between the leaflet stops of the valve that increases the effective flow area. We have demonstrated that the DTE optimization methodology can be used as test bed for developing devices with significantly improved thombogenic performance.

von Willebrand factor self-association on platelet GpIbalpha under hydrodynamic shear: effect on shear-induced platelet activation

The function of the mechanosensitive, multimeric blood protein von Willebrand factor (VWF) is dependent on its size. We tested the hypothesis that VWF may self-associate on the platelet glycoprotein Ibα (GpIbα) receptor under hydrodynamic shear. Consistent with this proposition, whereas Alexa-488-conjugated VWF (VWF-488) bound platelets at modest levels, addition of unlabeled VWF enhanced the extent of VWF-488 binding. Recombinant VWF lacking the A1-domain was conjugated with Alexa-488 to produce ΔA1-488. Although ΔA1-488 alone did not bind platelets under shear, this protein bound GpIbα on addition of either purified plasma VWF or recombinant full-length VWF. The extent of self-association increased with applied shear stress more than ∼ 60 to 70 dyne/cm(2). ΔA1-488 bound platelets in the milieu of plasma. On application of fluid shear to whole blood, half of the activated platelets had ΔA1-488 bound, suggesting that VWF self-association may be necessary for cell activation. Shearing platelets with 6-μm beads bearing either immobilized VWF or anti-GpIbα mAb resulted in cell activation at shear stress down to 2 to 5 dyne/cm(2). Taken together, the data suggest that fluid shear in circulation can increase the effective size of VWF bound to platelet GpIbα via protein self-association. This can trigger mechanotransduction and cell activation by enhancing the drag force applied on the cell-surface receptor.

Device Thrombogenicity Emulator (DTE)--design optimization methodology for cardiovascular devices: a study in two bileaflet MHV designs

Patients who receive prosthetic heart valve (PHV) implants require mandatory anticoagulation medication after implantation due to the thrombogenic potential of the valve. Optimization of PHV designs may facilitate reduction of flow-induced thrombogenicity and reduce or eliminate the need for post-implant anticoagulants. We present a methodology entitled Device Thrombogenicty Emulator (DTE) for optimizing the thrombo-resistance performance of PHV by combining numerical and experimental approaches. Two bileaflet mechanical heart valves (MHV) designs, St. Jude Medical (SJM) and ATS, were investigated by studying the effect of distinct flow phases on platelet activation. Transient turbulent and direct numerical simulations (DNS) were conducted, and stress loading histories experienced by the platelets were calculated along flow trajectories. The numerical simulations indicated distinct design dependent differences between the two valves. The stress loading waveforms extracted from the numerical simulations were programmed into a hemodynamic shearing device (HSD), emulating the flow conditions past the valves in distinct 'hot-spot' flow regions that are implicated in MHV thrombogenicity. The resultant platelet activity was measured with a modified prothrombinase assay, and was found to be significantly higher in the SJM valve, mostly during the regurgitation phase. The experimental results were in excellent agreement with the calculated platelet activation potential. This establishes the utility of the DTE methodology for serving as a test bed for evaluating design modifications for achieving better thrombogenic performance for such devices.

Transcatheter pulmonary valve implantation using the Edwards SAPIEN transcatheter heart valve

BACKGROUND

Conduits placed in the right ventricular outflow tract (RVOT) have limited longevity which often requires increasingly complex reoperations. Transcatheter pulmonary valve implantation improves conduit hemodynamics through a minimally invasive approach. We present data for 7 patients treated with the Edwards SAPIEN transcatheter heart valve (THV).

PATIENTS

Patients' ranged in age from 16 to 52 years, one was female, and all had NYHA class II-III symptoms. Patients had pulmonary homografts that had been placed 2-25 years earlier during the Ross procedure (n = 4), repaired double outlet right ventricle with situs inversus (n = 1), or Rastelli repair for D-TGA, pulmonary atresia, and ventricular septal defect (n = 2). Patients had either severe pulmonary stenosis and/or moderate to severe pulmonary regurgitation.

RESULTS

All patients had successful percutaneous implantation of the 23 mm SAPIEN THV under general anesthesia. Fluoroscopy times ranged from 16 to 49 mins and procedure times ranged from 110 to 237 mins. The RV:systemic pressure ratio decreased from 78 +/- 18 to 39 +/- 8%, the RVOT gradient improved from 60.7 +/- 24.3 to 14.9 +/- 6.9 mm Hg, no patients had pulmonary insufficiency, and all patients had symptom improvement. At a maximum follow-up of 3.5 years (median 22.5 months), Doppler peak gradients ranged from 7-36 mm Hg, and there is no evidence of late stent fracture or structural valve failure.

CONCLUSION

The SAPIEN THV can be used successfully in the treatment of patients with right ventricle to pulmonary artery homograft failure. The valve is durable to at least 3.5 years without stent fracture or regurgitation. Clinical trials are underway to assess the long-term safety and efficacy of this valve.

In vitro model of platelet-endothelial activation due to cigarette smoke under cardiovascular circulation conditions

Cigarette smoke has been shown to increase platelet activation and endothelial cell (EC) adhesion molecule expression. In the present study, we utilized a hemodynamic shearing device (HSD) to investigate the above effects in vitro in a combined system of platelets and cultured HUVECs (Human Umblical Vein ECs) under physiological shear stress. We investigated the alteration of E-selectin expression on ECs upon exposure to: (1) platelets and nicotine-free smoke extract (NFE), (2) platelets alone, (3) NFE alone, under physiological shear stress. We additionally confirmed the protective effect of nicotine on platelet activation. We found that: (i) surface expression of E-selectin on ECs was significantly increased upon simultaneous exposure of ECs and platelets to NFE relative to exposure of ECs to either platelets or NFE alone (p < 0.05). (ii) Platelet activation was significantly increased in the presence of NFE (p < 0.05). (iii) Nicotine (200 nM) when added to NFE, significantly reduced platelet activation due to NFE (p < 0.05), an effect additionally confirmed by conventional cigarette extracts which contain nicotine (p < 0.05). We therefore conclude that: (a) NFE and platelets additively increase EC E-selectin surface expression, and (b) nicotine modulates platelet activation regardless of ECs.